Agronomy

1. Key crop management issues (summary)

• Choose a variety that is recommended for your region, buy fresh seed of high quality and plant it at the optimum time to maximise yield potential and grain quality.

• Take a soil test prior to planting. Choose the correct fertiliser to correct nutritional deficiencies of phosphorus, sulfur, potassium and trace elements (in particular zinc on heavy clay soils). Potassium is commonly deficient in the sugarcane system on many soil types (refer to Crop nutrition Module of this manual). 

• Plan weed control measures carefully selecting appropriate herbicides (pre-emergent and/or post-emergent) and/or use inter-row cultivation. Always try to control weeds before you plant.

• Wherever possible, plant soybean into a full profile of soil moisture unless irrigation is available. Irrigated soybean fields should preferably be pre-irrigated and have an irrigation budget of 6–8 ML/ha. Avoid moisture stress from flowering to physiological maturity.

• Check seed germination and purity – insist on a current germination test certificate.

• Always inoculate seed correctly using the soybean-specific strain of Group H inoculant (strain CB 1809). Do not mix anything with inoculants unless safety is specified on the inoculant label.

• Seed size varies widely between varieties and seasons. Check the bag for seed size and use a formula to calculate the planting rate based on the seed size and target plant population.

• Establish and maintain a uniform plant stand at the recommended plant population for your climatic and soil conditions.

• Inspect crops for insect pests and beneficials at least once a week in the vegetative stage and then twice a week from flowering to maturity (refer to the Pest management Module of this manual).

• Reduce the risk of Phytophthora root and stem rot by using resistant varieties and selecting paddocks with good drainage and a disease-free history (refer to the Disease management Module of this manual).

• Harvest soybean crops as soon as they are ready to maximise grain quality by reducing the risk of weather damage or harvest losses from over-dry grain (refer to the Harvesting Module of this manual)..

2. Soil requirements

2.1 Soil pH, acidity and liming

Soybean is adapted to a wide range of soils from sands to heavy clays. The plant prefers a pHCaCl₂ in the range of 5.2–6.5. As pH levels drop below 5, increasing amounts of toxic aluminium and manganese can enter the soil solution. This effect is common in the coastal soils of NSW and is greatest in soils that are low in organic matter as indicated by an organic carbon soil test. For soybean keep aluminium saturation levels less than 15% and manganese less than 20 mg/kg. If barley is also grown, depending on variety, keep aluminium saturation levels below 5% and manganese less than 50 mg/kg. Soils with a pHCaCl₂ of 4.5 or less are unsuitable for growing soybean.

For example, many soils on the wet tropical coast of Queensland are marginal for soybean without the application of lime to raise the pH. Most Burdekin district soils are primarily in the range of pH 6–8 while those in the Mackay district range from pH 5–6.5. Where the soil is acidic liming is an important consideration. However, it should only be carried out after a soil test indicates lime is needed. In sugarcane areas mill mud can be a valuable source of calcium and other nutrients. Refer to the Crop nutrition Module of this manual for important information on nutrient recommendations for soybean.

2.2 Soil salinity

Soybean plants are intolerant of salt and yield reductions occur at soil salinities greater than 2.0 dS/m and irrigation waters greater than 1.5 dS/m. Salinity impairs nodulation and plant growth. Paddocks with known salinity problems should be avoided. Numerous Burdekin Delta farms in Queensland may have issues with soil salinity from saline irrigation (bore) water. This is usually less of an issue for summer crops (which are primarily rain grown), however, may be a significant issue for winter crops that are usually 100% irrigated.

3. Variety selection

Well established soybean growing regions in Australia have a range of agronomically suitable varieties available for planting. Growers and agronomists should select a preferred variety according to planting window, disease resistance, maturity, yield potential and suitability for the target market. The Rotations and varieties Module of this manual lists the current commercial varieties recommended for the major production regions of NSW and Queensland, along with explanations about photosensitivity and regional adaption in soybean.

The Australian Soybean Breeding Program (GRDC/CSIRO/NSW DPI) delivered new clear hilum varieties for Australian growers until 2023. The development of varieties with a clear hilum, large seed size and high protein enables growers to target a wider range of markets based on the quality they achieve. Recently released ASBP varieties such as Hayman*, Richmond*, New Bunya*, Kuranda*, Mossman*, Gwydir* and Burrinjuck* also have improved yield, disease resistance and broader regional adaptation.

In terms of risk management, if a large area of soybean is planned, consider selecting varieties of different maturity to spread planting, maintenance and harvesting operations. When trying a new variety for the first time, plant an area with a variety you are familiar with to enable differentiation of seasonal and varietal factors affecting performance. In regions affected by Phytophthora it is also important that the industry does not become reliant on one variety only, as resistance to Phytophthora could break down through the occurrence of new races.

4. Planting time

Planting windows and varieties vary widely across the soybean production regions of Australia.  Please refer to the Rotations and varieties Module of this manual and your local agriculture department or crop agronomist to obtain the recommended planting times and varieties for your region. If you choose to plant a variety outside its recommended window accept that it is unlikely to reach full yield potential.

4.1 New South Wales (and northern Victoria)

In northern Victoria, southern NSW and Central West NSW the optimum planting window is from early November to mid-December, so that crops can mature as early as possible, preferably by late March/early April. Planting in this window maximises plant dry matter production pre- and post-flowering, setting the crop up for maximum yield. Planting in late December shortens the vegetative growing time before the crop begins to flower and reduces total plant dry matter considerably, resulting in plants maturing in cooler overnight temperatures, which delays harvest and reduces yield potential. It also reduces flexibility in timeliness of farming operations, and often crops are exposed to greater insect pressure. Planting in early January in southern NSW is not recommended.

In northern inland NSW the planting window for maximum yield potential commences in mid November. Yield potential declines with late plantings. The critical cut-off date varies from mid December in the Macquarie and Namoi Valleys and tablelands, to late December in the irrigated northern border areas. By mid-January yield potential declines by 30% and other summer crop options are preferred.

In the Manning, Hastings and Macleay Valleys, the recommended planting times range from early November to the end of December.

In the North Coast region of NSW, correct variety choice and suitable conditions enable soybean crops to be planted from spring in October through to mid-February, with different varieties servicing each window (refer to the NSW DPI Summer Crop Production Guide for the latest variety and planting time recommendations and the Rotations and varieties Module of this manual).

4.2 Queensland – Southern and inland

December is the preferred planting time for soybean in southern Queensland. Crops planted in mid-December mature in 115–125 days. To avoid problems with excessive vegetative growth from early plantings, earlier maturity varieties such as New Bunya HB1* and Soya 791 are the best choice in this region. Conversely, for plantings late in the window choose varieties such as Hayman*, Kuranda HB1* or Stuart because they will help extend the length of the vegetative phase, which has a strong correlation with potential yield.

Avoid planting any variety after mid-January as plant growth, stature and crop yield are likely to be restricted. Leichhardt or Kuranda HB1* are the latest planting variety options and will extend a couple of weeks longer in warmer situations but is not a preferred option in inland production areas.

4.3 Queensland – Central and North Coastal

For summer planting, mid- to late-December is the preferred planting time in central and northern coastal Queensland. Refer to the Rotations and varieties Module of this manual for variety recommendations and descriptions for this region. Varieties differ in the time taken to reach harvest maturity, for example, Stuart matures around 2 weeks earlier than Leichhardt. Typically, Leichhardt planted in mid-December will be ready for harvest around mid-May (approximately 150 days), whereas Stuart will be ready for harvest 2 weeks earlier (~135 days).

The ideal time to plant summer grown soybean crops intended for grain production is mid-December to early January. Planting earlier than this may result in tall, large biomass crops that have increased risk of lodging, and increased difficulty in insect and disease control d and harvest. Very early plantings are best confined to green manure crops for biomass rather than grain production.

For autumn/winter planting in north Queensland, Kuranda HB1*, Mossman HB1*, Stuart and Leichhardt can be successfully be planted through summer and then in late autumn/early winter (up to 30 June). Further investigation is required to understand recent diseases issues and irrigation management for winter crops of Hayman* in the Burdekin region. The ideal time to plant dry season soybean crops is from early May–June 30. Plantings at the start of this window (May) will generally yield better than late June plantings, particularly in cooler winters. 

Do not plant soybean crops after 30 June because the combination of day length and temperature may result in a commercial failure as a grain crop. Late plantings into autumn and winter are obviously not suited to frost-prone areas.

5. Planting depth

It is very important to plant soybean seed at the correct depth for the soil type and conditions. Compared to other grain crops soybean has a comparatively large seed, which is not be able to emerge effectively if planted too deep or through a hard crusted surface. Figure 1 shows soybean seed emerging effectively in a minimum tillage system after planting at 3 cm depth. In general, soybean seed should be sown into moist soil as shallow as possible whilst allowing maximum contact between the seed and moist soil.

In dryland (rainfed) situations aim to place seed at a depth of no more than 5 cm. Where soil moisture is deeper than this, drilling to 7.5 cm has been successful but emergence can be poor, particularly if heavy rain causes the soil to pack or crust before seedlings emerge. Restrict seed depth to around 3 cm on medium clay soils, up to 5 cm on light sands and 2.5 cm on heavy clays or hard-setting soils. In irrigated situations shallower planting (2–3 cm) is preferred (see comments below in section 9 Irrigated soybean).

The use of rollers or heavy press-wheels that press soil directly over the seed row after planting is generally not advised. Planters with press wheels that press the soil onto the sides of the seeds whilst leaving a crown of uncompacted soil for easy seedling emergence are preferred.

It is best to avoid planting if heavy storms are forecast within 24 hours of planting. This is often difficult in the tropics but growers should consider this when making a decision to plant. Waterlogging or over irrigation immediately after planting is also not conducive to germination and usually results in patchy crop establishment.

6. Planting rates

6.1 Plant population and planting rates

Recommended plant population for soybean varies widely depending on region and planting time. Consult your local agronomist for the recommended plant population for your region and farming system. When the planting rate is higher than recommended for the planting conditions, individual plants become crowded. Over-crowding of soybean can result in:

• less branches and less pods produced per branch and less pods developed at lower nodes

• greater moisture stress to the plants during dry periods

• plants growing tall, thin and producing weaker stems leading to lodging

• greater risk of disease development (eg. Sclerotinia or white mould fungus, which favours humid, dense or lodged soybean crops where stems are in contact with each other). White mould disease can lead to further lodging and unfilled pods as the infection rots the stem.

• difficulties in applying crop protection products such as insecticides or fungicides due to poor penetration of the canopy

• difficulties in harvesting a lodged crop and a higher risk of picking up soil that can reduce the quality of the grain and reduced access to higher value markets.

Higher than recommended planting rates can, therefore, be costly due to the extra seed, lost returns in lower yield, harvesting difficulties and reduced grain quality.

For New South Wales the target plant populations for the major production regions are listed in Table 1 below. Note that lower plant populations are preferred when planting is early in the recommended window, while the higher densities are preferred for planting later in the recommended planting window.

For Queensland, summer plantings in coastal Queensland (Bundaberg, Mackay, Burdekin districts) aim to establish plant populations of 250,000 to 300,000 plants/ha. Anecdotal evidence would suggest that the extremely vigorous irrigated growing conditions of the Burdekin may allow the plant population to be reduced to 200-250,000 plants/ha to limit lodging. Do not use high plant populations under summer tropical conditions as lodging commonly occurs, resulting in difficulties with insect and disease control from poor spray penetration and greater risk of developing fungal diseases such as white mould (Sclerotium sclerotiorum). 

For autumn/winter in the Burdekin, increase this population to 400,000—450,000 plants/ha as conditions are cooler and plants will not grow as quickly or as large as during summer.  Always use the highest recommended population if planting at the end of the planting window (late June).

6.2 Calculating seed requirements

As seed size can vary considerably between varieties and between seasons, you must adjust the seeding rate accordingly to avoid over or under planting. It is far too risky to guess how many bags to the hectare should be planted as seed size (number of seeds/kg) and germination (% of germinative seeds) vary widely every season, as do the planting conditions.  

Refer to the seed packaging label for an accurate seed count (number of seeds per kg). All seed offered for sale must clearly state the germination percentage. Use the best quality seed available. It is not recommended to use seed lower than 85 per cent germination percentage.  Increasing the seeding rate to compensate for low germination seed is risky as low germination seed commonly also has low vigour.

To calculate your planting rate you will need to know these figures:

A  the targeted plant population to establish for your region – get this from your local agronomist (the example below uses 250,000 plants/ha)

B   a seed germination test result (%) – this should come with the planting seed (the example below uses 92%)

C  establishment rate of the germinative seed (assume 85% in a well-prepared seed bed in reasonable conditions. Reduce this value if planting into adverse conditions.)

D  number of seeds per kg for the variety to be planted – this seed count should be stated on the bag of planting seed (in this example the variety has 6400 seeds/kg)

Use one of the following formula to calculate planting rates:

6.3 Seed testing for germination and vigour

Soybean seed can sometimes have variable and poor establishment, however, 80–90% establishment is usually achievable in friable, non-crusting soils, using fresh, high quality seed. 

While a poor seedbed can contribute to poor crop establishment, poor seed quality is often more likely to be the cause. 

Key message:  Using low quality seed is the primary reason for germination difficulties and poor establishment, which leads to lost yield potential. 

Growers must insist on high quality planting seed!

The accelerated ageing test is an internationally accepted method of assessing seed vigour in soybean, and is available in Australia.  It should be interpreted in conjunction with the standard germination test.  

Use only seed testing laboratories that follow ISTA (International Seed Testing Association) seed testing rules, for example:

• Australian Seed Labs (formerly SGS Agritech), 214 McDougall St, Toowoomba, Qld 4350. Phone 07 4613 9052

• Futari Technology Services, 34 Francis Street, Narrabri NSW 2390. Phone 02 6792 4588

• SGS also operates in some major cities in Australia for seed testing services.

Be sure to:

• strongly consider purchasing industry approved seed from suppliers with a current (<3 months) germination certificate

• treat seed carefully and avoid dropping bags onto the ground, which causes splits

If you choose to use seed from non-industry approved sources: 

• avoid planting seed with hairline cracks in the seed coat. This usually indicates mechanical damage

• avoid using seed damaged or deformed by pod sucking insects (e.g. green vegetable bug)

• avoid using seed with any sign of mould or weather damage

• avoid using seed that is older than last season especially if it has not been stored appropriately

• the only long-term storage conditions recommended for soybean seed in the tropics is cooled and dried storage, i.e. cooled storage that is also de-humidified. Refrigeration alone is not acceptable as it is moist and condensation will damage seed viability.

If in doubt, check the rate of emergence of a small quantity of seed in soil prior to planting the crop.

Report poor seed quality to the Soy Australia Seed Committee. 

7. Row spacing and configuration

Soybean has a large seed size relative to other crops and a delicate, thin seed coat. It is, therefore, not ideal to broadcast through a spreader and disc in as this will destroy much of the seed. Soybean is best grown in defined rows using a seeder to achieve uniform seed depth and placement along the row.  

Row cropping allows more options for weed control including the use of banded sprays, shielded sprayers and inter-row cultivation. It also facilitates the use of directed sprays for insect control and aeration of the crop, which is particularly important to minimise the development of fungal diseases such as soybean leaf rust and Sclerotinia in humid environments. Row cropping enables soybean to be planted between the rows of the preceding crop as in the example in Figure 2.

The developments in row cropping and controlled traffic and GPS guidance systems in Australia have enabled developments in other aspects of crop management including use of shielded sprayers (to deliver different products to the plant row and the interrow space in one pass), and pressurised water irrigation systems (such as centre pivots or lateral move). This provides a very flexible system for soybean growers. For example, pre-watering to fill the soil profile (for both single and double cropping) and then to sow on time into a moist seedbed. In double cropping systems where a winter and summer crop is produced on the same paddock in a 12–15 month period, soybean can be planted into the inter-row of the previous crop or skip rows can be used to achieve a wider row spacing for the soybean crop (eg. wheat planted at 30 cm and soybean at 60 cm as illustrated in Figure 2). Stubble removal is not required in these systems and is considered highly beneficial for reducing soil water evaporation and water use by the crop. 

Row spacing considerations are covered in more detail in the Rotations and varieties Module of this manual.

8. Inoculation

If soybean seed is effectively inoculated with the correct strain of the nitrogen-fixing Rhizobia at planting, the plants rarely need any additional nitrogen (N) at planting or during the crop as they have the ability to fix far more nitrogen (N₂) from the atmosphere than they require to grow and produce grain. Soybean plants produce high protein grain and, therefore, have a high nitrogen requirement, but when inoculated correctly they fix more nitrogen than their own needs.

Soybean inoculum is a living bacterium Rhizobium strain specific to soybean (Group H, Bradyrhizobium japonicum strain CB 1809). It can be mixed in a peat culture or other media to protect and deliver it to the seed. Correct inoculation enables the live bacteria and the soybean root to form nodules, which are essential for fixing nitrogen from the atmosphere. 

Soybean is among the highest N fixing grain crops available, provided it is inoculated correctly. Estimates of N fixed by crop legumes in Australia (Unkovich et al., 2010; Herridge, 2013) is summarised in Table 2.

Inoculation of every soybean crop is recommended as the soybean-specific bacteria that facilitate the nodulation process are largely absent in Australian soils, especially soils that have not recently grown soybean crops or that have been waterlogged or experienced severe moisture stress since the previous soybean crop. Even if some Rhizobium cells have survived in the soil from previous soybean crops, the distribution will be patchy and may not be at a sufficiently high level to ensure good nodulation of the crop.

Avoiding proper inoculation will not save money as poor nodulation and reduced N fixation will have a negative effect on crop vigour, grain yield, protein content and the profitability of the crop. Soybean inoculant is among the lowest input cost (often <$10/ha) with arguably the highest benefit to the current and subsequent crops.

In 2022, the GRDC published updated guidelines on inoculation of legume crops Inoculating legumes: practice and science, which is available online and as a pdf.

A one-page summary of ‘do’s and don’t s’ to optimise inoculation is available in this GRDC poster.

General advice for use of inoculants in acidic soils is contained in this GRDC factsheet. 

8.1 Starter nitrogen?

In most situations, the addition of nitrogen fertiliser at planting is not necessary for soybean crops. Adding a small amount of starter fertiliser N (up to 20 kg N/ha) at planting may help establishment in situations where large amounts of organic matter from the preceding crop (e.g. cane trash or stubble from the previous crop) have been incorporated prior to planting soybean, temporarily ‘tying up’ some of the soil nitrogen available to the seedling. 

Care must be taken not to add too much N fertiliser at planting as this will interfere with timely formation of the N-fixing nodules on the roots commencing at the two to three trifoliate leaf stage (growth stage V2–V3, see the Plant growth Module). While additional nitrogen application may initially produce taller or greener looking plants, trial results to date show no economic yield response from the addition of nitrogenous fertiliser to soybean crops in dryland situations. It is far more cost-effective to inoculate the seed correctly at planting time than to risk nodulation failure and then supply additional N fertiliser during the crop.

8.2 Keeping inoculum alive 

Rhizobia are living organisms and the number of live cells will decrease rapidly in hot or dry conditions or if mixed with chemicals or other seed dressings. Do not lime-coat soybean seed after inoculation and do not tank-mix other liquids, pesticides, fertilisers or products with liquid inoculant unless the inoculant label expressly states that it is safe to do so. Large scale nodulation failures are known due to tank-mixing inoculant with other products.

Do not use inoculum if the expiry date has passed or if it has not been stored correctly in dry and cool (preferably refrigerated) conditions. The Australian Inoculants Research Group introduced the ‘Green Tick’ logo to identify inoculants produced with quality controls and verified testing (Figure 3).

Store inoculum refrigerated (but not frozen). If inoculating seed in the paddock, store inoculant in an esky with ice to keep it cool but not frozen. Likewise, after the seed is inoculated it should be kept in a cool shady place out of direct sunlight and planted as soon as possible after inoculation. A good rule of thumb is to only inoculate seed in small batches, e.g. a batch that you can plant in 3 hours. This avoids inoculated seed not being planted due to breakdowns or other interruptions at planting.  

High summer temperatures in most of Australia also mean that you should only treat enough seed to plant a small area at a time. Do not inoculate large quantities of seed prior to planting and leave standing outside in high temperatures and plant within 12 hours at the maximum.

Exercise caution when using air seeders, as hot air in the distribution system of some air seeders (eg. older style air seeders with the oil cooler in front of the air intake) can kill the inoculum. Temperatures greater than 30°C can rapidly kill Rhizobium cells.

8.3 Methods of inoculation

Methods of inoculation vary, but in general the better the job is done, the more effective the nodulation and the more nitrogen the crop can fix.  Methods include:

• peat-based slurry inoculation

• water injection and in-furrow sprays of liquid inoculant

• pelleted seed

• Nodulator® granules.

8.3.1 Peat-based slurry

This is the most common form of inoculation and is also proven very reliable and effective in a wide range of conditions. The peat inoculant contains live Rhizobium spores and is mixed with cool water to make a thick slurry, which is then gently mixed to evenly coat the seed (follow label instructions for mixing rates). Do not use too much water as the seed coat will crinkle. 

A benefit of peat inoculum is that it is dark brown in colour making it easy to see the amount and evenness of the inoculum coating on the pale coloured soybean seed. Generally, around 2.5 g of peat inoculant coats 1 kg of seed, or a 500 g pack should treat 200 kg of soybean seed of average size.

Small cement mixers on a very slow speed have been used for mixing soybean seed with the inoculum slurry provided that care is taken to avoid seed damage (e.g. excessive agitation causing seed to fall and hit the metal can crack the seed coat or the seed itself). Do not use augers. 

Peat slurry inoculated seed should ideally be sown within 2-4 hours of treatment, however, if stored in cool conditions out of direct sunlight it may be viable for a few hours longer depending on the conditions. Remember the inoculant contains high numbers of a living organism and the number of live cells will begin to drop almost immediately that it is applied to the seed. The aim is to ensure that as many live cells are available to the soybean seedling roots when they are required to form nodules.

8.3.2 Water injection and in-furrow sprays of liquid inoculant

For this method the inoculum (either from a peat, freeze-dried or liquid source) is suspended in water and injected/sprayed into the furrow over the seed or just below the seed in the furrow at planting before covering over with soil. When the germinating seed roots grow through the inoculum in the soil, nodules can form.  

The results from this form of inoculation are generally good, except where the seedbed is very dry or the water jet is not directed properly or becomes blocked. Cautions for using liquid and reconstituted inoculants are covered in this GRDC inoculation publication (page 20–23).

Water rates vary according to row spacing but use at least 300 L/ha to ensure plenty of inoculum is delivered onto and below the seed before being covered by soil. A continuous flow of water from each outlet, without blockages, is obviously essential for success. Conventional water injection equipment is suitable for this inoculation method. This method can save time when planting large areas. Follow inoculant label instructions for rates and cautions when preparing the liquid. 

8.3.3 Nodulator® granules

This was a clay-based granular form of the soybean inoculant (Group H) released by Becker Underwood in Australia in 2007 and subsequently discontinued in the Australian market when the company was bought by BASF. The Nodulator® granules in Australia are now only available for clover (Group C), field pea/vetch (Group E), lentil/faba bean (Group F), lupin (Group G) and chickpea (Group N). Nodulator Solid Core Granules® for soybean are still available in the USA and Canada, where they claim to contain 8 x 107 viable cells of Bradyrhizobium japonicum per gram. 

Loss of supply of granulated inoculant for soybean (Group H) in Australia is regrettable as it provided several advantages when planting large areas of soybean. It was placed straight into planting equipment via a dedicated box (e.g. seeders with separate additional seed boxes or granular insecticide boxes). Placement was as close as possible to the seed in the furrow before coverage to allow maximum contact between the granule and the seedling roots. As an alternative to granules, growers with large areas of soybean to plant have modified planting equipment to allow application of liquid inoculant onto the seed in the furrow at planting before coverage.

8.3.4 Pelleted seed

This method was offered for soybean by some commercial seed companies in Australia (e.g. CSD) but is not commercially available at present. Seed can be pre-pelleted with inoculum as well as fertiliser, insecticides, or fungicides. Pelleting increases the bulk of the seed, therefore, appropriate adjustments need to be made to planting rates and possibly also to planters. This procedure is expensive and is not always effective for living inoculum due to the time delay between pelleting and planting. Proper application and storage techniques are vital to ensure survival of the Rhizobium. Do not lime coat soybean seed after inoculating as this will adversely affect the Rhizobium.

8.4 Rhizobia, molybdenum and checking nodules

Rhizobia cells are living and fragile; hence fertilisers, insecticides, fungicides should not be mixed with inoculum or inoculated seed as many pesticides are toxic to Rhizobium. Unless the inoculum label specifies the safety of other chemicals they should never be mixed with the inoculum.

Molybdenum (Mo) is essential for soybean nodules to fix nitrogen from the atmosphere. Adding a fertiliser containing Mo (for example, Mo Super 0.025% or 0.05% Mo) can aid the efficiency of N-fixation in legumes especially when soil deficiencies exist, and the soil is acidic (pHCaCl2 less than 6). Alternatively, molybdenum trioxide can be applied at a rate of 70 g/ha every two years, for example in the form of Molybdenum trioxide (60% Mo).

Note that sodium molybdate is toxic to Rhizobia.  Ammonium molybdate and Mo trioxide are not toxic to Rhizobia.

Molybdenum is also available in several commercial foliar sprays (e.g. CoMo®). To be effective the molybdenum contained in these products must be translocated from the leaves and made available at the root surface at the time when the Rhizobium and the plant root are forming the nodule. 

To establish if nodules are functioning, growers are encouraged to dig up plants to check on the success of their inoculation procedures. Check several locations within the crop. Carefully dig up a group of plants that are at least 4 and preferably 5–6 weeks old, wash the root systems carefully and cut open several nodules. Functioning nodules should be large, firm and white on the outside with a pink—orange colour on the inside (Figure 4). Nodules that are very small in size, or that have a green, grey or white colour inside are not functional and you should seek the advice of your local agronomist.

Environmental conditions such as hot, dry weather at planting can kill the Rhizobium bacteria before they come into contact with the plant roots. This is one of the primary reasons why soybean seed must be planted into moist soil or irrigated immediately. Planting into dry soil and waiting for rain significantly increases the risk that the inoculant will die before the seed can germinate. Accordingly, in irrigated districts, only plant as much as can be irrigated in one set/cycle. 

Considering that temperatures of over 50°C have been recorded in surface layers of black soil in summer, planting into these conditions carries a degree of risk in terms of inoculant survival. Consider planting very early in the morning or later at night when soil temperatures are cooler to reduce mortality of the rhizobia.

During the growing period, Rhizobia can also die if the roots become waterlogged, however, soybean plants have remarkable tolerance to waterlogging especially if grown on raised beds that allow the root zone to drain and maintain some aeration. Careful field selection and improvements to drainage are critical to ensure adequate nodulation and N-fixation as well as good root growth. Consider growing soybean crops on raised beds or hills in fields prone to waterlogging.

9. Irrigation management

Soybean crops are suited to a range of irrigation systems including raised beds, furrow, full flood and overhead lateral or centre pivot irrigation. Regardless of the type of irrigation system, soybean plants have a peak water demand during flowering and early pod filling.

In the subtropical and tropical coastal production regions, summer-grown soybean crops are usually grown on rainfall alone, however, irrigation (if available) is also used to supplement growth at critical periods in the crop cycle. 

Winter-grown soybeans in the tropical north (e.g. Burdekin) rely almost fully on irrigation. High yielding soybean crops receiving little or no rain typically use 6–8 ML irrigation water/ha depending upon soil type, variety, paddock/irrigation layout and seasonal conditions.

In irrigated cropping systems (e.g. southern and central Queensland, northern inland and southern New South Wales and northern Victoria), the quantity of irrigations applied will vary depending on season, soil type and target yield. Implementing a soil moisture monitoring system to help identify when the crop is approaching water stress is recommended.

Pre-irrigating fields one to three weeks prior to planting is recommended. This allows accurate planning of your planting time as it allows the soil to become dry enough to plant, consolidation of formed beds or hills, and controlling weeds prior to planting. Watering after planting is possible but is not recommended as it poses many more risks including soil surface crusting before germination, Rhizobia on inoculated seeds dying in hot dry soil, and weeds germinating at the same time as the crop. It is also likely to result in uneven establishment compared with a crop sown into correct soil moisture. 

In the Riverina region of southern NSW, soybean is typically grown on raised beds using furrow irrigation on slopes of 1:1500 or flatter with run lengths of 400–800 m. This allows for better drainage around the root zone, less waterlogging problems, and less disease build-up (e.g. the water-spread disease Phytophthora root rot).

When double cropping on irrigated, raised beds, management of the winter crop stubble prior to irrigating for soybean can be an issue. Winter crop stubble may need to be removed by baling or mulching, for example, to allow better water flow and minimise stubble blocking water in the furrows.

Mild moisture stress in the early vegetative stage of the crop has little impact on grain yield and can encourage the plant to seek moisture with a deeper root system. Intermittent moisture stress throughout the crop should be avoided if possible as it may reduce grain protein content. When soybean plants start to become moisture stressed, they will firstly shutdown the nodules in the root system and will not activate them again until they receive adequate moisture. However, the delay between crop irrigation and fully functional Rhizobia can be several days. The cumulative effect over the season can lower the total nitrogen content within the plant and ultimately the grain protein concentration.

It is critical that the plants do not experience moisture stress from the start of flowering, through pod development and filling to physiological maturity. Soybean crops planted at the end of November to early December are often flowering and filling pods during the hottest part of the year (late-January to March). Moisture stress during flowering and pod-fill can reduce yield by reducing the number of retained pods and by reducing seed size and weight.

For a crop planted in late November-early December in southern NSW, this means the peak water demand is from mid-January to late March, equating to 50–110 days after emergence depending on variety. For a crop planted in mid-late December in north Queensland, the peak water demand is from mid-February to early April. This equates to 50–110 days after emergence depending on variety.

The timing of the final irrigation is also critical, as it needs to be timed to ensure adequate water supply for the plant until it reaches physiological maturity yet not causing the field to remain too wet for harvesting. Many growers stop irrigation too soon and lose valuable yield. As a very general guide, apply the final irrigation when the first maturing (pale yellow) leaves appear in the crop. Consider choosing a shorter-season (fast maturing) variety if one is available for your region as this may reduce the number of irrigations required for the crop.

Ideally soil moisture and irrigation monitoring equipment (e.g. evaporation pans, enviroscans, soil moisture probes, EM38 scans etc.) should be used to support irrigation decisions and monitor effectiveness. The Australian cotton industry has invested heavily in irrigation management research and water use efficiency and have many technical resources available to assist crop growers to understand and evaluate irrigation practices. Overwatering is not only expensive but can cause disease issues to develop in the crop and cause long term damage the soil structure.

It is not possible to make one definitive statement about irrigation scheduling that will be appropriate for all soil types or farming systems in all seasons. Grower experience of the soybean crop’s needs, seasonal conditions and understanding of soil type, soil structure of each paddock is essential to underpin crop management decisions, especially irrigation. Links to some regional best practice irrigation guidelines are provided here:

Soy Australia grower guides 

Cotton My BMP – Water 

https://cottoninfo.com.au/water-management

10. References and further reading

NSW Summer Crop Options – short guide 2022-23 

NSW DPI Summer Crop Production Guide (2019), available at NSW Department of Primary Industries offices or website  

Managing legume and fertiliser N for northern grains cropping (2013), Herridge, D.  Available from GRDC:     

Inoculant information

Inoculating legumes

Inoculating legumes in acidic soils 

Innoculant selector

Inoculant Group Chart Dec 2021

2023 BASF Inoculants brochure

Useful websites

Soy Australia www.soyaustralia.com 

Cotton Research and Development Corporation  www.crdc.com.au 

Grains Research and Development Corporation www.grdc.com.au 

NSW Department of Primary Industries www.dpi.nsw.gov.au 

Pulse Australia www.pulseaus.com.au 

Qld Department of Agriculture Fisheries and Forestry www.dpi.qld.gov.au 

https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/crops/field/broadacre