Tillage, mineralization and leaching: phosphate
T.M. Addiscott
*, D. Thomas
Soil Science Department, IACR, Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK
Accepted 28 July 1999
Abstract
Phosphate is usually the limiting nutrient for the formation of algal blooms in freshwater bodies, so tillage practices must minimize phosphate losses by leaching and surface run-off from cultivated land. Mineral soils usually contain 30±70% of their phosphate in organic forms, and both organic and inorganic phosphate are found in the soil solution. Some organic phosphates, notably the inositol phosphates, are as strongly sorbed by soil as inorganic phosphates, and this decreases their susceptibility to mineralization. The strength with which both categories are sorbed lessens the risk of their being leached as solutes but makes it more likely that they will be carried from the soil on colloidal or particulate matter, and the greatest losses of phosphate from the soil usually occur by surface run-off and erosion. Recent studies at Rothamsted have, however, shown substantial concentrations of phosphate in drainage from plots that have long received more phosphate as fertilizer than is removed in crops. These losses probably occurred because preferential water ¯ow carried the phosphate rapidly from the surface soil to the ®eld drains. For lessening losses of phosphate by leaching and run-off, the prime requirement of tillage is that it should encourage ¯ows of water through the soil that help it to retain phosphate. Primary and secondary tillage should ensure that the surface roughness and porosity of the top-soil encourage the ¯ow of water into the soil matrix where it will move relatively slowly and allow phosphate to be sorbed, thereby avoiding problems from run-off and preferential ¯ow. Inversion tillage can be useful for lessening the loss of phosphate by run-off and erosion. Secondary tillage could be used to decrease the size of the aggregates and increase the surface area for sorption. Although tillage will increase the mineralization of organic phosphate, pulses of mineralization are unlikely to be so rapid or to lead to such large losses as with nitrate. The strength with which phosphate is sorbed also lessens the problem. As with nitrate, the key to managing phosphate is basically good husbandry.
#2000 Elsevier Science B.V. All rights reserved.
Keywords:Leaching; Surface run-off; Organic and inorganic phosphate; Erosion; Tillage; Mineralization
1. Introduction
Public concern about the effects of plant nutrients lost from agricultural land to the wider environment
has concentrated mainly on nitrate. This has resulted from fears that nitrate in public water supplies can cause methaemoglobinaemia in infants and stomach cancer in adults, both of which have proved unjusti®ed (Addiscott et al., 1991). These concerns, though understandable, have tended to distract attention from two facts: (1) the main problems caused by nutrient losses from agriculture are to our environment rather
*Corresponding author. Tel.: 44-1582-76-31-33; fax: 44-1582-76-09-81.
E-mail address: tom.addiscott@bbsrc.ac.uk (T.M. Addiscott).
than to our health, and (2) the main environmental problem, algal blooms, often has phosphate rather than nitrate as the limiting nutrient, particularly in freshwater (e.g., Reynolds, 1992; Sharpley et al., 1994; Ferguson et al., 1996). Whether nitrogen or phosphate is the limiting nutrient depends on the type of water body and the nature of the phosphate source, which may be a point source (often sewage ef¯uent) or a non-point source (usually agricultural land). We clearly need to pay at least as much attention to the behaviour of phosphate in soil as to that of nitrate and to extract as much information as possible from existing reports on the behaviour of phosphate. This paper attempts to do so with particular reference to the effects of tillage on the mineralization and leaching of phosphate.
This paper begins by listing and discussing the various chemical entities found in the soil that contain phosphate. It then considers the processes that these species may undergo and the interactions that occur between them. It asks next how tillage affects these processes and interactions and ®nally what its overall effect on the leaching of phosphate is likely to be.
2. Different forms of phosphate in the soil
Those who refer to soil phosphorus or soil P need to keep in mind that virtually all the phosphorus in the soil is there as phosphate (strictly as orthophosphate), PO4 (Frossard et al., 1995). Both organic and
inor-ganic phosphates are to be found (Table 1), but neither category is ever present to the exclusion of the other, and neither could be said to be the dominant category in soils worldwide. This is in contrast to nitrate, which
is found in the soil only as an anion and is never combined chemically with organic matter. There is also usually about 50 times as much organic nitrogen in a soil as there is nitrogen as nitrate.
2.1. Inorganic phosphate
Orthophosphoric acid is tribasic, but the ®rst dis-sociation constant is very much greater than the second or third. Durrant and Durrant (1962) reported disagreement about their values but gave the following approximations:
H2PO42HH2PO4ÿ; K1910ÿ3 (1)
H2PO4ÿ2HHPO42ÿ; K2610ÿ8 (2)
HPO42ÿ2HPO43ÿ; K3110ÿ12 (3)
The proportions of the three orthophosphate ions depend on the pH of the solution but all are likely to be present at the pH values likely to be found in most soils (Aslyng, 1954). All the dihydrogen phosphates are soluble in water, but of the other orthophosphates, only those of the alkali metals (except lithium) are water-soluble. Plants reportedly show a preference for the dihydrogen phosphate (Moser et al., 1959), and Aslyng (1954) gave a table showing the proportion of the total orthophosphate present in this form at various pH values. This proportion will be relevant to phos-phate leaching where the dihydrogen phosphos-phate is sorbed or precipitated preferentially or when plant uptake is likely to diminish leaching signi®cantly.
Inorganic phosphate is found in a variety of inso-luble forms, of which the commonest in the earth's crust is apatite (Frossard et al., 1995). This has the general formula Ca10X2(PO4)6, where X is OH
ÿ
or Fÿ
,
Table 1
Some categories of phosphate in the soil
Category Subcategory Examples References
Inorganic Ionic PO43
ÿ , HPO42
ÿ , H2PO4
ÿ
Aslyng (1954) Mineral Apatite, tinticite Frossard et al. (1995) Organic Monoesters Inositol hexaphosphate Anderson et al. (1974)
Diesters Phospholipids Newman and Tate (1980); Hawkes et al. (1984) Nucleic acids Newman and Tate (1980); Hawkes et al. (1984) Biomass P Microbial P Brookes et al. (1982, 1984)
Adenosine triphosphate Jenkinson et al. (1979)
giving hydroxyapatite or ¯uoroapatite, respectively. Calcium may be substituted by sodium or magnesium and phosphate by carbonate. Monocalcium, dicalcium and octocalcium phosphates are also found in soils in which calcium predominates over aluminium and iron. Where the latter metals dominate the system, the phosphate compounds formed are not usually well crystallized (Frossard et al., 1995). Reaction with aluminium oxides may give an amorphous phosphate or an organized phase such as sterrerite, (Al(OH)2)3
H-PO4H2PO4, while iron oxides may give tinticite,
Fe6(PO4)4(OH)67H2O or griphite, Fe3Mn2(PO4)2
-(OH)2.
2.2. Organic phosphate
The organic component usually comprises 30±70% of the phosphate in mineral soils (e.g., Dormaar, 1972; Hedley et al., 1982; Tiessen et al., 1984). It is found in a wide range of forms in the soil (Table 1), which is not surprising, given its role in metabolic energy transfer and other life processes. Some forms are clearly de®ned from a chemical point of view, others less so. Organic phosphate is often fractionated using chemical extractants, giving fractions that are de®ned in terms of the extractant (e.g., Hedley et al., 1982; Sharpley, 1985a), but there does not seem to be a generally accepted procedure. We discuss two chemi-cally identi®able forms and two with less speci®c identities.
2.2.1. Monoesters
The term monoester-phosphate is used to describe compounds with the general structure ROH2PO3, of
which the commonest in soils is inositol hexapho-sphate. Inositol is essentially a hexane ring on which each carbon atom carries a hydrogen atom and a hydroxyl group. Its hexaphosphate, also known as phytin or phytic acid, results from the esteri®cation of each hydroxyl group. It has been known to soil scientists for many years (Anderson, 1955; Arnold, 1956), and Arnold (1956) studied its hydrolysis, ®nd-ing that the ester linkages were not all broken at the same time. The presence of the organic group in the molecule does not prevent the phosphate group from being sorbed by the soil, and this plays an important part in the compound's behaviour in the soil (Ander-son et al., 1974).
2.2.2. Diesters
Diesters have the general structure (RO)(R0
O)-HPO3, but this simpli®ed structure covers a wide range
of compounds that include fragments of RNA (Ander-son, 1970; Newman and Tate, 1980), phospholipids (Newman and Tate, 1980) and teichoic acid (Ander-son, 1980), a compound which consists of sugar units linked by phosphate groups and which may originate from bacterial cell walls (Ward, 1981).
2.2.3. Microbial biomass phosphate
The term microbial biomass has evolved as a col-lective term for the bacteria, fungi and small soil animals that between them effect the turn-over of organic matter in the soil (Jenkinson and Powlson, 1976). Phosphate is a constituent of phospholipids, DNA and RNA in these organisms and is also involved in their metabolic energy transfers. Brookes et al. (1984) estimated the microbial biomass phosphate in six arable soils to be between 6 and 24 kg haÿ1
, i.e., about 3% of the organic phosphate, and that in eight grassland soils to be between 18 and 101 kg haÿ1
, about 14% of the organic phosphate.
2.2.4. Humic phosphate
The term humic phosphate is used here to describe phosphate associated with dead organic matter that does not fall into either of the ester categories. In terms of the extraction procedure described by Hedley et al. (1982), this would be phosphate left after treatment with chloroform and sodium bicarbonate. Some of it will be susceptible to mineralization by microbes in the soil and some inert.
3. Processes
3.1. Inorganic and physical chemical processes
3.1.1. Dissolution and precipitation
phosphate, but it has tended to be replaced by more modern compound fertilizers containing monoammo-nium or diammomonoammo-nium phosphate.
Phosphate precipitation is of interest in the present context because it lessens the vulnerability of phos-phate to leaching, but in this respect it is usually less important than sorption. The process has to be dis-cussed in terms of the dominant metal oxides and hydroxides in the soil. In calcium systems, it can occur following the sorption of the phosphate on to a calcite surface (Cole et al., 1953; Freeman and Rowell, 1981), which leads to the precipitation of monocalcium phosphate, Ca(H2PO4)2. This changes to dicalcium
phosphate dihydrate, CaHPO42H2O, and thence to
octacalcium phosphate, Ca8H2(PO4)65H2O, and
hydroxyapatite, Ca10(OH)2(PO4)6. The last compound
is least soluble in water and should in theory control the phosphate concentration in the soil solution. In practice, apatite is not usually among the calcium phosphates found in the soil after application of fertilizer, possibly because other ions in the soil solution interfere with the formation of its crystals (Frossard et al., 1995). In soils dominated by alumi-nium or iron the precipitation cannot be described with the same detail and leads to the less well crystallized compounds discussed in Section 2.1.
3.1.2. Sorption and surface reaction
Phosphate sorption is again of interest in the present context because it lessens the vulnerability of phos-phate to leaching. It is a complex process that cannot be considered as independent from other physico-chemical processes in the soil. Physical chemists sometimes make the distinction between physical sorption, in which the sorbed substance is simply attracted to the sorbing surface, and chemical sorption, in which there is a surface reaction. The sorption of phosphate is best seen as a continuum, with some ions loosely held but most of them strongly (chemically) sorbed. Ions move within the continuum, and equili-brium will be reached if the system is left undisturbed. Both inorganic and organic phosphates are sorbed.
Because of the heterogeneous nature of the sorbing surfaces in most soil, detailed studies of the mechan-isms of sorption are often made on soil minerals rather than whole soils. Freeman and Rowell (1981) studied the sorption of inorganic orthophosphate ions by calcite and showed by scanning electron microscopy
that the sorption involved a precipitation reaction which produced hemispherical coral-like crystalline growths on the surface of the calcite. These underwent the sequence of chemical changes described earlier in the section.
The oxides and hydroxides of iron and aluminium play an important part in the sorption of phosphate (e.g., Yuan and Lavkulich, 1994).The role of goethite (FeOOH) has been studied in detail by Ognalaga et al. (1994) and Frossard et al. (1995). When it comes into contact with orthophosphate ions in aqueous solution there is a very rapid reaction involving exothermic ligand exchange between the ions and the reactive surface groups. A hydroxyl ion or a water molecule is released from the surface, and a phosphated surface complex is formed (Par®tt, 1978; Goldberg and Spo-sito, 1985; Torrent et al., 1990). The sorption of two monoester phosphates, inositol hexaphosphate and glucose-1-phosphate, on goethite was found by Ogna-laga et al. (1994) to show a very similar mechanism to that of the orthophosphate. Indeed, plotting the quan-tity of phosphate sorbed, the pH and the zeta-potential against the equilibrium concentration gave very simi-lar curves for all three sorbates. This ®nding accords with the observation by Anderson et al. (1974) that the sorption sites for inositol hexaphosphate and inorganic orthophosphate in acid soils were the same. Ognalaga et al. (1994) proposed a conformation for sorbed inositol hexaphosphate in which the hexane ring was parallel to the surface of the goethite, with four of the six phosphate units attached to it and the other two oriented in the other direction. This would be a very stable conformation and it would account for the observation of Anderson et al. (1974) that inositol hexaphosphate was sorbed in preference to inorganic orthophosphate in soils.
are considerable differences between the various clays in their ability to sorb phosphate, and ranking the clays and the iron and aluminium hydrous oxides for their capacity to sorb phosphate gives the following order (Sollins, cited by Frossard et al., 1995):
Montmorillonite<KaoliniteHematite
<Gibbsite<Goethite <Ferrihydrite<Allophane
Organic matter does not sorb phosphate directly in all circumstances, but it can have a strong in¯uence on the sorption or desorption of phosphate by other soil components. Carboxylates originating from organic matter may improve the availability of soil phosphate to crops (Staunton and Leprince, 1996) and presum-ably its vulnerability to leaching, but other reactions and reaction sequences involving organic matter may enhance its sorption. These reactions were covered recently by Frossard et al. (1995) in a comprehensive review of the reactions of phosphate in soils.
The relative importance of the various sorbing surfaces for phosphate in the soil will obviously vary greatly between soils. In the highly weathered soils of the tropics the oxides and hydroxides of iron and aluminium are likely to dominate phosphate sorption. Many African and South American soils, e.g., support only very small concentrations of phosphate in the soil solution and supply very little to crops unless fertilizer is applied (Le Mare, 1981, 1982; Warren, 1992). The strong sorption of inositol phosphate might have serious implications in such soils, because many
farm-ers are not easily able to afford mineral phosphate fertilizers. If they depend on organic fertilizers whose phosphate is mainly in organic forms, the sorption of such phosphate may limit its availability to crops. Allophane also sorbs phosphate very strongly and is likely to dominate phosphate sorption in soils in which it is present in appreciable amounts.
Calcareous soils are fairly widespread, so we need to assess the relative importance for phosphate sorp-tion in these soils of the sorbing surfaces associated with calcium, iron and aluminium. Holford et al. (1974) developed a Langmuir two-surface equation to describe phosphate sorption in soils in which differ-ing bonddiffer-ing energies were to be found, and Holford and Mattingly (1975) applied this to eight calcareous soils in UK, plots of which had received three differing amounts of superphosphate for at least 2 years. They inferred from the results the high-energy and low-energy Langmuir sorption capacities for the soils and computed multiple regressions of these capacities on the dithionite-extractable iron (Bascombe, 1968), the pH, the surface area of calcium carbonate (Talibudeen and Arambarri, 1964) and the percentage of organic matter in the soils. The high-energy sorption capacity was related highly signi®cantly to the dithionite-extractable iron but not to the other three variables (Table 2), suggesting that it was associated with the hydrous oxides of iron. The low-energy sorption capacity was highly signi®cantly related to both the surface area of calcium carbonate and the percentage of organic matter (Table 2), suggesting that both types of surface contributed low-energy sites. The
relation-Table 2
The main categories of site for phosphate sorption in soilsa
Category Calcareous soils Acid soils
High energy Low energy Britishb Tropical
Hydrous oxides
Ironc *** ± *** **
Aluminiumd * ± *** ***
Clay nte nte * *
Calcium carbonate ± *** ± ±
Organic matter ± *** ** **
aImportance shown by the number of asterisks from Holford and Mattingly (1975) and Lopez-Hernandez and Burnham (1974a,b). bRelationships for British soils depended on drainage. See text.
cDithionite-extractable Cof®n (1963); Bascombe (1968). dMcLean et al. (1958).
ship with the percentage of calcium carbonate, rather than its surface area, was not signi®cant, and Bakheit Said and Dakermaiji (1993) found the same in some Syrian soils. Holford and Mattingly (1975) calculated that phosphate was sorbed much less strongly on the calcium carbonate in the soils than it was in Jurassic limestone and suggested that this was because organic anions occupied many of the sites that would other-wise have sorbed phosphate. This conclusion seems to be supported by the effects of such anions reported by Staunton and Leprince (1996) in a calcic luvisol.
In acid soils, no contribution to phosphate sorption is expected from calcium carbonate, and extractable aluminium emerged as the factor dominating sorption in some acid Scottish soils (Williams et al., 1958). Lopez-Hernandez and Burnham (1974a) examined the behaviour of phosphate in 20 tropical and 20 British acidic soils, using `anion exchange capacity' of Piper (1942) and phosphate sorption index of Bache and Williams (1971). They related these indices to soil pH, percent clay, percent carbon, free iron oxides (dithio-nite-citrate extraction) and extractable aluminium (acidi®ed ammonium acetate), ®nding no differences between the British and the tropical soils (Table 2). Sorption of phosphate was well correlated with extrac-table aluminium and free iron oxides, the correlation with free iron oxides being the stronger in the freely drained British soils but not in the poorly drained ones. Sorption also correlated well with percent carbon in the poorly drained British soils and in the tropical soils when sorption was estimated using a large phosphate concentration. The relationships with pH and percent clay were not strong. However, when Lopez-Hernan-dez and Burnham (1974b) examined a group of ped-ologically similar soils differing mainly in pH, they found a highly signi®cant decrease in phosphate retention with increasing pH. This was associated to some extent with decreases in exchangeable and acetate-extractable aluminium. This accords with pre-vious studies on hydrous oxides, such as that of Bache (1964).
Ions not discussed in other contexts may in¯uence phosphate sorption through their contribution to the ionic strength of the soil solution. Ryden and Syers (1975) found that increasing the ionic strength enhanced sorption. This they attributed to the effect of ionic strength on the surface charge of sorbing surfaces and the thickness of the diffuse double layer,
but this effect depends on the ions contributing to the ionic strength (Choudhary et al., 1993). The dominant cation on the cation exchange complex also in¯uences phosphate sorption. Curtin et al. (1992) reported that sodium-saturated soil sorbed less phosphate than cal-cium-saturated soil and that the sorption was also in¯uenced by the pH of the soil. They could not explain the difference between the cations in terms of precipitation or surface reactions involving calcium phosphate, but they were able to deduce that it occurred during the rapid initial sorption rather than the time-dependant sorption discussed below.
The parent material from which the soil is formed is another in¯uence on phosphate sorption. Toreu et al. (1988) examined phosphate sorption on highly weath-ered soils derived from four parent materials in tro-pical Queensland. They found that the sorption capacity was greatest on those formed from basalt and lowest on those from granite, with metamorphic and alluvial material intermediate between them. Whether the soil is in a virgin or cultivated state may also be important. Mehadi and Taylor (1988) found that virgin soils sorbed more phosphate than those which had been cultivated, apparently because they had a lower pH and contained more exchangeable aluminium and free iron oxide. The effects of cultiva-tion on the mineralizacultiva-tion of phosphate are discussed in Section 4.2.
3.1.3. Time dependence of sorption
movement of a reaction front. Barrow (1983) devel-oped a computer model in which sorption was speci-®cally dependant on time. Another approach (Van Riemsdijk et al., 1984; Van der Zee et al., 1989) treats the slow sorption as a function of the `exposure integral'. This is the integral of phosphate concentra-tion with respect to time, the area under the curve obtained by plotting the concentration to which the soil is exposed against time (Fig. 1a). The fractional phosphate saturation of the soil is then related (Fig. 1b) to this exposure integral by a sigmoid curve (e.g., Freeze et al., 1995). This approach is useful because it allows parameters describing the slow sorption to be
derived from experimental data and used to simulate the outcome of the process at various times.
3.1.4. Desorption
Desorption is usually of interest because it is a process that makes phosphate available to plants. In the present context, we are concerned with it as one of the processes that make it vulnerable to leaching. Desorption usually occurs in response to a lessening of the phosphate concentration in the soil solution, most commonly as a result of uptake by plants. The concentration will also be lessened when rainfall percolates through the soil, and if appreciable deso-rption results, phosphate leaching may occur. In many soils the fact that the concentration of phosphate in solution has to be small for desorption to occur implies that phosphate leaching following desorption is unli-kely to be appreciable. The concentration of phos-phate in solution supported by a soil can be estimated by shaking it with 0.01 M calcium chloride solution. Johnston (1969) measured by this method the phos-phate concentration supported by the soils of the Broad-balk Experiment at Rothamsted. He also extracted these soils with 0.5 M sodium bicarbonate solution (Olsen et al., 1954), which is regarded as a good measure of phosphate available to plants. A plot of the concen-tration in the calcium chloride solution against the amount extracted by the method of Olsen et al. (Fig. 2) showed two interesting features: (a) the phosphate concentration in solution was very small when the bicarbonate-extractable phosphate was less than about 40 mg kgÿ1
but increased sharply when it was greater, and (b) soils from the two plots that receive farmyard manure supported larger phosphate concentrations than those from plots getting mineral fertilizer. Brookes et al. (1997) showed that data from soils from several experiments on broadly similar soils at Rothamsted could be combined in a similar relationship.
Three factors seem likely to be important in both sorption and desorption:
The proportion of the soil through which the water passes and, in particular, whether there is prefer-ential flow.
The residence time of the water in the soil.
The quantity of phosphate can be desorbed rapidly, as opposed to phosphate that can be desorbed only after a diffusion process.
Measurements of phosphate sorption often involve shaking the soil with more than its own volume of solution, so that a large proportion of the possible sorption sites are exposed to the phosphate in the solution. Similarly, in desorption studies, a large proportion of the phosphate in the soil is exposed to the solution. When water, with or without dissolved phosphate, percolates through the soil, it makes con-tact with only a limited proportion of the sorption sites. The extent of contact will depend on the nature of the soil (Section 3.3). The duration of contact is also important, bearing in mind the exposure integral dis-cussed above, so the residence time of the water in the soil becomes a relevant factor, and this again is in¯uenced by preferential ¯ow.
The third of the factors listed above, the quantity of phosphate that can be desorbed rapidly was shown by Heckrath et al. (1995) to be very important. They measured the phosphate concentrations in water drain-ing from the plots of the Broadbalk Experiment at Rothamsted and related them to the amount of phos-phate extracted from the soils by the extractant of Olsen et al. (1954). The concentrations in drainage were very small up to a `break point' above which they increased sharply with the amount of phosphate
extracted by the reagent (Fig. 3). This break point was at about 60 mg kgÿ1
of extractable phosphate, and phosphate seemed to be desorbed much more readily above it than below it. The relationships for the total and molybdate-reactive phosphate were similar, but the total phosphate concentrations were greater because some phosphate was transported on particu-late matter. Whether the phosphate extracted by the Olsen reagent corresponds exactly with the rapidly-desorbable phosphate is open to question, but it is clearly a useful measure of it.
Hawkes et al. (1984) used 31P Nuclear Magnetic Resonance to detect changes in phosphate fractions in the soil. They too found that phosphate from diesters was the most susceptible to mineralization, and that when old grassland was ploughed and left bare for about 25 years, the proportional decrease in diester phosphate was much greater than that of monoester phosphate.
3.2. Microbiological processes
3.2.1. Mineralization
Mineralization is not usually as signi®cant a con-tributor to inorganic phosphate in the soil as it is to
mineral nitrogen. Chater and Mattingly (1980) found that mineralization released between 0.5 and 8.5 kg haÿ1
per year of P as phosphate, the largest amounts coming from soils recently ploughed out of permanent grass or regularly given large dressings of farmyard manure and the smallest from soils long in arable cultivation without organic amendments. They calculated that the largest releases by mineralization represented about one-half of the annual phosphate uptake by an average cereal crop (in 1980, a crop yielding 5 Mg haÿ1
of grain). They added, by contrast, that mineralization could supply rather more than the whole nitrogen requirement of such a crop. This suggests that mineralization probably contributes less proportionally to phosphate losses from arable land than it does to nitrate losses. Sharpley (1985a) con-cluded that mineralization contributed about as much phosphate during the growing season as was supplied by fertilizer.
The various forms of organic phosphate found in soils were discussed in Section 2.2 and it is important to know which categories are most susceptible to mineralization. Condron et al. (1990) investigated
the amounts of monoester and diester phosphates and teichoic acid in three soil environments in Sas-katchewan, Canada. They found the greatest propor-tion of diesters in the part of the landscape least favourable for mineralization, and both diesters and teichoic acid were found only in native, uncultivated soils. In soils cultivated for 70±80 years, only monoe-sters were found. They concluded that diester phos-phates and teichoic acid are more readily mineralized than monoester phosphate. Monoesters such as inosi-tol phosphate are probably protected from mineraliza-tion by the strength with which they are sorbed by mineral components in the soil (Anderson et al., 1974). The distinction between organic and inorganic phosphates is probably not as sharp as for nitrogen compounds because of the stability of the monoester phosphates and because their sorption mechanism on goethite is so similar to that of inorganic phosphate. The diester phosphates include fragments of RNA and phospholipids which probably originate from dead microbial biomass. Bowman and Cole (1978) incubated various nucleotide phosphates in soil, including those of adenine, guanine, cytosine and
uracil, a mixture of nucleotides, and RNA. Sodium phytate, the sodium salt of inositol hexaphosphate, was incubated in the same way to provide a monoester phosphate for comparison. The net mineralization of phosphate from the nucleotide phosphates was between 60 and 75%, but from the inositol phosphate it was 3%.
A substantial proportion of the component described above as humic phosphate must also be mineralized if, as Sharpley (1985a) concluded, miner-alized phosphate contributes as much available phosphate for crops as fertilizer. Bowman and Cole (1978) concluded that diesters alone did not constitute a large proportion of the organic phosphate pool, and even if inositol phosphate and other monoesters are present in appreciable quantities, the strength with which they are sorbed by the soil limits their miner-alization. A substantial contribution from the humic component can also be inferred from the observation that where tillage has caused a decline in organic matter, the concentrations of carbon, nitrogen and phosphate have declined to similar extents (e.g., Bowman et al., 1990). Hedley et al. (1982) concluded that phosphate was released from both `extractable' and `stable' forms of organic phosphate. Tiessen et al. (1984) found conversely that the accumulation of organic matter depended of the availability of phosphate.
3.2.2. Immobilization
It is clear from the dependence of organic matter accumulation on phosphate that the latter must be immobilized in some circumstances, but the literature on the immobilization of phosphate is not extensive. Part of the reason must be that mineralization and immobilization occur simultaneously and both show considerable spatial variability. Hedley et al. (1982) found evidence of immobilization when soil was incubated, both when cellulose and nitrogen were added and when they were not. Rewetting and incu-bating an air-dried Rhodesian (now Zimbabwean) soil also immobilized phosphate (Salmon, 1965). How-ever, when Addiscott (1969) incubated a rewetted Tanzanian hillsand soil for 10 days, phosphate seemed to be immobilized during the ®rst 4 days and then re-released during the next 6 days, so that the ®nal concentration of phosphate differed little from that at the start.
3.2.3. Rapid changes in phosphate concentration
Mineralization and immobilization are slow pro-cesses when compared with another category of phos-phate transformation. All living cells, including those in the soil's microbial biomass, use the ADP±ATP energy shuttle to link supplying and energy-requiring reactions in their metabolism (Lehninger, 1965). The turn-over time of the terminal phosphate group on an ATP molecule is measured in fractions of a second (Lehninger, 1965), suggesting that phosphate concentrations could change rapidly during periods of intense microbial activity. White (1964) examined the changes in phosphate concentration and microbial respiration when soils were shaken in 0.01 M calcium chloride solution, and concluded that microbial uptake of phosphate became signi®cant after 2 h. Addiscott (1969), using the same extractant, measured a two- to three-fold decline in the phosphate concentration supported by a Tanzanian hillsand within 3 h. Because of the very small concentrations involved, between 10ÿ6
and 10ÿ7
M, the quantity of phosphate involved was small, about 0.1 mg P kgÿ1
soil or 0.2 kg P haÿ1
.
3.3. Transport processes
We found rather few papers concerned with phos-phate leaching from cultivated soils. This probably re¯ects the widespread perception that phosphate is a strongly sorbed species that is not vulnerable to leaching. This is a perception that had to be revised when Heckrath et al. (1995) found the relation shown in Fig. 3 and discussed above. It moves by leaching, surface run-off, erosion and diffusion. The last of these processes is so slow for phosphate in soil that it is of interest only at the scale of the soil aggregate or the cylinder of in¯uence around a root (Rowell et al., 1967). Its main interest in the present context is as a part of sorption. With the convective processes, we need to consider both the pathways for water over or through the soil and the in¯uences that determine the carrying capacity of the water for phosphate.
3.3.1. Water pathways
surface run-off on a sloping one. Accumulation causes water-logging and anaerobicity, while surface run-off carries nutrients and soil particles with it, leading to erosion in severe cases. Sharpley (1985b) found that in surface run-off the rainfall interacted with a thin layer of surface soil (10±25 mm) before leaving the ®eld. The fate of rain that in®ltrates the soil depends greatly on the type of soil. Water moves fairly uniformly through homogeneous sandy soils, although water-repellent organic coatings on the soil particles can cause `®ngering' of the water and thence non-uniform ¯ow. Silty soils generally show more distinction than sandy soils between mobile and immobile water in the soil, as evidenced by the patterns of chloride move-ment observed by Barbee and Brown (1986), but this does not usually extend to preferential ¯ow. Mobile and immobile categories of water are also found in many types of soil because of aggregation.
In clay soils patterns of ¯ow become more com-plicated in two ways. One form of ¯ow not usually found in the other soil types is the horizontal move-ment that occurs at the base of the plough layer. This results from the fact that the unploughed soil is far less permeable than the ploughed soil above it, and this effect can be intensi®ed by the compaction resulting from the vertical pressure exerted by the mouldboard of a plough. Clay soils are also far more likely than the others to show preferential ¯ow (Bouma and Dekker, 1978; Beven, 1981). This may arise because many clays develop cracks as a result of their propensity for swelling and shrinking. Continuous channels left by worms and roots also tend to be more durable in clay soils than in others. Clay subsoils are frequently very impermeable and need to have arti®cial drainage for the land to be usable for agriculture. If the preferential ¯ow pathways through the soil connect with the drainage system, a very ef®cient conduit is established from the soil surface to the point where the drain discharges into a ditch or stream. Thus, heavy rainfall can carry solutes and particulate, colloidal or organic material very rapidly from the soil surface to surface waters causing substantial phosphate losses (e.g., Duxbury and Peverly, 1978; Miller, 1979).
3.3.2. The carrying capacity of the water for phosphate
We saw in Section 3.1 that for phosphate to desorb from the soil surface the concentration in the soil
solution usually needs to be very small, implying that phosphate concentrations in water ¯owing through the soil are likely to be small. If, however, the sorbing phase is carried in the water much more phosphate can be transported. Matter varying considerably in size, composition and sorption capacity may be carried, depending on the type of soil and the intensity of the rainfall causing the ¯ow.
Particulate matter(>0.45mm). Substantial amounts
of large particulate matter may be carried in both horizontal and vertical ¯ows during heavy rain, but its relatively large size compared with other material carried and its consequently relatively small surface area to volume ratio may mean that it is not a particularly important carrier of phosphate.
Clay(<0.02mm). Clay is well known to be carried
downwards in soils. (Translocated clay is a frequently reported pedological feature of soil pro®les.) It is also carried laterally in surface run-off. The amount of phosphate it carries depends on the degree to which it is dispersed. Partially aggregated clay does not have as large a surface area as clay which has formed a colloidal suspension in the water carrying it. Clay in colloidal suspension should be an effective carrier for phosphate, but although the concept of colloidally facilitated transport of phosphate and pesticides has attracted considerable interest, we are not aware of any de®nite experimental evidence that it contributes sub-stantially to the downward leaching of phosphate. Surface run-off does carry both soil and phosphate with it, but it is dif®cult to de®ne where colloidally facilitated transport ends and erosion begins. During erosion, clay and colloidal organic matter are trans-ported preferentially, so that the eroded material becomes richer in phosphate than the original soil (Sharpley, 1985c). Sharpley et al. (1994) concluded that run-off and erosion are the main mechanisms by which phosphate is lost from agricultural soils, and the same group (Sharpley et al., 1992) has also provided equations for estimating these losses. Catt et al. (1994) measured phosphate losses in an erosion experiment, ®nding small (1 kg haÿ1
) annual amounts in the run-off water but 0.1±13 kg haÿ1
organic matter and dissolved organic matter of high molecular weight. Phosphate can be sorbed on organic matter (Section 3.1), but Holford and Mattingly (1975) found that organic matter contributed only to low-energy sorption sites, while Lopez-Hernandez and Burnham (1974a) found sorption on organic matter mainly in poorly drained soils. Providing that the organic matter carried in soil water has surface proper-ties similar to those of organic matter in the soil matrix, transport on organic matter seems likely to contribute to phosphate movement but this may not be as signi®cant as for those pesticides whose sorption is related speci®cally to organic matter. Also, appreci-able concentrations of organic phosphates may be dissolved in water ¯owing through both arable (Han-napel et al., 1964) and grassland (Haygarth et al., 1998) soils.
3.4. Interactions between processes
3.4.1. Microbial and physical chemical processes
We saw above that sorption on the soil surface makes organic matter less accessible to microbes and so tends to inhibit mineralization. Precipitation would have a similar effect. Anderson et al. (1974) suggested that the strength with which inositol phos-phate was sorbed and the resultant protection from mineralization was one of the reasons why this com-pound was so widespread in the soil environment. Bowman and Cole (1978) showed that it was miner-alized to a far smaller extent than diester phosphates, probably for this reason.
3.4.2. Physical chemical and transport processes
That precipitation and sorption lessen phosphate leaching by withholding it from the water ¯owing through the soil is self-evident. Conversely, leaching lessens the physical chemical processes, by removing the phosphate from the reaction surfaces. This empha-sizes the importance of the distinction between rapid and longer term slow reactions and the relevance of the concept of the `exposure integral' discussed in Section 3.1. The above is true, of course, only as long as the soil material remains immobile. Colloidal or particulate matter suspended in the water increases its phosphate-carrying capacity (Section 3.3) and thus increases losses, particularly during surface run-off.
3.4.3. Mineralization and leaching
As with nitrate, mineralization increases the quan-tity of phosphate vulnerable to leaching. Mineraliza-tion, however, does not need to proceed as far as inorganic orthophosphate to contribute to phosphate leaching. We saw in Section 2.2 that a substantial proportion of the phosphate in the soil is in organic forms, and the same has long been known to be true of the soil solution (Pierre and Parker, 1927). Even ear-lier, Dyer (1902) pointed out that the downward penetration of phosphate through soil at Rothamsted was greater in plots receiving farmyard manure than in those receiving mineral fertilizers. More recent data from long-term experiments at Rothamsted (Johnston, 1976; Johnston and Poulton, 1992) also show that, where farmyard manure or grassland has led to increases in the organic matter in the top-soil, there is greater movement of phosphate down the soil pro®le (Table 3). Dyer attributed this movement to earth-worm activity, possibly having preferential ¯ow in mind, but (laboratory) soil column experiments by Hannapel et al. (1964) suggest a more direct role for the organic residues in the manure. They found that adding barley (Hordeum vulgareL.) and bean ( Pha-seolussp.) residues or sucrose increased the amount of phosphate moving down through the columns. This increase was accounted for by organic phosphate (Table 4). By contrast, adding inorganic phosphate equivalent to that in the residues, but without residues, did not increase the amount moving. Studies using32P showed that a large proportion of the extra organic phosphate was mobilized by soil microbes from the
Table 3
Percentage increase in total phosphate, relative to plot receiving no phosphate, in layers of arable soil receiving inorganic (superpho-sphate) fertilizer (IF) or farmyard manure (FYM), and grassland receiving inorganic fertilizera
Soil layer (cm) Percentage increase
Arable land Grassland
IF FYM IF
0±23 66 76 148
23±30 13 40 41
30±46 8 27 20
46 0 11 ±
indigenous soil organic phosphate. These laboratory studies were supported by measurements on soil pro-®les in the ®eld by Frossard et al. (1989), who found that several forms of organic phosphate all had greater mobilities than inorganic phosphate in the B-horizons of a soil sequence covering the transition from grass-land to forest. They concluded that the leaching of organic phosphate through the soil pro®le and phos-phate losses in surface run-off were both played an important part in the fate of phosphate in these soils.
4. Effects of tillage
Tillage operations fall into three categories, primary tillage, secondary tillage and subsoiling. Primary til-lage usually aims to loosen compacted soil and it often involves total or partial inversion of the top 250 mm of soil, which buries weeds and incorporates crop resi-dues so that they can be broken down by microbes. Secondary tillage causes further soil fragmentation intended to produce a seedbed. Both types of tillage are part of the annual routine for many farmers, but subsoiling is usually done only on an occasional basis. It goes well below the depth of the other operations to loosen dense or compacted subsoils or to provide more drainage channels.
Tillage has a very important effect on the structure of the soil (e.g., Dexter, 1988). In particular, it changes the size distribution of the aggregates in the top-soil
and the water pathways through it. The fragmentation may also lessen the porosity of the aggregates; their mass is conserved, but their porosity is not (Currie, 1966). It also usually compacts the soil at the base of the plough layer, making it more impermeable, and this can have signi®cant consequences for ¯ows of water and pollutants from the soil.
4.1. Tillage effects on physical chemical processes
Tillage obviously does not have a direct effect on the mechanisms of precipitation, sorption or deso-rption, but it can alter considerably the nature and area of the surfaces on which these processes occur. Both primary and secondary tillage, particularly the latter, will greatly increase the surface area on which sorption and desorption can occur, as illustrated in the simple calculation in Table 5. (If the soil contains calcium carbonate, there will also be a larger area on which phosphate can be precipitated.) Tillage will also expose new surfaces on which these processes can occur, and it reorganizes the soil. Thus, preferential ¯ows are switched from the surfaces previously exposed to them to fresh surfaces. The above is, of course, true only for the plough layer unless subsoiling is done. This operation could have an appreciable effect on phosphate leaching in some circumstances, as is discussed later, because it creates new prefer-ential ¯ow pathways. The deliberate creation of a compacted layer, or `plough pan' (Addiscott and Dexter, 1994) could also have interesting conse-quences because the delay to water ¯ow would increase the `exposure integral' for phosphate sorption discussed in Section 3.1, so that more phosphate was sorbed and less leached.
Table 4
Cumulative amounts of organic and inorganic phosphate displaced from columns of sandy loam soil after incorporating barley or bean residues equivalent to 25 Mg haÿ1, adding phosphate equivalent to that in the bean residues as phosphoric acid or potassium hydrogen phosphate or adding sucrose and ammonium nitrate to give C and N equivalent to those in the bean residues
Material Cumulative displacement of phosphate (mg P)
Inorganic Organic Total
None (control) 74 79 153
Barley residues 78 662 740
Bean residues 98 789 887
H3PO4 62 95 157
K2HPO4 41 116 157
SucroseNH4NO3 120 1077 1197
S.E.a 7.9 38.9 42.0
aS.E.standard error from Hannapel et al. (1964).
Table 5
Simple illustration of the effects of `tillage' on the total surface area of cubic aggregatesa
Length of side (mm)
Number of cubes
Total surface area (mm2)
100 1 6104
10 103 6105
1 106 6106
4.2. Tillage effects on microbial processes
Tillage is well known to enhance the mineralization of organic matter in the soil (e.g., Rovira and Greacen, 1957; Powlson, 1980). Ploughing up old grassland, in particular, results in the mineralization of large amounts of organic nitrogen, 4 Mg haÿ1
in one ®eld at Rothamsted, and Whitmore et al. (1992) showed that the grassland ploughed during and after World War II made a substantial contribution to nitrate concentrations in natural waters. Losses of organic phosphate were not recorded in these studies, but estimates of the losses resulting from cultivating virgin land have been made at several sites in North America. These were obtained by comparing the amounts of organic phosphate in virgin land with those in land cultivated for various periods of time (Hedley et al., 1982; Bowman et al., 1990; Condron et al., 1990). The results, summarized in Table 6, show that the losses vary considerably with the type of soil, as do those of organic nitrogen. Considering three different periods, Bowman et al. (1990) showed that a substantial proportion of the loss occurred during the ®rst 3 years, and that the loss was approximately proportional to the square root of time. The loss of organic nitrogen at the Rothamsted site showed a broadly similar pattern.
The fate of the phosphate that was mineralized is not clear. The decline in organic phosphate was not matched by an increase in inorganic phosphate, and some of the cultivated soils seemed to have lost clay, and in some cases silt, when compared with their virgin counterparts. Thus, erosion may have
contrib-uted to the loss. Minimum tillage should decrease these losses, whatever their mechanism.
4.3. Tillage effects on transport processes
We are concerned here with phosphate losses by leaching and surface run-off, so we need to ask ®rst whether tillage in¯uences the partitioning of water between these processes. On a smooth soil surface, run-off can occur as soon as water begins to accumu-late on the soil surface, and tillage can lessen its likelihood by increasing the roughness of the soil (Dexter, 1977). It does so in two possible ways: it can increase the ability of the soil surface to store water temporarily in the depressions, so delaying the onset of run-off, and it can lessen the velocity of the run-off water. Once the depressional storage has been ®lled, the water will run-off unless there are large pores or other preferential pathways through which it can move rapidly into the soil, so the effect of tillage on such pathways is a factor in the partitioning. Sur-face sealing or `capping' of the soil needs to be avoided because it favours run-off and delays in®ltra-tion. Tillage will break up a soil cap, but we need to remember that such caps result from a particular distribution of particle sizes in the soil and that injudicious tillage can bring about this size distribu-tion and so aid the formadistribu-tion of a cap.
4.3.1. Leaching
Addiscott and Dexter (1994) reviewed the interac-tion between tillage and leaching and identi®ed sev-eral ways in which tillage was likely to in¯uence water ¯ow through the soil.
Tillage increases surface roughness and thence the capacity to store water in surface depressions, as described above. This will encourage in®ltration into the soil matrix rather than surface run-off or prefer-ential ¯ow, both of which are likely to carry solutes, including phosphate, rapidly from the soil surface into water bodies in which they are not wanted. Tillage also lessens the bulk density of the top-soil and thus increases its overall porosity, which will have broadly the same consequences. These effects will lessen phosphate leaching and will also interact in a bene-®cial way with the likely increase in surface area for sorption discussed in Section 4.1. We need to note, however, that in some circumstances, encouraging
Table 6
Percentage losses of organic phosphorus from virgin soils during various periods of cultivation
Soil type Cultivation period (years)
Percent loss
Reference
Black Chernozem 65 33 Hedley et al. (1982) Brown Chernozem 70 36 Condron et al. (1990) Gleysol 70 53 Condron et al. (1990) Luvisol 70 80 Condron et al. (1990) 3 39 Bowman et al. (1990)
Sandy loam 20 47
¯ow through the soil matrix may increase the leaching of nitrate.
Tillage and other operations leave wheelings in the soil in which cracks form subsequently. (They form because the soil in the bottom of the wheeling is wetter, and therefore physically weaker than the sur-rounding soil. When the soil shrinks, the crack forms at the weakest point.) Cracks so formed could act as preferential ¯ow pathways for phosphate loss.
Ploughing can leave a smeared or compacted layer at the base of the top-soil. This could have a bene®cial effect, as suggested in Section 4.1, by delaying down-ward water movement and increasing the `exposure integral' for phosphate sorption. It will, however, encourage horizontal ¯ow of water, and if this ¯ow is fairly rapid, phosphate losses to surface waters could be accelerated in some circumstances. Much will depend on the topography and the intensity of the rainfall.
Ploughing can seal-off the larger continuous pores in the soil, while direct drilling or minimum tillage leaves them open and should encourage preferential ¯ow, making any phosphate on the surface vulnerable to rapid leaching, whether it comes from fertilizer or other sources.
Tillage that inverts the soil moves any phosphate or crop residues on the surface to a depth of up to 250 mm (Fig. 4). This could be signi®cant in some circumstances. What may be more signi®cant, how-ever, is that practising minimum tillage and not invert-ing the soil while continuinvert-ing to broadcast phosphate fertilizer can lead to a build-up of phosphate in the soil at the surface. Such phosphate is very vulnerable to loss in the preferential ¯ow described in the preceding paragraph or to being carried off in surface run-off. Grif®th et al. (1977) found that the phosphate con-centration in the surface layer increased six-fold within few years when the soil was not inverted.
Clearly, good tillage practice can lessen phosphate losses by leaching, just as bad practice can make them worse. Fortunately, practices that retain water in the soil matrix and so help to store water in the soil for use by crops also seem likely to restrain losses of phos-phate by leaching.
4.3.2. Surface run-off
Sharpley et al. (1994) considered run-off and ero-sion to be the main mechanisms by which phosphate is lost from agricultural land, so the effects of tillage on these processes are important. Sharpley et al. (1992) showed that the concentration of `particulate phos-phate', that carried in solution on particulate matter (0.45mm), was 30±40 times greater in run-off from
fertilized, conventionally tilled wheat (Triticum sp.) than in that from unfertilized grassland. Whether the increased concentration came mainly from the tillage or the fertilizer is not certain, but the amount of soil eroded was about 100 times greater from the tilled land than from the grassland suggesting that the tillage was probably the major factor. This gives a reminder that the differences between conventional and mini-mum tillage also need to be considered. Three factors discussed in this paper seem particularly relevant.
Appropriate tillage should lessen run-off by increasing the surface storage capacity for water and lessening the run-off velocity (Section 4.3). The word `appropriate' is italicized because some tillage practices, such as tilling up and down rather than across the slope, increase run-off and erosion (Catt et al., 1994).
Run-off and erosion occur as rainfall interacts with a thin layer (10±25 mm) of surface soil (Section 3.3.1; Sharpley, 1985b).
Without tillage that inverts the soil (Fig. 4), phos-phate accumulates at the surface (Section 4.3.1; Griffith et al., 1977).
The last two points suggest strongly that inversion tillage can be useful as a means of removing phosphate from the critical top 25 mm of soil, where it tends to accumulate and is also at risk of being carried away in run-off. Sharpley et al. (1994) cite several papers showing that incorporating phosphate, from fertilizer or manure, beneath the soil surface lessened the loss by run-off. This practice will remain effective, of course, only until long-term fertilizer use and
tion bring about a uniformly large concentration of phosphate in the top-soil, as may have happened in some of the plots of the Broadbalk Experiment at Rothamsted (Heckrath et al., 1995). The risk of losses by run-off should generally be lessened by tillage practices designed to retain water within the soil. The most basic of these is simply to plough across the slope rather than up and down it (Catt et al., 1994).
5. Conclusions
We feel that the best form of conclusion is to suggest how tillage practice can most effectively lessen phosphate losses by leaching and surface run-off. This involves optimizing the interactions between the main processes that control losses, and we need to consider all three forms of tillage, primary, secondary and subsoiling.
The prime requirement is that tillage should opti-mize the flow of water through the soil. Primary and secondary tillage need to be adjusted to ensure that both the surface roughness and the porosity of the top-soil encourage the movement of water into the soil matrix where it will move relatively slowly and provide the opportunity for phosphate to be sorbed. This will avoid problems from run-off and preferential flow, both of which can move phos-phate rapidly into water bodies in which it is not wanted. Unwanted preferential flow could also, as noted above, be restricted by the adjustment of primary tillage to smear and compact the base of the top-soil, but this is an option that should be exercised with great care.
Inversion tillage clearly has a useful role in lessen-ing the susceptibility to loss by run-off and erosion of phosphate that has accumulated in the surface soil. Such accumulations are most likely to occur with minimum tillage, suggesting that where there is a risk of run-off and erosion it may be useful to interrupt periods of minimum tillage with at least one inversion tillage.
Tillage also needs to optimize the sorption of phosphate by the soil. Secondary tillage can be used to lessen the size range of the aggregates and thereby increase the surface area for sorption.
Caution may be needed in some soils, because too fine a tilth could be at risk from erosion by wind or water, and the wrong distribution of sizes may lead to soil capping.
Subsoiling may have a role too. Heckrath et al. (1995) reported enhanced phosphate concentra-tions in drainage from some plots of the Broadbalk Experiment at Rothamsted in which phosphate applications had exceeded removals by crops. One of the reasons they suggested for the penetra-tion of these enhanced concentrapenetra-tions through the subsoil was that, although the subsoil as a whole had ample sorption capacity for the excess phos-phate, water draining from these plots was moving through preferential pathways whose walls had become saturated with phosphate. The most appro-priate response seems to be to use subsoiling techniques to create new preferential pathways with pristine walls and thence unused sorption capacity.
Tillage practices adopted for other purposes will inevitably lead to mineralization of organic matter, and should therefore be used only in soils in which they are needed and are likely to be effective. It may be, however, that this is more important for avoiding losses of carbon and nitrogen than those of phosphate. The nitrate problem is dominated by mineralization (e.g., Addiscott et al., 1991), but surface run-off is probably the main factor in phosphate problems (Sharpley et al., 1994).
The comments above are essentially about careful management of soil resources, and they bring out the point that in controlling phosphate losses, as in many other land stewardship issues, there is ultimately no substitute for good husbandry.
Acknowledgements
References
Addiscott, T.M., 1969. A method for measuring the phosphate potential of a Tanzanian soil. East Afr. Agric. For. J. 35, 21±27. Addiscott, T.M., Dexter, A.R., 1994. Tillage and crop residue management effects on losses of chemicals from soils. Soil Tillage Res. 30, 125±168.
Addiscott, T.M., Whitmore, A.P., Powlson, D.S., 1991. Farming, Fertilisers and the Nitrate Problem. CAB International, Wall-ingford, 176 pp.
Aharoni, C., Sparks, D.L., Levinson, S., Ravina, I., 1991. Kinetics of soil chemical reactions: relationships between empirical equations and diffusion models. Soil Sci. Soc. Am. J. 55, 1307± 1312.
Anderson, G., 1955. Paper chromatography of inositol phosphates. Nature 175, 863±864.
Anderson, G., 1970. The isolation of nucleoside diphosphates from alkaline extracts of soil. J. Soil Sci. 21, 96±103.
Anderson, G., 1980. Assessing organic phosphorus in soil. In: Khasawneh, F.E., Sample, E.C., Kamprath, E.J. (Eds.), The Role of Phosphorus in Agriculture. American Society of Agronomy, Madison, WI, pp. 411±431.
Anderson, G., Williams, E.G., Moir, J.O., 1974. A comparison of the sorption of inorganic orthophosphate and inositol hexapho-sphate by six acid soils. J. Soil Sci. 25, 51±62.
Arnold, P.W., 1956. Paper ionophoresis of inositol phosphates, with a note on the acid hydrolysates of phytic acid. Biochim. Biophys. Acta 19, 552±554.
Aslyng, H.C., 1954. The lime and phosphate potentials of soils. Solubility and Availability of Phosphates. Royal Agric. Vet. Coll. Yearbook, Copenhagen.
Bache, B.W., 1964. Aluminium and iron studies relating to soils. II. Reactions between phosphate and hydrous oxides. J. Soil Sci. 15, 110±116.
Bache, B.W., Williams, E.G., 1971. A phosphate sorption index for soils. J. Soil Sci. 22, 289±301.
Bakheit Said, M., Dakermaiji, A., 1993. Phosphate adsorption and desorption by calcarous soils of Syria. Commun. Soil Sci. Plant Anal. 24, 197±210.
Barbee, G.C., Brown, K.W., 1986. Comparison between suction and free drainage soil solution samplers. Soil Sci. 141, 149±154. Barrow, N.J., 1983. A mechanistic model for describing the
sorption and desorption of phosphate by soil. J. Soil Sci. 34, 733±750.
Bascombe, C.L., 1968. Distribution of pyrophosphate-extractable iron and organic carbon in soils of various groups. J. Soil Sci. 19, 251±268.
Beven, K.J., 1981. Micro- meso- and macro-porosity and channelling phenomena in soils. Soil Sci. Soc. Am. J. 45, 1245. Bouma, J., Dekker, S.W., 1978. A case study on in®ltration into dry clay soil. I. Morphological observations. Geoderma 29, 27±40. Bowman, R.A., Cole, C.V., 1978. Transformation of organic phosphorus substrates in soils as evaluated by NaHCO3 extraction. Soil Sci. 125, 49±54.
Bowman, R.A., Reeder, J.D., Lober, R.B., 1990. Changes in soil properties in a central plains rangeland soil after 3, 20 and 60 years of cultivation. Soil Sci. 150, 851±857.
Brookes, P.C., Powlson, D.S., Jenkinson, D.S., 1982. Measurement of microbial biomass phosphorus in soil. Soil Biol. Biochem. 14, 319±329.
Brookes, P.C., Powlson, D.S., Jenkinson, D.S., 1984. Phosphorus in the soil microbial biomass. Soil Biol. Biochem. 16, 169± 175.
Brookes, P.C., De Smedt, J., Hofman, G., Vanderdeelen, J., 1997. Losses of phosphorus in drainage water. In: H. Tunney, O.T. Carlton, P.C. Brookes and A.E. Johnston (Editors) Phosphorus losses from Soil to water. CAB International, pp. 253±271. Catt, J.A., Quinton, J.N., Rickson, R.J., Styles, P., 1994. Nutrient
losses and crop yields in the Woburn erosion reference experiment. In: Rickson, R.J. (Ed.), Conserving Soil Resources: European Perspectives. CAB International, Wallingford, pp. 94±104.
Chater, M., Mattingly, G.E.G., 1980. Changes in organic phosphorus contents of soils from long-continued experiments at Rothamsted and Saxmundham. Report of the Rothamsted Experimental Station for 1979, Part II, pp. 41±61.
Choudhary, O.P., Arora, B.R., Hundal, H.S., 1993. Phosphate sorption characteristics of arid and semiarid soils of Punjab, India as in¯uenced by ionic composition of the supporting medium. Arid Soil Res. Rehabil. 7, 293±301.
Cof®n, D.E., 1963. A method for the determination of free iron in soils and clays. Can. J. Soil Sci. 43, 7±17.
Cole, C.V., Olsen, S.R., Scott, C.O., 1953. The nature of phosphate sorption by calcium carbonate. Soil Sci. Soc. Amer. Proc. 17, 352±356.
Condron, L.M., Frossard, E., Tiessen, H., Newman, R.A., Stewart, J.W.B., 1990. Chemical nature of organic phosphorus in cultivated and uncultivated soils under different environmental conditions. J. Soil Sci. 41, 41±50.
Currie, J.A., 1966. The volume and porosity of soil crumbs. J. Soil Sci. 17, 24±35.
Curtin, D., Syers, J.K., Bolan, N.S., 1992. Phosphate sorption by soil in relation to exchangeable cation composition and pH. Aust. J. Soil Res. 31, 137±149.
Dexter, A.R., 1977. Effect of rainfall on the surface micro-relief of tilled soil. J. Terramech. 14, 11±22.
Dexter, A.R., 1988. Advances in characterization of soil structure. Soil Tillage Res. 11, 199±238.
Dormaar, J.F., 1972. Seasonal pattern of soil organic phosphorus. Can. J. Soil Sci. 52, 107±112.
Durrant, P.J., Durrant, B., 1962. Introduction to Advanced Inorganic Chemistry. Longman, London, p. 725.
Duxbury, J.M., Peverly, J.H., 1978. Nitrogen and phosphorus losses from organic soils. J. Environ. Qual. 7, 566±570.
Dyer, B., 1902. Results of investigations on the Rothamsted soils. US Dept. Agric. Off. Exp. Sta. Bull. 106, 98.
Ferguson, A.J.D., Pearson, M.J., Reynolds, C.S., 1996. Eutrophica-tion of natural waters and toxic algal blooms. In: Agricultural Chemicals and the Environment (Eds R.E. Hester and R.M. Harrison), Issues in Environmental Science and Technology, No. 5, pp. 27±41. Cambridge, UK: The Royal Society of Chemistry.
Freeze, D., Van Riemsdijk, W.H., Van der Zee, S.E.A.T.M., 1995. Modelling phosphate-sorption kinetics in acid soils. Eur. J. Soil Sci. 46, 239±245.
Frossard, E., Brossard, M., Hedley, M.J., Metherall, A., 1995. Reactions controlling the cycling of P in soils. In: Tiessen, H. (Ed.), Phosphorus in the Global Environment. Wiley, New York, pp. 107±137.
Frossard, E., Stewart, J.W.B., St Amaud, R.J., 1989. Distribution and mobility of phosphorus in grassland and forest soils of Saskatchewan. Can. J. Soil Sci. 69, 401±416.
Goldberg, S., Sposito, G., 1985. On the mechanism of speci®c phosphate adsorption by hydroxylated mineral surfaces: a review. Commun. Soil Sci. Plant Anal. 16, 801±821. Grif®th, D.R., Mannering, J.V., Moldenhauer, W.C., 1977.
Con-servation tillage in the eastern Corn belt. J. Soil Water Conserv. 32, 20±28.
Hannapel, R.J., Fuller, W.H., Bosma, S., Bullock, J.S., 1964. Phosphorus in a calcareous soil. I. Predominance of organic forms of phosphorus in phosphate movement. Soil Sci. 97, 350±357.
Hawkes, G.E., Powlson, D.S., Randall, E.W., Tate, K.R., 1984. A 31P nuclear magnetic resonance study of the phosphorus species in alkaline extracts of soils from long-term ®eld experiments. J. Soil Sci. 5, 35±45.
Haygarth, P.M., Hepworth, L., Jarvis, S.C., 1998. Forms of phosphorus transfer in hydrological pathways from soil under grazed grassland. Eur. J. Soil Sci. 49, 65±72.
Heckrath, G., Brookes, P.C., Poulton, P.R., Goulding, K.W.T., 1995. Phosphorus leaching from soils containing different phosphorus concentrations in the Broadbalk experiment. J. Environ. Qual. 24, 904±910.
Hedley, M.J., Stewart, J.W.B., Chauhan, B.S., 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci. Soc. Am. J. 46, 970±997.
Hillel, D., 1977. Computer Simulation of Soil Water Dynamics: A Compendium of Recent Work. International Development Research Centre, Ottawa, 214 pp.
Holford, I.C.R., Mattingly, G.E.G., 1975. The high- and low-energy adsorbing surfaces in calcareous soils. J. Soil Sci. 26, 408±417. Holford, I.C.R., Wedderburn, R.W.M., Mattingly, G.E.G., 1974. A Langmuir two-surface equation as a model for phosphate adsorption by soils. J. Soil Sci. 25, 242±255.
Jenkinson, D.S., Powlson, D.S., 1976. The effects of biocidal treatments on metabolism in soil. V. A method of measuring soil biomass. Soil Biol. Biochem. 8, 209±213.
Jenkinson, D.S., Davidson, S.A., Powlson, D.S., 1979. Adenosine triphosphate and microbial biomass in soil. Soil Biol. Biochem. 11, 521±527.
Johnston, A.E., 1969. Plant nutrients in Broadbalk soils. Rep. Rothamsted Experimental Station for 1968, Pt II, pp. 93±115. Johnston, A.E., 1976. Additions and removals of nitrogen and phosphorus in long-term experiments at Rothamsted and Woburn and the effect of residues on total soil nitrogen and phosphorus. In: Agriculture and water Quality, Ministry of agriculture, Fisheries and Food Technical Bulletin No. 32, pp. 111±114.
Johnston, A.E., Poulton, P.R., 1992. The role of phosphorus in crop production and soil fertility: 150 years of ®eld experiments at Rothamsted, United Kingdom. In: Phosphate Fertilizers and the Environment. Proceedings of the IFDC workshop, March 23± 27, 1992, Tampa, Florida, USA.
Lehninger, A.L., 1965. Bioenergetics. Benjamin, New York, pp. 61±63.
Le Mare, P.H., 1981. Phosphorus sorption and release. In: Greenland, D.J. (Ed.), Characterization of Soils in Relation to their Classi®cation and Management for Crop Production: Examples from Some Areas of the Humid Tropics. Clarendon Press, Oxford, pp. 97±134.
Le Mare, P.H., 1982. Sorption of isotopically-exchangeable and non-exchangeable phosphate by some soils of Columbis and Brazil, and comparison with soils of southern Nigeria. J. Soil Sci. 33, 691±707.
Lopez-Hernandez, I.D., Burnham, C.P., 1974a. The covariance of phosphate sorption with other soil properties in some British and tropical soils. J. Soil Sci. 5, 196±206.
Lopez-Hernandez, I.D., Burnham, C.P., 1974b. The effect of pH on phosphate adsorption in soils. J. Soil Sci. 5, 207±216. McLean, E.O., Heddleston, M.R., Bartlett, R.J., Holowaychuk, N.,
1958. Aluminium in soils. I. Extraction method and magnitudes in clays and Ohio soils. Soil Sci. Soc. Am. Proc. 22, 382±387. Mehadi, A.A., Taylor, R.E., 1988. Phosphate adsorption by two
highly weathered soils. Soil Sci. Soc. Am. J. 52, 627±632. Miller, M.H., 1979. Contribution of nitrogen and phosphorus to
subsurface drainage water from intensively cropped mineral and organic soils on Ontario. J. Environ. Qual. 8, 42±48. Moser, U.S., Sutherland, W.H., Black, C.A., 1959. Evaluation of
laboratory indexes of absorption of soil phosphorus by plants. I. Plant Soil 10, 356±374.
Newman, R.H., Tate, K.R., 1980. Soil characterised by31P nuclear magnetic resonance. Commun. Soil Sci. Plant Anal. 11, 835± 842.
Nye, P.H., 1966. The measurement and mechanism of ion diffusion in soils. I. The relation between self-diffusion and bulk diffusion. J. Soil Sci. 17, 16±23.
Ognalaga, M., Frossard, E., Thomas, F., 1994. Glucose-1-phosphate and myo-inositol hexaGlucose-1-phosphate adsorption mechan-isms on goethite. Soil Sci. Soc. Am. J. 58, 332±337. Par®tt, R.L., 1978. Anion adsorption by soil and soil material. Adv.
Agron. 30, 1±50.
Pierre, W.H., Parker, F.W., 1927. Soil phosphorus studies. II. Soil Sci. 24, 119±128.
Piper, C.S., 1942. Soil and Plant Analysis. University of Adelaide, Adelaide.
Powlson, D.S., 1980. Effect of cultivation on the mineralization of nitrogen in the soil. Plant Soil 57, 151±153.
Reynolds, C.S., 1992. Eutrophication and the management of planktonic algae: what Vollenwieder couldn't tell us. In: Sutcliffe, D.W., Jones, J.G. (Eds.), Eutrophication: Research and Application to Water Supply. Freshwater Biological Association, Ambleside, pp. 4±29.
Rowell, D.L., Martin, M.W., Nye, P.H., 1967. The measurement and mechanism of ion diffusion in soils. III. The effect of moisture content and soil-solution concentration on the self-diffusion of ions in soils. J. Soil Sci. 18, 204±222.
Ryden, J.C., Syers, J.K., 1975. Rationalisation of ionic strength and cation effects on phosphate sorption by soils. J. Soil Sci. 26, 396±405.
Salmon, R.C., 1965. Changes in phosphate potential on re-wetting air-dry soil. Nature 205, 316.
Sharpley, A.N., 1985a. Phosphorus cycling in unfertilised and fertilised agricultural soils. Soil Sci. Soc. Am. J. 49, 905±911. Sharpley, A.N., 1985b. Depth of surface soil-runoff interaction as affected by rainfall soil slope and management. Soil Sci. Soc. Am. J. 49, 1010±1015.
Sharpley, A.N., 1985c. The selective erosion of plant nutrients in runoff. Soil Sci. Soc. Am. J. 49, 1527±1534.
Sharpley, A.N., Chapra, S.C., Wedepohl, R., Sims, J.T., Daniel, T.C., Reddy, K.R., 1994. Managing agricultural phosphorus for protection of surface waters. J. Environ. Qual. 23, 437±451. Sharpley, A.N., Smith, S.J., Jones, O.R., Berg, W.A., Coleman,
G.A., 1992. The transport of bioavailable phosphorus in agricultural runoff. J. Environ. Qual. 21, 30±35.
Staunton, S., Leprince, F., 1996. Effect of pH and some organic anions on the solubility of soil phosphate: implications for P bioavailability. Eur. J. Soil Sci. 47, 231±239.
Talibudeen, O., Arambarri, P., 1964. The in¯uence of the amount and the origin of calcium carbonates on the isotopically-exchangeable phosphate in calcareous soils. J. Agric. Sci. Camb. 62, 93±97.
Tiessen, H., Stewart, J.W.B., Cole, C.V., 1984. Pathways of phosphorus transformations in soils of different pedogenesis. Soil Sci. Soc. Am. J. 48, 853±858.
Tiessen, H., Stewart, J.W.B., Oberson, A., 1994. Innovative soil phosphorus availability indices: assessing organic phosphorus. In: Soil Testing: Prospects for Improving Nutrient Recommen-dations. SSSA Special Publication No. 40. American Society of Agronomy, Madison, WI.
Toreu, B.N., Thomas, F.G., Gillman, G.P., 1988. Phosphate sorption characteristics of soils of the North Queensland coastal region. Aust. J. Soil Res. 26, 465±477.
Torrent, J., Barron, V., Schwertmann, U., 1990. Phosphate adsorption and desorption by goethites differing in crystal morphology. Soil Sci. Soc. Am. J. 54, 1007±1012.
Van der Zee, S.E.A.T.M., Leus, F., Louer, M., 1989. Prediction of phosphate transport in small columns with an approximate sorption kinetics model. Water Resour. Res. 25, 1353±1365. Van Riemsdijk, W.H., Boumans, L.J.M., De Haan, F.A.M., 1984.
Phosphate sorption by soils. I. A model for phosphate reaction with metal oxides in soil. Soil Sci. Soc. Am. J. 48, 537±541. Ward, J.B., 1981. Teichoic and teichuronic acids: biosynthesis,
assembly and location. Microbiol. Rev. 45, 211±243. Warren, G.P., 1992. Fertilizer phosphorus: sorption and residual
value in tropical African soils. NRA Bulletin No. 37. Natural Resources Institute, Chatham, UK.
White, R.E., 1964. Studies on the phosphate potentials of soils. II. Microbial effects. Plant Soil 20, 184±193.
Whitmore, A.P., Bradbury, N.J., Johnson, P.A., 1992. The contribution of ploughed grassland to nitrate leaching. Agric. Ecosyst. Environ. 39, 221±233.
Williams, E.G., Scott, W.M., McDonald, J.M., 1958. Soil proper-ties and phosphate sorption. J. Sci. Food Agric. 9, 551±559. Yuan, G., Lavkulich, L.M., 1994. Phosphate sorption in relation to
