Crop nutrition

1. Introduction

Well grown soybean crops can accumulate large quantities of plant nutrients, in particular nitrogen, phosphorus and potassium. At harvest, a higher proportions of those nutrients are removed in harvested grains than other tropical and subtropical grain legume crops like peanut and mungbean. Any decisions about fertiliser requirements for a proposed soybean crop will be determined by the nutrient availability in the soil relative to the critical concentration determined from soil testing, and particularly for the less mobile nutrients like phosphorus and potassium, the distribution of those available nutrients in different soil profile layers. While soil testing multiple profile layers is an additional cost, it is an investment that can return many times that cost by ensuring nutrients are available in a part of the profile that allows efficient root access when crop nutrient demand is high. These considerations are part of the 4Rs of good nutrient management – apply the Right rate of the Right product in the Right place at the Right time. Getting all four Rs correct is critical for achieving efficient use of expensive fertiliser inputs.

While there are many factors that go into determining the right nutrient application rates, including uptake efficiencies and the available fertiliser budget, an important consideration should always be that the nutrient inputs at least balance the rates of removal in harvested product – either grain or hay. These concepts are particularly important in low fertility soils.

2. Essential nutrients

2.1 Nitrogen (N)

Once established, the nitrogen fixing bacteria (rhizobia) in the root nodules can supply soybean plants with a large proportion of their nitrogen requirements, provided soil mineral N is low. The proportion of soybean crop N derived from fixation tends to decrease as soil mineral N increases, as the plant will accumulate ‘easy’ soil nitrate rather than divert photosynthate to support a population of Rhizobium to fix atmospheric N₂. High mineral N in the early stages of crop establishment can delay early nodule development, possibly causing an N deficiency to occur later in the season once the soil mineral N is exhausted. This may be overcome by delayed nodule establishment, but this may occur too slowly or be insufficient to meet crop demand, resulting in yield loss. Soybean plants start to shut down their N fixation capability once seed filling commences, as the assimilate that was used to maintain healthy functioning nodules gets diverted to filling grains. This contributes to yellowing/canopy senescence, and while a naturally occurring characteristic of the crop, this can cause yields of potentially large crops to become limited by N availability. Interestingly, in soils where N mineralisation from crop residues and soil organic matter occurs later in the season (e.g. from a decomposing sugarcane trash blanket), the crop can utilise this soil N to maintain crop N supply, ensuring larger seed size and higher grain protein.

A small amount of ‘starter’ nitrogen (15–20 kg N/ha) may be beneficial when the crop is sown into soils where there are large amounts of crop residue that have high C:N ratios (e.g. wheat straw or sugarcane trash), as the soybean seedlings need access to some soil N to grow and establish while the root nodules are developing. Another situation where some starter N may benefit is in late sown crops, or crops grown outside their optimum daylength period, as these crops tend to flower earlier and set pods closer to the ground. Some starter N in this instance ensures good early growth of seedlings and maximises the height to the lowest pod. However, don’t apply too much starter N as this will raise soil mineral N concentrations and have a detrimental effect on the growth of nodules that supply nitrogen to the plant later in its growth cycle.

Phosphorus drilled with, or banded close to, the seed is generally the most effective way to supply this nutrient to the soybean plant, although this banding does reduce the volume of soil that is enriched by the fertiliser application, and so limits the proportion of the crop root system that can access this P. Soybean plants minimise this effect by being very efficient at developing roots in and around a P fertiliser band, but banding will still limit the amount of P that the crop can recover from a band. Therefore, for practical and economic reasons most growers with soils of low to moderate P status (as indicated by Colwell-P) and a low P sorbing ability (indicated by PBI) broadcast and incorporate the entire fertiliser requirement prior to sowing. This strategy will not be appropriate in soils with a strong P sorbing capacity (i.e. high PBI), and P fertiliser will need to be banded to maximise crop recovery. In these situations, reducing the distance between fertiliser bands will help to maximise root access to high P while minimising the exposure to the P sorbing soil surfaces. 

Some P fertilisers can be placed in the seeding row with the seed at planting, but there are limits to how much can be applied in this fashion. Both the germinating seedling and the Rhizobium inoculant can both be damaged if rates are too high. The maximum safe rate with seed will depend on the amount of soil mixing occurring at planting, the distance between the seed and the fertiliser and the type of fertiliser product used. When direct drilling with minimal soil disturbance (e.g. disc seeders), safe rates will be lower than with tine planters that cause more soil disturbance. Local advisors or fertiliser resellers can help with the maximum safe rates of the fertiliser product you intend to use. The amount of P that can be applied with or near the seed will be reduced if the fertiliser product contains N. If it is not possible to apply sufficiently high rates in the planting furrow, pre-plant band applications, or additional tines that place fertiliser below and beside the seed can be effective.

2.4 Potassium (K)

Soybean crop yields may be limited by potassium deficiencies on sandy soils and those soils with a long history of intensive cropping where heavy export of K in hay, silage, grain or sugarcane has occurred. Potassium behaves similarly to P in soils, in that it is relatively immobile in all except coarse sands, so fertiliser K tends to stay where it is put in the soil, unless it is taken up by the crop. In contrast to P, crops tend to accumulate large amounts of K in their vegetative tissues – often much more than that required to support growth. While much of this K can be returned to the field in crop residues (grain K concentrations remain relatively similar, regardless of crop K status), it does mean that available K in soil can appear quite high in the topsoil but may be much lower in the subsoil. A two-layer soil sampling regime is therefore recommended for K as well as P.

Soil analyses for K are typically based on exchangeable or Colwell-extractable K, with both producing similar results in most cropped soils. Exchangeable K is typically used in eastern Australia while Colwell K is commonly used in Western Australia. Both these tests measure K that is loosely held on negatively charged soil or organic matter surfaces, with this K readily released into the soil solution in response to crop K uptake. Soils can also contain some slow-release forms of K that are not measured in these commercial soil tests, and while some of these tests are commercially available (e.g. nitric K), the results cannot be easily linked to crop K availability. The greatest amounts of these reserves (which are present as slowly dissolvable soil minerals) tend to be in ‘young’ alluvial soils in creek flats or at the lower end of river valleys, while these reserves are not present in granitic soils, soils produced from in situ weathering of basalt (i.e. eastern Darling Downs) or on red volcanic soils. In these situations, and perhaps more generally, it is safer to assume that exchangeable K is all there is and fertilise accordingly.

A guide to potassium fertiliser requirements for soybean crops is provided in Table 4, although the critical soil test value for exchangeable K is affected by things like the soil cation exchange capacity and the relative availability of sodium (Na) to potassium in some heavy clay soils. The apparent effect of high exchangeable Na on K availability is probably caused by poor soil structure restricting the soil volume from which roots can extract K as well as their ability to take up K. The result is that higher soil K concentrations are needed for optimum yield.

When sowing, never place any muriate of potash (MOP) fertiliser in contact with the seed, as plant establishment will be impaired by a ‘salt’ effect (rapid dissolution of the fertiliser, resulting in a high concentration of dissolved ions around the fertiliser band and drawing of water away from the germinating seed). There is a little more flexibility in application if using potassium sulfate (SOP), which does not produce as strong a salt effect. However, SOP application rates need to be 20% higher to apply the same amount of K and are generally much more expensive than MOP. There is no need to band K fertilisers, or to put them in close proximity to the seeding trench, so application and incorporation during land preparation or banding away from the immediate seeding area in minimum tillage systems are preferred. However, it is worth noting that plant roots do not proliferate around a band of K fertiliser of either type, and so crop uptake of banded K may be limited. This can be overcome by adding some P into the K band (e.g. by creating a blend of ammonium phosphate and K fertiliser), so that when roots multiply around the P band they can also acquire K quite efficiently.

Rates of K removal in soybean grain are as high or higher than any other grain or grain legume crop, with ~ 20 kg K/t of grain removed at harvest. This compares to 3–4 kg K/t in cereal grains and 8–10 kg K/t in other grain legumes like mungbean and chickpea. Removal rates in high yielding soybean crops are comparable to that in irrigated cotton at 60–80 kg K/ha.

2.5 Zinc (Zn)

Zinc deficiency is widespread on the alkaline grey clays of inland irrigation areas and can also be low on some of the sandier soils in the coastal sugarcane areas. These differences may be due to low soil Zn (acidic, sandy soils) or to low availability of Zn to plants (alkaline clays), and each soil type responds differently to soil applications.

Although it is a well-known problem with soybean, zinc deficiency is still occasionally seen in commercial crops. Some varieties may be more sensitive than others. Soil tests provide an indication of plant available Zn status but are not very precise and more work is needed to define a soil test critical value. At this stage, critical values lie somewhere between 0.4 (acidic soils) and 0.8 (alkaline soils) mg DTPA-extractable Zn/kg soil, with the higher critical value in the alkaline soils reflecting the reduced plant availability at higher soil pH.

Zinc can be applied either to the soil or to the foliage, although the effectiveness and residual value of soil applications will vary depending on whether the application is to fix a low soil Zn situation in an acidic sand or to make some Zn available to plants in an alkaline clay. In lighter textured, neutral to acidic soils applications to soil are very effective and have a good residual value, e.g. an application of 30 kg/ha of zinc oxide to the soil every 5–7 years will fix any problems. However, in heavier alkaline clays it is more effective to apply small amounts with the starter P fertiliser in the seeding trench to ensure early plant access. Large applications mixed through the soil may be rendered unavailable to plants, limiting residual values. Foliar applications of zinc sulfate heptahydrate at 4 kg/ha 6–8 weeks after planting are always effective, but in soils with very low Zn, deficiencies may be evident well before the plants get enough leaf area to absorb this foliar application. In these low Zn soils, some Zn in the starter fertiliser (e.g. Zn-fortified ammonium phosphates) in the seeding row will help.

2.6 Molybdenum (Mo)

Root nodule bacteria require molybdenum as part of an enzyme to convert atmospheric nitrogen to a form that is used by the plant. Most soils on the NSW North Coast and Northern Tablelands are acidic and deficient in plant available molybdenum, and levels can also be marginal in many of the acidic, sandier coastal sugarcane soils in Queensland. Soil tests for Mo are not reliable, and so low-rate applications of products like sodium molybdate (dissolved and sprayed onto the soil before planting, or as a foliar spray early in the season) can be an effective insurance policy to ensure good N fixation. A seed dressing of molybdenum trioxide can also be used (although check for effects on efficacy of inoculants if considering this method), or as a blend with products like superphosphate fertilisers, which are used to enhance N fixation of legumes in pastures. Application rates of 50–100 g Mo/ha are only required every 2–3 years.

2.7 Other trace elements

Except for molybdenum on the NSW North Coast and Tablelands, and zinc on the grey clays of inland irrigation areas, fertiliser trace elements are generally not required for soybean crops. The exceptions may be on the sands and Wallum soils found in coastal areas, where copper, zinc and manganese deficiencies may occur. In these situations, the lack of reliable soil test diagnostics means that foliar applications of a mix of trace elements can sometimes provide useful insurance.

3. Nutritional disorders and deficiencies

3.1 Boron (B) deficiency

Description: Plants are not very efficient at remobilising B from older plant tissues to the developing growing points and seeds, so the most severe symptoms are typically seen in growing points, flowers and developing seeds/pods. Symptoms develop first on younger leaves, with tissues turning pale green and yellow. Later, dark brown necrotic spots develop in the yellow tissue. The tips and edges of young expanding leaves often curl down and under. Older leaves remain dark green. The buds at the top of the plant and at the nodes may die and the young, underdeveloped leaves or flowers turn pale brown. Pollination and seed set are reduced, and in severe cases plants may die. 

Conditions: Boron deficiency can occur in sandy soils leached of boron, alkaline soils containing free lime, soils low in organic matter, acid peat soils and in soils of acid igneous or fresh-water sediment parent material.

Management: Soil tests for available boron are not reliable indicators of likely B responsiveness but can indicate broad categories of B availability. Boron may be applied to the soil as borax, boric acid and chelated boron compounds. Boric acid or chelated B compounds may also be applied as a foliar spray 5–6 weeks after seedling emergence or as soon as symptoms appear. Over fertilisation with B can result in toxicity, so be careful with applications to soil. Applying products in solution sprayed onto soil is an effective strategy of getting uniform distribution of relatively low rates of product.

3.2 Calcium (Ca) deficiency

Description: Calcium deficiency causes stunting of plants, with short internodes, stout stems and dark green leaves. Symptoms first appear on young leaves. The leaves fail to expand the tips, margins cup up or down and leaves may turn yellow, developing necrotic pale brown areas. The entire forming bud will often die if the deficiency is severe. On older plants the veins of the upper leaves die and turn brown, and the areas beside these veins turn yellow. Pod set can be reduced, and yields are low.

Conditions: Ca is important for normal plant growth and development. It is also involved in nitrogen fixation. Ca deficiency may occur in acidic sandy soils leached of Ca, strongly acid peat soils of low Ca, alkaline or sodic soils where sodium and pH are high, or soils with high aluminium levels and low exchangeable Ca. It is unusual to encounter Ca deficiency in most cropping soils.

Management: Apply lime or dolomite if the soil pH is low, or gypsum if only Ca is lacking. Over-liming can cause deficiencies in other nutrients such as potassium, magnesium, iron, zinc and copper.

3.3 Iron (Fe) deficiency

Description: Affected crops are unthrifty, lack vigour and are typically stunted with thin, spindly stems, with young leaves yellow and older leaves dark green. Symptoms develop first on young expanding leaves, with pale yellowing in the interveinal area. The veins remain green for a while but later turn yellow. On severely affected plants dark brown lesions develop on the veins and pale brown lesions develop on the leaf margins and on petioles. The underside of the leaflet develops more prominent symptoms. These lesions occur after the whole leaf has turned yellow, distinguishing Fe deficiency from sulfur deficiency. Older leaves tend to remain green.

Conditions: Iron deficiency occurs on alkaline soils with low soluble Fe levels, waterlogged soils, acid soils with excess levels of soluble manganese, zinc, copper or nickel, in sandy soils low in total Fe and in peat soils.

Management: Inorganic Fe salts can effectively control deficiencies but will quickly become insoluble and unavailable to plants if placed into soil. Foliar sprays of inorganic salts or chelates need to be applied every 10 to 15 days to provide Fe to the new leaves. Apply Fe salts of organic chelates to the soil to keep Fe in solution. Apply the chelate that is most appropriate for the soil type.

3.4 Magnesium (Mg) deficiency

Description: Affected crops are pale green, shorter than usual with thin spindly stems and pale green-yellow leaves. Symptoms start on the middle leaves as pale-yellow interveinal mottling, spreading to older leaflets and advancing up the plant to younger leaves. If deficiency is severe, light brown lesions develop in the interveinal areas spreading towards the margins, veins remain green, and the interveinal area may become puckered. 

Conditions: Mg deficiency inhibits nitrogen fixation. Deficiency is likely to occur in acidic sandy soils leached of Mg, strongly acidic peat soils and in lighter soils where high rates of Ca or K fertiliser have recently been applied.

Management: Dolomite can be broadcast and mixed into soils several months before planting. Apply magnesium sulfate or magnesium chloride in a band at or before planting. Apply soluble salts in irrigation water. Foliar sprays are not recommended.

3.5 Manganese (Mn) deficiency and toxicity

Deficiency: Symptoms are similar to the early stages of Fe deficiency. Affected plants tend to be in patches, and are stunted with short, thin stems and pale green-yellow foliage. Young leaves turn pale green with a yellow mottle developing between the veins. The veins remain green. Later, small dark brown dead spots form in the yellow areas of leaves. If the deficiency becomes severe the youngest leaves may fall off. Older leaves remain green.

Toxicity: Mn toxicity develops first on older leaves. Plants are stunted with short stout stems and dark green leaves. Pinpricked red-brown dead lesions develop on the upper surface between the veins. With severe toxicity, red-brown areas develop on the veins and are more prominent on the underside of the leaves. The younger leaves are smaller than usual and develop yellow mottling that changes into red-brown necrotic lesions.

Conditions: Mn deficiency may occur in strongly alkaline soils, poorly drained soils with high organic matter, strongly acid soils leached of Mn and in soils formed from rocks low in Mn.

Mn toxicity can develop on strongly acid soils or in waterlogged soils where the lack of oxygen has increased the availability of the soil Mn. Plants growing quite normally can rapidly develop Mn toxicity symptoms in response to periods of excessively wet soil but may recover quite rapidly when soils dry out. 

Management: Deficiency can be controlled using foliar sprays or soil dressings of Mn salts, such as manganese sulfate. Foliar sprays should be applied 3–5 weeks after emergence and again if symptoms occur.

Toxicity can be controlled by applying lime to acid soils to raise pH and thus reduce Mn availability to plants. Improve drainage and avoid over fertilising with Mn fertilisers. Applying irrigation water low in Mn or mulching with organic materials can also remove soluble Mn from the soil and decrease Mn toxicity.

3.6 Nitrogen (N) and molybdenum (Mo) deficiency

Description: N deficiency makes older leaves turn pale or yellowish-green, and in severe cases the whole plant can appear a pale yellow-green colour. If N deficiency occurs during seed filling, the ‘self-destruct’ process seen during maturation (i.e. leaf browning and shedding) is accelerated and seed fill can be affected.

Conditions: Deficiency can occur during early growth, before the nodules are well developed and capable of fixing N. This is often the situation when soil mineral N reserves are low – perhaps as a result of microbial immobilisation of available N during decomposition of heavy crop residues like sugarcane trash. In later crop stages deficiency develops commonly because of poor nodulation, which can be the result of ineffective inoculation practices (i.e. low numbers of Rhizobia added or surviving long enough to infect roots), molybdenum deficiency, extreme soil acidity (leading to toxic levels of aluminium or manganese) or wet or cool soil conditions. Typically, if soybean crops have been grown in the field in the recent past, there should not be a problem related to insufficient bacteria present in the soil to properly inoculate the roots.

Management: If N deficiency or nodulation problem in the past is suspected, it is important to plant inoculated seeds for future crops. Most often, inoculation problems are related to poor growth conditions. Soybean crops do not grow very well in periodically waterlogged or very acidic soils. Ensuring adequate soil pH and good drainage are probably the best ways to reduce the problem of poor nodulation.

The problem of heavy residue loads and low soil N availability can be addressed by applying a low rate of starter N fertiliser to aid early stages of crop growth (until nodules are functional). If nodulation is poor and N deficiency becomes obvious later in crop growth (e.g. after a waterlogging event), topdressing with N fertiliser may provide some benefits – especially if applied early in crop development (i.e. before seed filling).

3.7 Phosphorus (P) deficiency

Description: Phosphorus deficiency may cause stunted growth, dark green coloration of the leaves and leaf cupping. Symptoms occur first on older leaves. Severe deficiency can result in fine yellow spotting on older leaves, with these yellow spots subsequently coalescing into necrotic patches. Older leaves and petioles will sag and be shed prematurely.

Phosphorus deficiency can delay flowering, although not as much as in some cereal crops, but conversely can accelerate senescence and maturation during pod filling. This results in small seed size and reduced yields. The combination of stunted plants and premature senescence can result in greater difficulty recovering grain at harvest.

Conditions: Phosphorus deficiency often occurs in early stages of growth, when root systems are small and only able to access small soil volumes. The deficiency can be accentuated when soils are cool and wet, due to slower root system development and decreased P uptake. Long bare fallows can also reduce populations of mycorrhizal fungi that assist plants in uptake of phosphorus, increasing the reliance on P fertilisers.

Management: Phosphorus deficiency is best prevented by regularly soil testing to monitor soil P status and applying the appropriate rate and form of P fertiliser at or before sowing. It is difficult to address a P deficit once the soybean crop has established.

3.8 Potassium (K) deficiency

Description: Potassium deficiency is observed as yellowing or browning and necrosis (death) of the edge of older leaves. When the problem persists, this deficiency will continue to move up from older to newer leaves, while the top leaves may look completely green.

Conditions: Potassium deficiencies develop more often at early stages of development when the root system is small. However, symptoms may also start to appear during seed filling, when the plant is trying to redistribute K in the vegetative plant parts to meet the demand of the developing seeds. Soil K concentrations are often highest in topsoil layers, due to accumulation of K from crop residues and fertiliser – K will only leach in light, sandy soils. As a result, K deficiency may appear in crops during a dry spell (when topsoil root activity is reduced, and roots are active in subsoils with lower K status) but disappear after rain when topsoil moisture (and root activity) is restored. While this may be visually pleasing, yield may already have been compromised.

Management: Management is similar to that for P. The best way to manage K fertility is by understanding the soil K status and applying the appropriate rate of K fertiliser to the soil prior to sowing. Fertiliser K has excellent residual value in soil so applying excess K fertiliser (e.g. if the yield is lower than expected) is still money in the soil fertility bank, although luxury crop uptake may mean this K rapidly concentrates in the topsoil layers.

3.9 Sulfur (S) deficiency

Description: Symptoms appear first on the youngest leaves. In young crops the whole plant may become pale green. Older crops are stunted with thin stems, the young leaves are pale green to yellow, and the older leaves remain green.

Conditions: Deficiency occurs in soils low in organic matter, soils formed from parent material low in S or acidic sandy soils leached of S.

Management: Elemental S should be applied to soils about 4 months prior to sowing, while S can also be supplied more rapidly as pre-planting applications of gypsum and /or sulfur fertilisers (e.g. potassium sulfate, or sulfate of ammonia). Soluble salts can be applied in irrigation water. If P is also low, application of single superphosphate may correct both deficiencies.

3.10 Zinc (Zn) deficiency

Description: Affected plants are stunted with thin short stems, lack vigour, mature slowly and have pale green to bronze foliage. The first symptoms are a pale mottling in the interveinal areas of the middle leaves. In later growth stages older leaves become affected while the younger leaves remain green. If the deficiency is severe the older leaves develop small bronze spots in the interveinal areas, the leaf edges cup downwards and the leaflets point towards the ground. The leaflets die and drop off.

Conditions: The deficiency is likely to occur in strongly alkaline soils, leached sandy soils and soils that have been recently levelled, exposing Zn deficient subsoils. Heavy fertilisation with P can induce Zn deficiency.

Management: Soil dressings of Zn chelates, sulfates or oxides should be mixed into deficient soils 2–3 months prior to planting. Compound fertilisers fortified with additional Zn can be applied in a band at planting. Deficiencies identified in-crop can be treated effectively with foliar sprays provided the deficiency is identified within 6 weeks of emergence.

4. Environmental disorders

4.1 Heat canker

In temperatures above 35°C, the seedling stems are girdled at or just above ground level, forming a red-brown ring. The affected seedlings usually die, and re-planting may be required if a significant proportion of the stand is affected.

4.2 Salinity

Soil salinity tends to occur in patches across a paddock. Plants are stunted, wilted on hot days but recover at night, have small pale to grey leaves and reduced flowering and seed production can occur. Older leaves tend to show symptoms first. In severe conditions younger leaves turn pale brown and the plant dies.

Salinity occurs in soils high in sodium and chloride ions in the soil solution and/or soils that were previously fertile but flooded or heavily irrigated with water high in salt. Irrigation water quality is important, as soybeans are more sensitive to saline conditions than other summer crops.

Salinity can be managed by testing soils to identify saline areas and by checking the quality of irrigation water. Apply gypsum and leach the soluble sodium and chloride beyond the rooting depth of plants in better drained soils. Rotate to deeper rooting plants such as perennials or more tolerant crops such as lucerne to help lower the watertable.

4.3 Soil acidity

Optimum soybean yields cannot be achieved in very acidic soil conditions. Soybeans are more sensitive to strongly acidic soils than most other field crops. The optimum pHw for soybean crops on sandy and clay-textured soils ranges from 5.5 to 6.5, while yields often decrease as soil pH falls below pHw 5.5.

If soils are acidic, applications of lime or dolomite should be made well before planting. Remember that lime is slowly soluble in soil, and so time and soil moisture are necessary for the lime to be effective. It is preferable to develop a liming program to maintain soil pH in or near the optimal range, rather than trying to overcome extreme acidity in the months before planting. In other words, soil pH should be managed across the farming system, rather than as a specific input for a particular crop.

The effectiveness of lime or dolomite is generally controlled by purity and by fineness. The finer the lime particles and the higher the purity (described as neutralising value), the more rapidly and effectively soil acidity can be countered.

4.4 Sooty mould

Sooty mould is a black powdery fungal growth that forms spots on the leaves, flowers and branches. It is not a disease of soybean, but a secondary infection. Sap sucking insects, such as silverleaf whitefly and aphids secreting honeydew cause conditions for the fungi to develop. The condition is more likely to occur in dry conditions as rain can wash the honeydew off the plant.

The mould hinders plant photosynthesis and may cause poor growth. High populations of sap sucking insects can cause the mould to expand to cover the entire plant. The mould is easily rubbed off the plant and it may dry and flake off if insect numbers are reduced. Overhead irrigation and rainfall can also wash the mould from plants.

4.5 Sunburn

This condition develops when leaves turn over and are exposed to intense sunlight. The lower leaf surfaces tend to develop a reddish colour. Only a small proportion of leaves are typically affected, so there is very little impact on yield.

4.6 Waterlogging

Plants in fields that are flooded for extended periods of time often die. Factors such as air and water temperature and whether the water is still or moving can influence the mortality rate. Poor drainage, low lying areas and compacted soil can all contribute to the period of inundation.

Waterlogging of soil during periods of heavy rain or flooding can leach vital nutrients from the soil and promote root diseases, as well as depleting the oxygen supply to roots and nodules, which will reduce nitrogen fixation. These effects can cause stunting and yellowing, and in severe cases, plant death.

Waterlogging effects can be minimised by a combination of variety selection and improved land management. Sow varieties tolerant to phytophthora if waterlogging is expected, avoid planting in low lying areas, improve paddock drainage and avoid extended periods of water run during furrow irrigations.

5. Inoculation

Soybeans need little or no additional nitrogen if the seed is effectively inoculated at planting with the correct strain of the nitrogen-fixing Rhizobia. Inoculum is a mixture of the sensitive living bacteria (Bradyrhizobium spp.) that can be supplied to seed in a peat culture, in a clay granule or even as freeze-dried spores. Inoculation of soybean seed with Rhizobia helps maximise nodulation and N-fixing ability. Effective nodulation also maximises the residual nitrogen carryover for the following crop (at least 40–60 kg N/ha in a harvested crop, and possibly as high as 120 kg N/ha).

Refer to the Agronomy module in this manual for important information about inoculation.