Plant population
In dryland crops aim for 200,000 plants/ha (and up to 250,000 plants/ha under favourable dryland conditions). On lighter granite soils 180,000 - 200,000 plants/ha is better.
In irrigated crops aim for 300,000 plants/ha or up to 400,000 plants/ha for high yielding situations, or for late plantings made in January. Some varieties of small or upright stature respond to higher planting rates.
Average seeding rate
Seed size can vary considerably so adjust the seeding rate accordingly.
Refer to the seed packaging label for an accurate count, usually around 5000 to 7000 seeds/kg for most varieties. Assuming an average seed size of 5700 seeds/kg and 80% establishment. The table below provides a guide to suitable seeding rates.
Seeding rates
Target population Seeding rate 200,000 plants/ha 42 kg/ha 300,000 plants/ha 63 kg/ha
Germination percentage
All seed offered for sale must clearly state the germination percentage of that seed line. Use the best seed quality available. It is not recommended to use seed lower than 80% viable. Strongly consider using seed from the ´industry-approved scheme´ endorsed by the Northern Australia Soybean Industry Association (NASIA).
Establishment percentage
Soybean has a reputation for poor establishment; however, 80 - 90% establishment is achievable in friable non-crusting soils, using quality seed. While a poor seedbed environment can contribute to poor establishment, seed quality is often more important. Be sure to:
- avoid using planting material with hairline cracks in the seed coat. This usually indicates mechanical damage
- use belt elevators in preference to augers when handling seed crops
- avoid using seed damaged by pod-sucking insects (e.g. green vegetable bug)
- avoid using old seed. The recommended storage life for soybeans is only about six months, even under good conditions
- avoid using seed with any sign of mould or weather damage
- only keep seed with an after-harvest germination test of more than 90% to plant your next crop. Keep the seed in a cool room
- consider purchasing industry-approved seed from suppliers at least every three years.
If in doubt, check the seed’s rate of emergence by planting a small quantity of seed in soil prior to planting the crop. A high germination percentage in soybeans does not necessarily mean a high level of seedling vigour. The accelerated ageing test is an internationally accepted method of assessing seed vigour in soybeans, and is now available in Australia.
It should be used in conjunction with the standard germination test. For further details contact SGS Agritech.
Row spacing
Row spacing of 70-100 cm is standard practice. Narrower row spacing of 20-30 cm can be an advantage in a high-yielding irrigated situation, or with a late planting where smaller bush size is likely to limit yield.
Seed placement depth
Plant seeds 30-50 mm, preferably into moisture. Sowing deeper than 50 mm can significantly reduce emergence, especially in hard-setting soils after rain.
Precision planters should aim to plant at 30 mm. Avoid watering the crop up, as this can lead to establishment problems such as soil crusting and weed germination as the crop emerges.
Inoculum
Soybeans do not need additional nitrogen (N) if the seed is effectively inoculated at planting with the correct strain of the nitrogen-fixing rhizobia; however, there is evidence that applying approximately 30 units of N at planting can assist with the plant’s early vigour. Inoculum is a mixture of the sensitive living bacteria (rhizobia) mixed in a peat culture.
Inoculation helps maximise nodulation and N-fixing ability. Effective nodulation also maximises the residual nitrogen carryover for the following crop (approximately 40-60 kg N/ha in a harvested crop). Poor inoculation or avoiding inoculation will not save money or time as poor nodulation and reduced nitrogen fixation will have profound effects on both yield and protein content of the crop.
While nitrogen application may produce taller, greener plants, trial results show no economic yield response from the addition of nitrogenous fertiliser to soybeans in dryland situations.
The rhizobia strain specific to soybeans is the group-H rhizobium. Refer to the container label for inoculant application rates and check the expiry date. Do not use inoculum if the expiry date has passed or if it has not been stored correctly. Rhizobia are living organisms and very sensitive to hot, dry conditions. Store in a cool place, preferably the refrigerator (but not the freezer). Likewise, inoculated seed should be kept in a cool shady place out of direct sunlight. Only treat enough seed for each day´s planting and sow treated seed immediately.
Exercise caution when using air seeders, as hot air in the distribution system of some air seeders can affect the inoculum. Do not use old air seeders with the oil cooler in front of the air intake. Temperatures greater than 30#C can kill the rhizobia. Methods of inoculation vary, but in general the better the job is done, the more effective the nodulation. Methods include:
- slurry inoculation
- water injection and in-furrow sprays
- pelleted seed
- Nodulator® granules.
Slurry inoculation
This is the most common form of inoculation and is also the most reliable and effective. The peat inoculant is mixed with cool water then introduced to the soybean seed. It is gently mixed to evenly coat the seed (follow label instructions for mixing rates). Cement mixers and recirculating grain driers are suitable for mixing seed with the inoculum slurry.
Take care to avoid seed damage (e.g. bouncing seed off metal can crack the seed coat). Do not use augers. Slurry inoculated seed should be sown within 12 hours of treatment, however if stored properly (5#C or lower and out of direct sunlight) can be kept for longer, provided a peat inoculum and sticker have been used.
Water injection and in-furrow sprays
For this method place a band of inoculum, suspended in water, just below the seed. The germinating seed´s roots grow through the band of inoculated soil, leading nodules to 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.
Water rates vary according to row spacing, but use at least 300 L/ha. A continuous flow of water from each outlet, without blockages, is essential. Conventional water injection equipment is suitable for this inoculation method.
Pelleted seed
This method is prepared by commercial companies. Seed can be pre-pelleted with inoculum and fertiliser, insecticides, or fungicides.
Pelleting increases the bulk of the seed, therefore appropriate adjustments need to be made to planting rates. This procedure is expensive and is not always effective due to the time delay between pelleting and planting.
Proper application and storage techniques are vital. Lime pelleting is another form of seed pelleting that is only recommended where soil acidity is below pH 6. Fine lime is added to form a protective coating around the seed.
Nodulator® granules
This is a clay-based granular inoculant released in autumn 2007 in eastern Australia. It can be placed straight into sowing equipment (air seeders, seed boxes or granular insecticide boxes) and is a free-flowing dust-free product. Generally, it is best not to mix fertilisers and insecticides with inoculum or inoculated seed as many pesticides are toxic to rhizobia. Check compatibility before use. In-field experience has shown that low rates of dust, granular and prilled fertilisers (e.g. MAP), do not reduce the efficiency of inoculation when applied at planting. Where fertilisers or insecticides are mixed with inoculum, avoid prolonged contact. Use immediately, do not allow the mixture to stand.
Water injection application of inoculum seems to have a buffering effect, protecting the inoculum from fertiliser damage. To test if nitrogen fixation is occurring, carefully dig up a plant, wash the root system and cut open a few of the nodules. A red or pink colouration indicates that N-fixation is taking place in the nodules. Be aware that in non-legumes, lumps on the roots that look like rhizobia nodules may be caused by root-knot nematodes.
Rhizobia can not survive in acidic soils (below pH 6). Lime pelleting can provide some protection for the inoculum under acidic conditions. Molybdenum (Mo) also aids in the nitrogen-fixation process. Adding fertiliser containing Mo can aid the efficiency of N-fixation in legumes especially when soil deficiencies exist and the soil is acidic (pH less than 6).
Environmental conditions such as hot, dry weather at planting can kill the bacteria before they come in contact with the plant roots. During the growing period, rhizobia can die if the roots become waterlogged. While these environmental factors are often unpredictable, careful field selection is critical to ensure adequate nodulation and N-fixation. Try growing soybeans on raised beds in fields prone to waterlogging.
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