Soybean Care Through the Season: Water, Support and Why You Do Not Feed

Feeding: the mistake that costs you the crop

Soybeans form nodules on their roots that host bacteria converting nitrogen from the air into a form the plant can use. A healthy plant in reasonable soil supplies its own nitrogen from a few weeks after emergence onwards and needs none from you. Give it nitrogen anyway and you get exactly what you would expect: a tall, dark green, leafy plant that flowers late, sets fewer pods and, on a dry-bean crop, may not finish ripening before the weather turns.

This is not a small effect. A bed dressed with general fertilizer or fresh manure in spring grows soybeans that look magnificent all summer and give a disappointing weight of beans in autumn. Improve the bed with compost the previous autumn instead, so the nitrogen has largely gone by sowing time.

Phosphorus and potassium are a different matter, and where a soil test shows either is low, a dressing of wood ash or a low-nitrogen fertilizer before sowing is worth doing. Potassium goes directly into filling the beans, and a potassium-short crop makes small, shriveled seed.

Dig up one plant a few weeks after emergence and look at the roots. You should find lumps the size of a pinhead to a small pea, and cutting one open should show pink or red inside. Pink means the nodule is fixing nitrogen. White or green inside means it is not, and that is the one situation where a light nitrogen feed is justified.

Nodules only form if the right bacteria are present in the soil. On a bed that has never grown soybeans, a seed inoculant applied at sowing is the difference between a crop that feeds itself and one that goes pale and stalls at knee height.

Water matters at two moments and barely at all in between

A soybean sends its taproot down two to three feet (60 to 90 cm) and copes with dry spells better than most vegetables. For the first six or seven weeks, from emergence until the flower buds show, it needs very little from you. Watering a young block heavily keeps the roots shallow and produces a plant that then collapses in the first real drought.

From the moment the first flowers open until the pods are full, that reverses completely. This is when the plant decides how many pods it will keep and how big the beans in them will be, and a dry fortnight in this window drops flowers and leaves you with flat, half-filled pods that are useless as dry beans. Aim for the equivalent of an inch (25 mm) of water a week over this period, delivered as one or two thorough soakings rather than a daily sprinkle.

Then, once the pods have filled and the leaves start to yellow, stop. A dry crop needs to dry, and late watering keeps the plants green, delays the whole ripening process and pushes the harvest into worse weather. Watering a soybean block in autumn is actively harmful.

Wilting in the afternoon heat is normal and not a signal to water. A soybean that is still wilted first thing in the morning is genuinely short of water, and that is the only reliable test.

Keeping a loaded block standing

A soybean carrying a full crop of filling pods is heavy and top-heavy, and a summer storm will lay a whole block flat. Plants that go down stay down, their lowest pods end up on wet soil, and those pods pick up mold that then spreads through the batch when you thresh. Preventing it is far easier than dealing with it.

The first defense is spacing: a tight block of plants leans on itself and stays up where widely spaced individual plants topple. The second is earthing up. When the plants are eight to ten inches (20 to 25 cm) tall, draw soil up around the base of the stems to two or three inches (5 to 8 cm). It anchors them, and soybeans respond by rooting into the drawn-up soil, which strengthens the anchorage further.

For a small block in a windy garden, run string down each side of the row at knee height, tied to canes at the ends, with a second line higher up as the plants grow. If a block does go over after a storm, lift the plants upright the same day while the stems are still flexible and firm soil around the bases; left a week they set into their new position and the lowest pods are already spoiled.

1. Earth up at eight to ten inches (20 to 25 cm) tall Draw soil from between the rows up to the stems with a hoe. Do it when the soil is slightly damp so it holds its shape rather than running back.
2. Run string lines before you need them Two canes at each end of the row and a line down each side at knee height. Add a higher line as the plants grow past it.
3. Do not stake individually Individual stakes damage the taproot going in and take an hour per row. A tight block plus string does the same job better.
4. Lift a flattened block the same day Stand the plants back up and firm soil at the base. Waiting a week means the lowest pods have already been lying on wet ground.

Weeds, mulch and when to stop touching the crop

The first six weeks are when weeds do the damage, because the soybeans are small and their roots are shallow and easily disturbed. Hoe shallowly between the rows and hand-pull anything close to the stems. Deep hoeing near a soybean cuts surface roots and sets the plant back further than the weed would have.

Once the block closes over, stop. The canopy suppresses weeds on its own, and walking through a flowering block knocks flowers off and spreads disease on your clothes. From flowering to harvest, leave it alone apart from watering.

A straw mulch between the rows, laid once the soil is thoroughly warm, holds moisture through flowering and keeps the lowest pods off bare soil. Do not mulch early, when you want the soil to warm, and do not use anything rich in nitrogen.

1. Hoe shallow, and only between rows Skim the top inch (2.5 cm). Anything deeper than that near the plants cuts feeding roots that the crop needs at flowering.
2. Mulch once the soil is warm Straw two inches (5 cm) deep between the rows after midsummer. It holds the flowering-period moisture and keeps low pods clean.
3. Stay out of the block from flowering Never work among wet plants. Fungal spores move on damp foliage and on your sleeves, and soybean leaf diseases spread exactly this way.

Flower drop, heat and keeping a useful record

Soybeans set far more flowers than they can carry and routinely shed half of them. Finding small flowers on the ground under a healthy block is normal and not a sign of anything wrong. What is not normal is losing nearly all of them, which happens when temperatures sit above 95F (35C) for several days during flowering, or when the block runs dry in that same window.

You cannot do much about the heat, but you can make sure the plants are not also short of water when it arrives. A well-watered block coming into a heatwave sheds noticeably fewer flowers than a dry one, so soak the bed thoroughly the day before one is forecast.

Note the date the first flowers opened, the date the pods were visibly filled, and any stretch of extreme heat in between. Over two or three seasons those notes tell you whether your usual sowing date puts flowering into your hottest fortnight, which is a problem you fix at sowing time and nowhere else.