2.3 Establishing the minimum dataset
2.3.7 Olsen P
Phosphorus (P) is an essential element and is a macronutrient. A measure of soil P as a soil quality indicator in a minimum dataset, particularly for environmental interaction, is necessary. It is recognised that relatively low concentrations, in an agricultural context, of P in surface water (< 20 µg/l) can cause detrimental effects through nutrient enrichment and subsequent eutrophication. Further, recent evidence suggests that significantly more P may leave soils and enter into adjacent
environmental compartments than previously thought, via eroded soil material and overland flow, and also through land drains and soil through-flow (Johnes and
Hodgkinson, 1998). Finally, excessive soil P is understood to alter plant successional dynamics and community composition and for semi-natural, nutrient-poor habitats this can reduce diversity (Roem et al., 2002).
Olsen P or phosphorus concentrations as extracted from soil by a solution of sodium bicarbonate (0.5M NaHCO3) have been or are likely to be used in a range of soil quality indicator programmes (De Clerk et al., 2003; Sparling et al., 2003). Its possible inclusion here is due to the need for a more responsive measure of P behaviour in soils than total P. As the total amount of P held in soil solution at any time is very small relative to the total P content of the soils, measuring total soil P content gives little indication of the labile amount of P in soils and likely
environmental behaviour of P. This is because of the strong sorption of P to soil matrices and the very low solubility of P compounds, leading both to very long P residence times in the soil (> 1000 years) and to soil solution P (the most labile form) concentrations in the order of < 0.002% (Tisdale et al., 1993). Therefore, the use of a dilute chemical soil extractant, with its well known drawbacks (Barber, 1995), does give a broad indication of an operationally defined measure of P availability and potential environmental mobility (McDowell et al., 2001). There are, however, many methods for measuring P availability (around 26) and the actual forms of P within this available fraction are numerous and temporally and environmentally highly variable.
Nevertheless, links have been made between Olsen P concentrations and surface water quality (Heckrath et al., 1995).
The key concern for this paper is the use of Olsen P as a soil quality indicator for the function ‘environmental interaction’. More specifically, the aims are to challenge the use of Olsen P as a soil quality indicator. What does change in soil Olsen P mean?
What triggers in Olsen P exist? Is it possible to integrate Olsen P with other
indicators? How much does Olsen P measurement cost and what is the sensitivity and likely variability of the method used? Again, as with pH, methods for Olsen P may be standardised but its interpretation would not be, depending on major soil group and land use. This is an acknowledgement that different soil conditions are desirable for different land uses; that is, the soil condition is fit for that particular land use (Schipper and Sparling, 2000).
What does change mean?
Phosphorus, as an essential macronutrient, is required by all living organisms. It is involved in energy transfer reactions and plays a role in plant photosynthesis, respiration, cell enlargement and division – indeed, almost every metabolic reaction of any significance proceeds via a phosphate derivative. Phosphorus also promotes
early root formation and growth, and as plants mature most P moves into seeds and/or fruiting bodies; hence the quality of grain, fruit and vegetable crops is greatly improved with an adequate supply of P (Tisdale et al., 1993). Phosphorus, along with water and nitrogen, tends to be the dominant yield-limiting factor for agricultural crop growth.
In agricultural systems, too little available soil P results in stunted crops and reduced yield, and in grazing livestock may cause reduced milk yield, poor live-weight gains, reduced food intake and fertility. Therefore, it is broadly accepted that in intensive agricultural systems there is a significant requirement, for most soil types, to apply appreciable amounts of P in fertilizer or manurial form.
Too much soil phosphorus
The need for appreciable P inputs to soils from mineral and organic fertilizers in UK agriculture over the last 45-50 years has led to a present day soil P surplus
compared to the agronomic requirement of use. Repeated, inefficient applications of P to soils or through feed to livestock and the behaviour of P on entering the soil has led to this accumulation. For example, between 1979 and 1985, an increase in topsoil (0 – 15 cm) P of over 200 mg/kg has occurred under both arable and grassland
systems, representing an increase of almost 20% (MAFF, 2000).
This shift in the balance between P input and crop offtake in the UK is mirrored across Europe, with Belgium and the Netherlands having the greatest annual surpluses of 40 kg P/ha (cf. UK 16 kg P/ha) (Brouwer et al., 1995). Cropping systems are regarded as having the greatest P surplus, with 269 kg/ha, while losses to drainage channels depend not only on surplus size but also on land management and farming system (Figure 2.3.7.1) (Edwards and Withers, 1998). One of the results of this imbalance between inputs and outputs is a build up in soil P to levels that are of environmental rather than agronomic concern (Daniel et al., 1998).
* Remaining includes mixed farms and horticulture.
Figure 2.3.7.1: The contribution of farming types to the UK annual P surplus, as a percentage of the total agricultural land (Edwards and Withers, 1998)
The transfer of P from agricultural systems to other environmental compartments is complex, but is dependent upon three factors: the source of P; the mechanism of release from soil to water; and the hydrological pathways by which mobilised P moves from the land. The source of P, for example manure or inorganic fertiliser, determines the P form within the soil, that is, whether it is in an organic or inorganic form. The predominant mechanisms of release from soil to water are solubilisation (release to solution), detachment (primarily the erosion of soil particles) and
incidental transfers (loss of P before incorporation into the soil). Detachment is the most important mechanism of release, accounting for approximately 60% of total P loss, whereas solubilisation and incidental losses account for 20% each (Defra, 2002).
The magnitude, form and extent of P transport along the pathways shown in Figure 2.3.7.2 will vary considerably with land use, catchment topography, soil physical and chemical properties (including P content), rainfall duration and intensity (and
preceding hydrological conditions) and proximity to stream corridors (Johnes and Hodgkinson, 1998). Losses of P are always greater for cultivated soils than
uncultivated and semi-natural habitats, and losses via subsurface pathways greatest on poorly drained, high organic matter and heavily manured soils. Leaching and subsurface loss of P from agricultural systems would suggest that P is being transferred primarily in the dissolved form. This is not always the case; finer soil fractions may also be transferred, with P sorbed to soil colloids, along subsurface pathways and overland flow. The proportion of P transferred in particulate form can range from 17-60% of the P lost via drainage water (Heckrath et al., 1995; Hooda et al., 1996).
0 5 10 15 20 25 30 35 40 45
Cropping Pigs and Poultry
Dairy Remaining*
Farm Category
Percentage
Land Area, %.
P Surplus, %.
It is estimated that 43% of the phosphorus entering UK waterways is from agriculture, with the rest coming from point source discharges, such as sewage treatment works (Morse et al., 1993). For Olsen P to be a useful soil quality indicator in a minimum dataset and have relevance to the soil function of environmental interaction, there must be an explicit link between measured values in the soil and likely impacts on broader environmental quality (cf. McDowell, 2001). However, it is important to stress that soil P surplus or Olsen P values are not the only factors that determine P loss from soils, although links between Olsen P content of soils and deleterious effects upon water quality have been made with varying degrees of success by a range of workers (Pote et al., 1996; Djodjic et al., 2004).
The ‘change point’ is the soil P concentration at which the solubility of P to soil solution markedly increases. In a study examining the existence and behaviour of a change point in soil P release from soils under a range of management systems from the UK, New Zealand and the USA, McDowell et al. (2001) examined the
relationship between quantity and intensity of P supply. The plots of intensity, as measured by calcium chloride extraction, against the quantity, as measured by Olsen P, showed change points ranging from 20 – 112 mg Olsen P kg-1. The change points are values above which release of P into soil solution occurs at a greater rate per unit increase in soil P concentration. Hooda et al. (2001) demonstrated the link between Olsen P and potential environmental implications of P in soils under intensive fertilizer and manuring practice. Saturation of the soil with P to 25% of its estimated total capacity was suggested to result in excessive P concentrations in run-off. In order to reduce this risk, an Olsen P trigger, again matching agronomic optimum values (25 < 45 mg Olsen P kg-1) was suggested, in particular on soil receiving inorganic P addition.
Agricultural crop Hillslope Riparian zone Water body Atmospheric deposition
Fertiliser Manures
Soil particles minerals and organic matter
Leaching Eluviation
Groundwater
Throughflow Selective transport of clays and silts
Subsurface quickflow along field drains Retention cycling and export
Downstream transport and transformation Cycling
routine test to predict particulate P loss, however, the DESPRAL project
(www.despral.org.uk) developed a water dispersion test to predict soil vulnerability to particle detachment and initial P mobilisation in fields. Further work is still required to examine changes in soil P dispersibility across different scales and to combine the test with a hydrological component for use in risk assessment.
At a larger scale, for example the catchment, a greater degree of complexity is introduced through consideration of hydrological, geographical and land use factors, making interpretation of Olsen P data for soils significantly more onerous with regard to water quality (cf. Edwards and Withers, 1998). Yet it should be the role of an indicator in the minimum dataset to highlight likely environmental risk, and it is clear that Olsen P values have been used to establish the potential for adverse effects on the broader environment (Jordan et al., 2000). Change in soil Olsen P values
indicate an increase in the available P content of the soil and likely risk of nutrient enrichment to aquatic systems. For example, He et al. (2003) observed an increase in Olsen P in a range of soils under intensive irrigated agriculture in response to fertilizer application and a positive correlation with loss of P via overland flow.
Further, organic carbon was noted as being the soil factor that explained 50% of the variability in total P load in surface run-off, followed in importance by Olsen P and then fertilizer rate.
Elevated soil P concentrations, as measured through Olsen P, may also have a detrimental effect upon ecosystems adapted to low P conditions. Such effects may include reductions in species richness (Reom et al, 2002), symbiotic plant
associations (Smith and Read, 1997) and organism diversity. The enrichment of soil P levels, as reflected in large or excessive values of Olsen P compared to controls, can occur at sensitive sites adjacent to farmland having received unintended fertilizer inputs (Stevenson, 2004), road sides (Brewer and Cralle, 2003) and reclaimed farm or horticultural land (Gough and Marrs, 1990).
Gough and Marrs (1990) noted that the re-establishment of semi-natural vegetation on previous agricultural land was limited by the excessive soil P concentrations, expressed as Olsen P. Stevenson (2004) suggested that increased values of Olsen P (<10 times compared to soils at control sites) reduced the long term stability of indigenous forest fragments located adjacent to permanent pasture. A reason for this was thought to be the limiting nature of P availability on microbial activity in the indigenous forests control sites, but not in the fragments of forest near pasture.
Phosphorus toxicity, while not common, may occur at excessive P concentrations appearing as interveinal chlorosis and necrosis in younger leaves and the shedding of older leaves (Reuter and Robinson, 1997). Long-term immobilisation (and hence system inefficiency) is likely to be more of a concern in the majority of cases when thinking about upper thresholds.
Selection of triggers for Olsen P
Total P concentrations in topsoils are generally in the range of 100-3000 mg/kg. Yet, as has already been outlined, only a very small fraction of this total is available for plant uptake or movement through soils in solution. The total concentration of P in surface waters, which is considered critical in terms of biological production and
potential eutrophication, is about 0.01- 0.1 mg/l. This is some 10 to 100 times lower than that required for nitrogen and 10 times lower than the concentration commonly thought to be required for crop growth. Therefore, what may be considered as a trivial solution P concentration in an agronomic context may be crucial with regard to water quality.
Agronomic requirements for P, developed through calibration against plant tissue analysis using Olsen P extraction, are widely available and indicate optimum P levels of fertilizer application to maintain P levels in soils at certain grain yields (MAFF, 2000). There is also strong positive correlation between inorganic and organic P fertilizer applications to agricultural soils and values of Olsen P (Rowell, 1994).
Differences in values of Olsen P between conventional farm managed systems and biodynamic farming systems on the same soils can be up to 20 mg/kg and should be relatively easy to discern (66 against 46 mg P/kg, respectively) (Reganold et al., 1993).
A linear relationship between Olsen P concentrations in soils of a plough layer and the concentrations of dissolved reactive P in underlying tile drains were observed by Heckrath et al. (1995) at Olsen P values above 60 mg/kg. Below this concentration – termed the change point – concentrations in the tile drains were below 0.1 mg/l.
From their work on change points, McDowell et al. (2001) suggested that most critical Olsen P concentrations, with regard to adverse effects upon water quality, were 40% of the optimum values for plant growth on a range of soil types and land uses. Therefore, remaining within the agricultural optimum should be protective and ultimately more economical than over-fertilizing. The source of fertilizer P, either organic or inorganic, has also been observed to have only a limited effect upon P loss from plots subject to simulated rainfall events, where change points were calculated for a range of soil types and were again close to the agronomic optimum for those soils (McDowell et al., 2003).
In an attempt to establish the sustainable soil Olsen P level that would minimise environmental effects on water quality but sustain optima crop growth, Jordan et al.
(2002) estimated the change point to be 22 mg Olsen P/l(a P index of two, MAFF, 2000). This was calculated from estimating catchment loads of P for 56 rivers across Northern Ireland and measured values of Olsen P from 5615 soil samples from corresponding sites in the catchments. Importantly, a P index of three (around 26-45 mg P/l) was noted as being excessive environmentally and for crop
production. From the Represetative Soil Sampling Scheme (RSSS) data, which solely considers agricultural land, 41 per cent (n = 396) of the sites sampled in 1969 showed Olsen P values at or above index three and in 2002, 43 per cent (n = 219) were in index three. Smith et al. (1998) suggested that in order to reduce leaching losses of P it was necessary to restrict topsoil Olsen P levels to below 70 mg/l. In practice it will not be possible to set absolute values that apply across the whole of
Table 2.3.7.1: Target limits for Olsen P (µg/ml) for five broad land uses and three major soil types in New Zealand for both agricultural production and environmental interaction (Sparling et al., 2003)
Pasture on sedimentary and allophanic soils
0 15 20 50 100 200
Pasture on pumice and organic soils 0 15 35 60 100 200 Cropping and horticulture on
sedimentary and allophanic soils
0 20 50 100 100 200
Cropping and horticulture on pumice and organic soils
0 25 60 100 100 200
Forestry on all soils 0 5 10 100 100 200
Very
Low Low Adequate Ample High Figures in shade represent upper and lower limits.
The relevance of the soils in Table 2.3.7.1 and the assigned values is limited in terms of a UK context, but what this table does demonstrate is the ability to produce such a table as a screen for use of Olsen P in a minimum dataset. It could be
anticipated that in the UK, if such a table does not already exist, it may be relatively straightforward to derive one.
The development of trigger values for Olsen P for use in low P habitats are not so readily available. Nevertheless, Walker et al. (2001) noted, in a study looking at semi-natural lowland grasslands, that the mean value of Olsen P at these sites was 2.2 mg/kg. It is thought that values of Olsen P greater than 5 mg/kg could indicate species-poor, nutrient-enriched pastures (Goodwin et al., 1998).
Sparling et al. (2003a) suggested that there were three possible methodologies by which trigger values for soil indicators could be derived. The first of these methods was to use the national soil baseline data – calculating median and lower quartile values for a parameter for each soil order. This approach can be followed with RSSS datasets from 1969 and 2002 to give an indication of the range and variability of Olsen P data across all land types sampled (Table 2.3.7.2). However, the justification for the use of the lower quartile is not entirely clear.
Table 2.3.7.2: Olsen P data from the RSSS data base from 1969 and 2002 Olsen P, mg/l
1969 (n = 397)
Olsen P, mg/l 2002 (n = 218)
Mean 28 27
Standard
deviation 20 16
95th Percentile 69 60
50th Percentile 22 23
25th Percentile 15 16
5th Percentile 7 10
Harrod and Fraser (1999) have also undertaken some work in the UK from National Soil Inventory (NSI) datasets on the comparison of total and Olsen P values in
topsoils under a range of land uses, including semi-natural (Tables 2.3.7.3 and 4).
The Olsen P median values from Tables 2.3.7.3 and 4 indicate contents by land use classes as follows: Horticulture > orchard > arable > ley> permanent grass > forestry and semi-natural.
Table 2.7.3.3: Total and [Olsen] P concentrations (mg/kg) in NSI topsoils under various landuses in England and Wales (Harrod and Fraser, 1999)
Arable Horti-
culture Ley Perman’t
grass Orchard Conifers Decid- uous
Mean 830
[32]
1023 [56]
922 [27] 939 [24] 882 [50] 527 [14] 608 [19]
Median 734 [26]
854 [45]
822 [20] 847 [17] 851 [44] 448 [11] 528 [12]
Standard
error 9.5
[0.5] 81.8
[6.3] 17.3 [1.0] 11.0 [0.6] 66.6 [5.8] 24.4
[0.9] 23.0 [1.5]
Range 246-
4189 [0-205]
280- 2312 [4-160]
170-4535
[1-274] 188-4529
[0-337] 177-1925
[1-123] 41-1685
[0-94] 108- 2636 [1-210]
Number 1888 [1868]
41 [39] 675 [668] 1560 [1538]
35 [34] 207 [203]
273 [267]
Table 2.3.7.4: Total and [Olsen] P concentrations (mg/kg) in NSI agricultural and semi-natural topsoils in England and Wales (Harrod and Fraser, 1999)
Arable, ley and
permanent grass Upland heath and
grass Deciduous and
coniferous woodland
Mean 887 [28] 762 [16] 572 [17]
Median 794 [21] 696 [12] 500 [11]
Standard error 6.7 [0.4] 20.3 [0.8] 16.9 [0.9]
Range 170-4535 [1-337] 142-2214 [1-157] 41-2634 [1-210]
Number 4111 [4111] 344 [344] 479 [479]
The final two methodologies suggested by Sparling et al. (2003a), by which trigger values for soil indicators could be derived, were modelling to set a limit values and using expert judgement to derive response curves for changes in the Olsen P level (on the x-axis) against soil quality (0-100%).
The link between soil quality indicator and the soil function of ‘environmental
Table 2.3.7.5: Potential trigger values for Olsen P (mg/l) for ‘environmental interaction’
Function Soil type
Mineral Peaty Calcareous
Metal retention Microbial function/
biofiltering
Gaseous emissions Soluble phosphorus leaching
>60 >60 >60
Hydrological?
Habitat support table
Calcareous grassland >16
Mesotrophic grassland >10 >10 Acid grassland >10 >10 Dwarf Shrub Heath >10 >10 Biomass production
Arable and horticultural 16–45 16–45 16–45
Improved grassland 16–25 16–25 16–25
Integration with other national monitoring schemes
The NSI data and RSSS contain data collected on Olsen P. Importantly, the reported data in RSSS is based on w/v criteria (mg/l) and not w/w (mg/kg). To ensure the greatest continuity with these relatively large datasets it is recommended that measurements of Olsen P are undertaken on w/v and w/w.
Cost and suitability for broad-scale monitoring
The Olsen P methodology is thought to be most suitable for predicting fertility and crop recommendations in soils of pH > 7, while acid extracts (sometimes including fluoride to complex Al3+ and Fe3+) frequently give better correlation with acid soils.
Costs of determination for Olsen P can be as much as £15 (National Laboratory Service, Environment Agency, Pers. Comm.), but is routinely less than £5 a sample.
Olsen P is commonly used in some, but not all, EU countries for the determination of P status in soils and fertilizer recommendations. Importantly, there is significant variability in methodologies and results obtained, with commensurate differences in interpretation (Sibbesen and Sharpley, 1997; Neyroud and Lischer, 2003).
Relative to soil pH determination, Olsen P is thought to be a more volatile or variable soil quality indicator. In a study across 29 sites on a range of land uses showed a coefficient of variation for Olsen P of 15.6% (cf. with the coefficient of variation for pH of 2.3%) (Schipper and Sparling, 2000). In a study of spatial variability of soil quality indicators in New Zealand across four land uses and one major soil order, Gilltrap