© CAB International 2003. Nutrients for Sugar Beet Production: Soil–Plant Relationships
(A.P. Draycott and D.R. Christenson) 139
In addition to being present in sufficient quantity, nutrients and water must be acces- sible to plant roots, so soil has to provide a suitable environment for roots to grow in.
The sugar beet crop, particularly just after sowing, is very sensitive to soil physical con- ditions. To establish and yield well, sugar beet roots must be able to extend rapidly.
Soil structure and tilth are terms used to describe the physical state of soil in respect of its suitability for seed germination, seedling emergence and root growth through to har- vest. Russell, E.W. (1971) stated, ‘good tilth is something a farmer can recognize with his boot, but scientists cannot describe’. Indeed, the literature contains many different mea- surements. There does not seem to be a single analysis or even several sufficiently sensitive analyses that adequately describe conditions for ideal crop performance. The following addresses aspects of some of the commonly used evaluations of soil physical properties and factors affecting nutrient requirement.
Plant roots must grow through the many pore spaces in well-structured soils.
Consequently, any factor that affects pore space in soil affects plant growth. The ‘ideal’
soil contains approximately 50% solid matter and 50% pore space. That pore space con- tains 50% water-holding micropores and 50% air-holding macropores. Compaction of soil removes the macropores first, forcing
water into the medium and smaller soil pores and thus reducing the supply of air for root growth. Limited air supply restricts nutrient uptake by roots because oxygen is required for metabolic processes involved in active nutrient uptake.
Soil texture and structure both influence percentage pore space. Soil texture is defined as the percentage of sand, silt and clay in a specific soil type. There is very little opportu- nity through common farming practices to change texture. Soil structure, however, is the aggregation of textural and organic com- ponents into larger and more dynamic parti- cles, easily compacted by traffic and tillage, which are very much under farmers’ control.
Physical conditions are also affected by rainfall, temperature variation, wind, crop rotation, field operations and tillage. Bulk den- sity, pore-size distribution, water-holding capacity, penetrometer resistance, sheer strength, hydraulic conductivity and aggregate size and stability have been used to describe some of the physical properties of soil.
Optimum Conditions Bulk density
Bulk density is commonly used as an indi- cator of the relative degree of compaction in
soils. General values range from 1.0 to 1.5 g ml1for fine- and medium-textured soils to 1.3 to 1.8 g ml1 for coarse-textured soils.
Miller and Donahue (1990) suggested that bulk densities should not exceed 1.4 g ml1 for fine-textured soils and 1.6 g ml1 for coarse-textured soils. These values corre- spond to 48 and 40% pore space, respec- tively. Work by Jaggard (1984) confirms this suggestion when he reported that sugar- beet yields declined as bulk densities of medium-textured soils increased above 1.45 g ml1(Fig. 10.1). Pabin et al.(1991), work- ing in Poland, showed a marked reduction in yields on a loamy sand soil with bulk densities above 1.51 g ml1. Håkansson (1990) proposed a ‘degree of compactness’
measurement, which is the ratio (%) of the dry bulk density of the soil and the bulk density of the same soil in a compacted state. On mineral soils, the maximum crop yield of barley was obtained at the same degree of compactness, irrespective of soil type. Unfortunately, no data were presented for sugar beet.
In summary, the maximum bulk density a sugar beet can withstand without limiting growth is about 1.50 g ml1. Generally, the bulk-density measurement is not sufficiently
sensitive to detect changes in soil structure affecting plant growth.
Pore size and distribution
Pore size and its distribution govern air and water movement, as well as the capacity of soil to hold both components. Kuipers (1955) felt that the soil structure of marine clays in The Netherlands could best be described by the amount of large pores. Independently, Baver and Farnsworth (1940) and Baver (1949) showed that, on a fine-textured soil, beet yield decreased sharply as the non-cap- illary pore space dropped below 10% on US soils. Pendleton (1950) found unrestricted root growth of sugar beet at 14 and 18% non- capillary pore space on a sandy loam and a silt loam soil, respectively. Conversely, root growth was restricted at 6.5 and 11.7%
macropore space for the two soils. He also reported improvement in the shape of roots as non-capillary pore space increased in field studies. Later work confirmed these values for sugar beet growth (Blake et al., 1960).
Thus, to grow sugar beet where pore space is a non-limiting factor, a total pore space of at least 45% and a non-capillary pore space of at least 12% are necessary.
70 80 90 100
1.3 1.4 1.5 1.6 1.7
Dry bulk density (g ml–1)
Loose Compact
Sugar yield (%)
Fig. 10.1. Effect of bulk density of a medium-textured soil on sugar yield expressed as a percentage of the highest yield (from Jaggard, 1977).
Aggregate stability and size distribution
Aggregate stability and size distribution are important considerations for evaluation of the physical conditions of soil. Sieving techniques are usually used to evaluate size distribution.
Van Bavel (1949) introduced a mean weight diameter (MWD) concept that quantifies aggregate size. De Boodt et al. (1961) advanced the methodology further, suggest- ing a ‘change in mean weight diameter’
(CMWD). A bulk sample of soil is air-dried under controlled humidity. The MWD is mea- sured for the dry aggregates. The aggregates are then moistened and MWD is measured again. The result gives an indication of the water stability of the soil aggregates.
Measurement on one soil was related to the yield of mangolds, as shown in Fig. 10.2.
The parabolic nature of this plot is surpris- ing. One might expect a curvilinear relation- ship, where a maximum yield could be determined, or possibly a linear relationship, showing a steady-state effect. De Boodt et al.
(1961) explained the parabolic relationship as follows:
At lower values of CMWD, the soil aggregates are sufficiently stable to act as gravel reducing the water holding capacity of the soil and
making it difficult for mangold roots to penetrate. At the higher values of CMWD, the soil is not stable causing the soil to slake with associated aeration problems and low yields.
At the intermediate values of CMWD, the stability of aggregates approached the optimum with appropriate yields.
Gummerson (1989) reported the impor- tance of fine aggregates for good seedling emergence. He showed an increase in emer- gence of sugar beet as the percentage of aggregates less than 5 mm increased from 40 to 60. Dürr and Aubertot (2000) found that the per cent emergence decreased exponen- tially with aggregate size over 10 mm. They also showed that the number of seedlings impeded increased markedly when the aggregate maximum length exceeded 25 mm. When the weight of the aggregate exceeded the force exerted by the sugar beet, emergence was decreased. The force exerted by sugar beet was in the range of 0.10 to 0.15 newtons (N). For comparison, wheat exerts about 0.30 N. Dürr et al. (1992) found that seedling size was greatly affected by rate of emergence, the largest seedlings being those which emerged first.
Whether aggregates are too small or too large, emergence is reduced. Small aggre- gates increase the strength of the surface
50 60 70 80 90
0.6 0.8 1.0 1.2 1.4 1.6
CMWD (mm) Yield mangolds (t ha–1)
Fig. 10.2. Comparison of yield of mangolds with change in mean weight diameter (CMWD) on a sand soil (from de Boodt et al., 1961).
while large aggregates are too heavy for the sugar beet seedling to force aside.
Aggregates in the range of 0.5–5 mm seem to be the most favourable but there is a need for additional work describing the pore-size dis- tribution needed for optimum sugar beet emergence and growth.
Saturated hydraulic conductivity
Saturated hydraulic conductivity (KS) is an indirect measurement of soil structural sta- bility. Undisturbed soil cores are placed in water and saturated for up to 48 h. Then sat- urated flow is measured by applying a small head of water above the surface of the soil contained by the core. Darcey’s law is applied to calculate KS. Relatively greater aggregate stabilities from different treat- ments can be compared for increasing KSval- ues. However, there are no data indicating an ideal value or range for sugar beet pro- duction. Probably the best use of this mea- surement is for comparing treatments within an experiment.
Soil strength
Strength of soil has an impact not only on root growth, but also on emergence.
Robertson (1952) used penetrometer resis- tance to show greater crust strength in plots without soil-improving crops. Smucker and Leep (1975) reported that sugar beet emer- gence could be doubled when soil-crust strengths exceeding 1.0 MPa were reduced to 0.57 MPa, when rows were banded with anti-crusting agents. Sugar beet root yield was increased by 630 kg ha1 for each 0.1 MPa reduction in crust strength. Taylor and Bruce (1968) presented the root yield of sugar beet in respect of increasing penetrom- eter resistance. There was a near-linear decrease in yield with increasing soil strength between 0.1 and 2 MPa resistance.
Individual tap root weight declined from 1.1 to 0.6 kg over this range. Pabin et al. (1991) found an increase in yield of 603 kg ha1for each 0.1 MPa reduction in resistance between 1.75 and 4.65 MPa.
Nutrition of Sugar Beet on Compacted Soils
Oxygen
Oxygen is not usually considered a limiting nutrient in the growth of crops since there is an abundant supply of the element in the atmosphere. Oxygen supply may be limiting for adequate growth of plant roots, particu- larly where there are dense subsoil layers. In some early work, Bertrand and Kohnke (1957) observed that oxygen diffusion was slower in compacted subsoil layers than in the looser counterpart. About the same time, Gill and Miller (1956) found that normal root growth is negatively affected when the oxygen content in soil air is reduced to 10%. However, they noted that roots continued to grow with as lit- tle as 1% oxygen, provided they were not sub- jected to a mechanical barrier.
Lemon and Erickson (1952) suggested that the oxygen diffusion rate through soil is the important factor and not necessarily the absolute amount in the soil air. Stolzy et al.
(1961) found that root growth was reduced when the oxygen diffusion rate was less than 38 µg O2 cm2 s1. Wiersma and Mortland (1953) reported that oxygen supply to roots is a limiting factor in the growth of sugar beet.
Scott and Erickson (1964) showed that oxy- gen supply to sugar beet roots limited their penetration of a dense soil layer. Supplying the root with an oxygen source (calcium per- oxide) promoted penetration of roots through the layer, which was compacted to a bulk density of 1.9 g ml1. Barley and oats are able to survive lower soil oxygen diffusion rates from 8 to 25 µg O2cm2s1, while sugar beet requires from 13 to 50 µg O2cm2s1. Thus, while oxygen supply is important, adequate diffusion is necessary and may be greater for sugar beet than for other crops.
Nitrogen
Most of the research work on the effect of soil compaction on nitrogen centres on the effect of additional fertilizer required for sugar beet under compacted conditions.
Kuipers (1955) pointed out that sugar beet
responded to higher nitrogen rates on com- pacted soil than on looser conditions of soil.
Draycott et al. (1970b) reported that increased fertilizer nitrogen was needed for sugar beet grown under compaction, which was confirmed by Jaggard (1977). The former showed an additional need of 75 kg N ha1 on compacted soils compared with looser soils. Jaggard’s results, averaged over 3 years, showed that soils with less com- paction did not respond to amounts above 75 kg N ha1, while on more compacted plots there was a response to 150 kg N ha1, thereby confirming earlier work.
Wiersum (1962) found that nitrate uptake by plants was independent of rooting den- sity, undoubtedly due to the mobility of the ion. The difference in the fertilizer nitrogen needed appears to be due to restriction of the amount of mineralization under the reduced aeration of compacted soils. Whisler et al.
(1965) and Clement and Williams (1962) found less mineral nitrogen in compacted than in uncompacted soils after incubation.
The latter’s work showed 16.1 mg N kg1 mineralized on compacted soils with 3% air- filled pores. Less than 10% of the mineral- ized nitrogen was in the nitrate form. On the uncompacted soil, there was 41.7 mg N kg1 mineralized, with nearly all of it as nitrate. In a study in Norway, Bakken et al. (1987) found from five to seven times as much nitrogen loss through denitrification in com- pacted compared with non-compacted soil.
These factors may explain the need for addi- tional fertilizer in compacted soils compared with their less compacted counterparts.
Phosphorus
Decreased phosphorus uptake by crops on soils with poor structure has been reported (Wiersum, 1962). Wiersum demonstrated the influence of soil structure on root growth and that uptake of nutrients depended on the mobility of the nutrient. More intensive rooting in finer soil improved the utilization of phosphorus, but phosphorus uptake diminished with coarse aggregates, when only few roots were developed. Lawton (1945), Flocker et al.(1959) and Flocker and
Nielsen (1962) also found that soil com- paction affected plant growth and decreased phosphorus uptake.
Shierlaw and Alston (1984), working with maize and annual ryegrass, found that com- paction decreased root length in compacted layers, but increased root length in the over- lying soil. They also showed that phospho- rus uptake per unit length of root was generally decreased with increasing com- paction. Draycott et al. (1970b) suggested that the amount of phosphorus fertilizer needed for sugar beet was affected little by compaction. Jaggard (1977) also reported that sugar beet did not respond to additional phosphate fertilizer under compacted condi- tions. These crops were grown on soils with high residual phosphorus concentrations and so would not be expected to respond much to applied fertilizer.
Prummel (1975) described an interaction between soil phosphorus concentration and soil compaction in respect of dry-matter yields of sugar beet. On compacted soils there was a response where soil concentration was more than 24 mg P l1 extracted in water.
However, on uncompacted soils there was no response with 13 mg P l1in the extract. Fried and Broeshart (1967) suggest that uptake is diminished by restricted root growth. Barber (1995) clearly demonstrated that uptake was influenced most by root surface area, which also increased with greater root length.
Voorhees et al. (1975) reported that the root elongation rate increased by nearly 80% as soil air-filled pores increase from 1 to 30%.
Therefore, as mechanical impedance of soil increases, causing root thickening and stunted roots, greater supplies of soluble phosphorus are needed to supply the uptake demands of plants having smaller root systems (Silverbush and Barber, 1983).
Adjustment of phosphorus application for sugar beet on compacted soils does not seem to be needed in most conditions. Under con- ditions of high residual phosphorus concen- tration in soil, no additional phosphorus should be needed. With low residual phos- phorus, it appears that normal amounts of phosphorus fertilizer should be adequate.
Recommendations based on soil analysis may already account for some degree of
compaction, since the correlation work has been done over a wide range of conditions, including varying degrees of compaction.
Potassium
Lawton (1945) showed that reduced aeration in water-retentive soils reduced the potas- sium concentration in maize. Merely forcing air through the soil increased uptake. Philips and Kirkham (1962) reported a reduction in total potassium in maize leaves due to trac- tor traffic, whether potassium was applied or not. Poor aeration was an important factor in potassium nutrition of sugar beet in experi- ments conducted in Montana, USA (Larson, 1954a). The results showed 31% less potas- sium in petioles when grown on soils con- taining a small number of large pores.
Potassium uptake seems to be affected to a greater extent by poor aeration than other nutrients. Lawton (1945) showed the follow- ing ratios of uptake on non-aerated to aer- ated cultures of a silt loam containing 50%
water: K, 0.3; N, 0.7; Mg, 0.8; Ca, 0.9; P, 1.3.
General considerations
Smucker et al.(1978) studied the interaction of soil compaction and sugar beet variety on nutrient concentration in beet leaves.
Averaged across three varieties, concentra- tions of nitrogen, phosphorus and calcium were decreased with increasing compaction.
Conversely, there was little effect on potas- sium and magnesium concentration due to soil compaction. Yield on the compacted plots was 11% lower than on the looser soil.
Russell and Goss (1974) provide an excel- lent discussion concerning physical aspects of uptake.
While restricted root growth generally reduces uptake of nutrients from soil reserves, spatial distribution of compacted regions within the soil profile greatly affects the functional efficiency of nutrient uptake by roots. For example, in fields where wheel tracks compact only portions of the root zone, the vertical distribution of roots is most noticeably affected. Compensatory
root growth into less compacted areas of soil that are more hospitable to root growth and function often results in greater vertical root growth. Consequently, some com- pacted conditions may not decrease the amount of nutrient taken up, especially if nutrients are uniformly distributed within the soil where the zone explored by fibrous roots has a sufficient concentration of the desired nutrient. Then no additional fertil- izer nutrient would be needed. However, at lower concentrations in soil or when com- paction excludes root growth in proximity to fertilizer bands, additional nutrients probably need to be added. These spatial considerations affecting soil compaction need to be included in conclusions on the nutritional needs of sugar beet grown on compacted soils. Consequently, a better description of rooting patterns, localized soil compaction, types of fertilizer applica- tions and the nutrient concentrations of the soil is needed. Such data could be incorpo- rated into a model proposed by Aubertot et al.(1999).
Previous Cropping
Improvement of soil structure by inclusion of forage legumes and/or grasses in a rotation is well documented. R.H. Eliot, as quoted by Low (1955), said: ‘four to six years good turf on old arable land would restore it to a con- dition comparable with old pastures’.
Following up on this, Low conducted a study to evaluate the time taken by a ley to change the physical state of an old arable soil to that of an old grassland. The process was found to be slow, taking possibly 50 years on some clay soils, but only 5–10 years on sandy soils. Barber (1959) measured the effect of lucerne, lucerne–brome grass, brome grass and maize on aggregation of a silty clay loam soil in Indiana, USA. The rank order of formation of aggregates was brome grass lucerne–brome grass lucerne maize.
Maize did not affect aggregation and yields were not affected by the improved aggrega- tion. Lucerne promotes aggregation on coarse-textured as well as fine-textured soils (Miller and Kemper, 1962).
Robertson (1952) reported the effects of several crop rotations on sugar beet produc- tion on a sandy clay loam. Growing a lucerne–brome grass sward in the rotation sig- nificantly increased sugar beet yields. One year was as beneficial as 2. Both total and non- capillary pore space was increased by produc- tion of this forage. Differences in pore space were greater during the latter part of the sea- son than at the beginning. Crusts formed on plots that did not have legumes, but were not present on plots with a legume in the rotation, as measured by a penetrometer. Since greater yield and improved soil structure were the result of rotation, Robertson concluded that some of the increase in yield was due to improved structure.
In Michigan, cropping patterns changed after this work by Robertson. A study was initiated to evaluate the use of non-forage crops for reducing the decline of soil organic matter due to row crop practices. The study was described in detail, including yields (Christenson et al., 1991b). Momen (1985) evaluated the soil structure after 10 years of cropping. MWD was significantly affected by the amount of maize present in the rota- tion. There was a steady progression of increased MWD with increasing amount of crop residue returned to the soil (Table 10.1).
All systems lost carbon over the course of the study, but those systems with more crop residues lost less. Other measurements including bulk density, KS, total porosity, air porosity and aggregate size distribution, were not affected by the amount of residues returned to the soil.
In this same study, the effect of residues on nutrient concentration in sugar beet leaves was striking (Table 10.2).
Concentrations of nitrogen, phosphorus, magnesium and iron were all reduced by returns of greater than 5 t ha1 year1 of crop residue, while the concentration of potassium increased. Where maize was included in rotations, it returned large residues.
Christenson and Butt (2000) reported that some of the differences could be accounted for with increased nitrogen on sugar beet following maize. However, there appear to be other factors when maize is included in the rotation, and their specific nature is not clear. Crookston and Kurle (1989) suggest that response was not due to the beneficial effects of decomposing above-ground residue. Some evidence sug- gests that maize residues might produce phytotoxic compounds, causing negative effects on subsequent crops (Guenzi and McCalla, 1966; Guenzi et al., 1967; Yackle and Cruse, 1983). While the effects have been measured on maize and soybean, there is no reason to suggest that similar effect would not occur on sugar beet fol- lowing maize. This has not been investi- gated because very little sugar beet production includes maize in the rotation.
Table 10.1. Mean weight diameter (MWD) and aggregate size as affected by amount of crop residue returned over 10 years of cropping (Momen, 1985; Christenson, 1997).
Crop Aggregate size range
residue MWD 1 mm 1–5 mm
(t ha1) (mm) (%) (%)
33 0.49 82 18
50 0.50 81 20
68 0.55 78 22
90 0.67 77 23
Table 10.2. Crop-residue effects on nutrient concentration in sugar beet leaves (Christenson et al., 1979; Christenson, 1997).
Crop residue (t ha1year1)
Nutrient 3.5 5.0
N % 4.0 3.7
P % 0.36 0.31
K % 4.6 5.1
Ca % 0.85 0.83
Mg % 0.74 0.66
B mg kg1 58 53
Fe mg kg1 205 122
Mn mg kg1 24 26
Zn mg kg1 36 37