How to Grow Soybean Cover Crops in Australia

Soybean (Glycine max, pea family, Fabaceae) is a bushy summer legume grown as a cover crop to rest and feed a bed between vegetable crops. It stands about 60 cm tall, fixes nitrogen with the right bacteria, and copes with wet ground better than lablab or cowpeas. Where gardeners go wrong is skipping the inoculant, because the bacteria soybean needs are largely absent from Australian soils.

Three mistakes that matter

1. Sowing without the inoculant. Soybean fixes nitrogen only with its own Group H rhizobia (root bacteria). Without them the plants turn pale and give the bed little. Coat the seed as Preparing the seed describes.
2. Sowing deep, or watering onto a dry, sown bed. The large seed has to push its seed leaves up through the surface, and a crust or over 50 mm of soil can stop it. Sow shallow into a pre-soaked bed, as Sowing direct describes.
3. Sowing late in summer. Soybean starts to flower as the days shorten after midsummer, so a late sowing flowers small and gives the bed far less growth. Sow early in your zone's window.

When to plant

Soil temperature and frost decide it. Sow once the danger of frost has passed and the soil has held at least 13°C for seven days and is still rising. Seedlings come up fastest in soil above 25°C. One sowing covers a bed for the summer, and earlier sowings in the window make more growth before flowering.

Where frost comes early, sow early enough to dig the crop in before the first autumn frost.

Site, soil and setup

Pick a sunny bed that will sit empty over summer; heavy clay suits it.

Sowing Seeds

Sow Green Manure Soybean seeds straight into the bed, in shallow rows rather than scattered.

Preparing the seed

Coat the seed with a Group H soybean inoculant (a peat culture of the rhizobia). Mix the peat at the packet's rate with a little cool water into a thick slurry, then stir the seed through gently. Too much water crinkles the seed coat. Keep treated seed in the shade and sow it within a few hours. Store the inoculant in the fridge, not the freezer, and use it before its expiry date.

Sowing direct

• Soak the bed a few days ahead, and sow once the soil is moist but crumbly.
• Draw shallow rows 20 to 30 cm apart.
• Sow about 5 g of seed per square metre, roughly 25 to 35 seeds, one every 10 to 15 cm along each row.
• Cover the seed 25 to 40 mm deep, shallower in heavy clay and up to 50 mm in sand, and firm gently.
• If the surface dries before the seedlings appear, water with a fine spray so no crust forms.
• Expect seedlings in 5 to 10 days.

Scattered seed lands at uneven depths and gives a patchy stand. The seed is also short-lived, so sow it the summer you buy it and keep any left over cool and dry.

Care by growth stage

Water decides this crop at both ends: steady moisture to establish, and again at flowering, when its demand peaks. Once inoculated it makes its own nitrogen, so it needs no nitrogen feed.

To check the inoculant, dig up a plant at 4 to 6 weeks and wash the roots. Firm nodules that are pink to orange inside are fixing nitrogen; green, grey or white ones are not. If a soil test shows zinc or molybdenum is short, a soluble trace element mix at the label rate corrects it.

Companion planting

Soybean pairs well with other summer cover crops and badly with crops that share its stem and root rots.

Grows well with

• Millet, a summer grass sown with soybean in cover crop mixes: the grass adds bulk while the legume adds nitrogen.
• Buckwheat, another quick summer cover crop that suits the same mixes and the same sowing months.
• French marigolds (Tagetes patula), a summer cover crop dug in to help control root-knot nematode.

Keep apart

• Sunflowers, because soybean sown after them can pick up Sclerotinia stem rot.
• Mustard and other brassicas, which also carry Sclerotinia into a following soybean crop.
• Beans and peanuts, legumes that build up the same diseases when soybean follows them.

Cutting and Digging In

Dig the crop in when about half the plants carry at least one open flower. In the south, flowering often starts 7 to 9 weeks after sowing. Early northern sowings take longer, because flowering waits for shortening days, and the timing also varies with variety.

• Slash or mow the plants low and chop the tops finely, so they break down faster.
• Dig them into the top 15 cm of soil, or leave them on the surface as mulch.
• Wait 4 to 6 weeks before sowing seed into a dug-in bed.
• If the bed is needed sooner, leave the tops as mulch and plant seedlings through it.
• Cut before any pods ripen, so no seed drops to sprout in the next crop.

Carting the tops off for mulch elsewhere can take much of their fixed nitrogen with them.

Weed lists, state by state

Soybean is a field crop and no state or territory declares it a weed. Volunteers (self-sown plants) can carry soybean rust from one season to the next.

Common problems

Gaps in the rows. Seed sown too deep, under a crust, into cold, wet soil or from an old packet often fails (see Three mistakes that matter). Slugs and snails can strip seedlings in prolonged wet weather. Seedlings cut off at soil level point to cutworms (Agrotis species), which feed at night and hide in the soil by day. If seed vanishes in cold, wet weather, dig for false wireworms (Pterohelaeus and Gonocephalum species). These shiny cream to tan larvae are 15 to 35 mm long. Read more

Seedlings collapsing. Young plants that topple with water-soaked, brown or reddish stems at soil level have damping off (a soil disease that rots seedlings). It strikes most in cool, wet soil, so wait for warm soil before sowing again. Read more

Pale, yellowish plants. Few or no pink nodules means the rhizobia died, often because the inoculant was old, heated in the sun or sown into dry soil. Waterlogged roots kill the bacteria too.

Red spots under the leaves. Small brick-red spots on the underside of leaves flipped up to strong sun are sunburn, which affects only a few leaves and needs no treatment.

Pests and diseases

Before flowering, soybean tolerates losing about a third of its leaves, so a cover crop rarely needs spraying.

Cotton bollworm (Helicoverpa armigera). It and native budworm (Helicoverpa punctigera) look alike and are managed the same way. Their caterpillars reach 35 mm and vary from green to brown or black. They chew rounded holes in leaves and shoot tips, worst on drought-stressed plants. Pick them off by hand, and keep the crop watered.

Common grass blue (Zizina labradus). Green, slug-like larvae up to 10 mm long window the leaves and eat shoot tips, often in young crops. Hoverfly larvae look similar but eat aphids, so leave those. Soybean looper (Thysanoplusia orichalcea) caterpillars chew more angular holes; pick them off as for cotton bollworm.

Two-spotted mite (Tetranychus urticae). Tiny mites speckle the leaves pale and spin fine webbing underneath in hot, dry weather, often after broad sprays have killed their predators. Keep plants watered and unstressed. Read more

Green vegetable bug (Nezara viridula). Bright green shield bugs, 13 to 15 mm long, suck young pods and seeds, so a crop dug in at half flowering mostly escapes them. Hand-pick adults and clusters of nymphs early in the morning. Read more

Phytophthora root and stem rot (Phytophthora sojae). Plants wilt and die in patches on wet, heavy ground, with a sunken brown-red lesion climbing the stem from soil level. It survives in soil for many years and attacks only soybean, so improve drainage and rotate beds.

Sclerotinia stem rot (Sclerotinia sclerotiorum). White, cottony growth covers stems, followed by black lumps (sclerotia) like rat droppings, in humid weather and dense stands. Bin affected plants rather than digging them in.

Prevention first

• Clear weeds and old crop trash before sowing, since they shelter slugs and snails.
• Check the crop weekly for caterpillars, windowed leaves and speckling.
• Avoid broad-spectrum sprays, which kill the predators that keep caterpillars and mites in check.
• Do not sow thicker than the rate in Sowing direct, because crowded plants invite Sclerotinia.

Tips

1. Keep lawn weed sprays well away: soybean is highly sensitive to hormone herbicides such as 2,4-D and dicamba, and drift can damage it.
2. In a heatwave, sow in the cool of the early morning or evening, because hot soil kills the rhizobia on inoculated seed.
3. Treat soybean as the summer half of a rotation, as Queensland cane growers do between sugarcane crops. Dig it in by late summer, then plant a hungry winter crop.

Due to quarantine restrictions and our shipping policy, we cannot send Green Manure Soybean seeds to Western Australia or the Northern Territory.