Black Soybean Care: Pinching, Support and Watering a Big Late Crop

Feed it nothing, and mean it

Like every soybean, this crop forms root nodules that host bacteria converting nitrogen from the air, and from a few weeks after emergence it supplies its own. Adding nitrogen makes a vigorous plant more vigorous, and on a type that is already chest high and already flowering late, that is a genuinely destructive thing to do. A fed black soybean makes an enormous dark green frame, flowers thinly at the top, lodges in the first storm and gives a fraction of the beans it should.

The effect is stronger here than on the standard crop precisely because the plant is bigger and the season longer. A bed dressed with general fertilizer or fresh manure in spring will look like the best thing in the garden all summer and be a disappointment in autumn. Improve the bed with well-rotted compost the previous autumn, so the nitrogen flush has passed before you sow.

Potassium is a different matter and it goes straight into filling large seed. Where a soil test shows potassium is low, a dressing of wood ash or a low-nitrogen feed before sowing is worth doing, and it shows up as plump beans rather than shriveled ones. Keep the pH between 6.0 and 7.0; below 6.0 nodulation falls away and liming takes months to act, so it belongs to the previous autumn.

A few weeks after emergence, dig up one plant and cut a nodule open. Pink or red inside means it is fixing nitrogen and you should leave the crop alone. White or green inside, on a pale, stalled plant, is the single case where a light nitrogen feed is justified, and it tells you to use a seed inoculant next season.

Pinching the tip: the one job that is specific to this crop

Because black soybeans put on a long run of stem before they flower, they respond extremely well to having the growing tip taken out early. Pinch out the top of the main stem when the plant has four or five true leaves, roughly knee high. The plant stops climbing, pushes out side branches from the leaf joints below, and ends up shorter, stiffer and carrying its pods on more branches rather than strung up one tall stem.

The gain is real on three counts. A pinched plant is far less likely to be flattened by an autumn storm, which on a crop that fills its pods this late is the difference between a harvest and a mess on the soil. It sets more pods, because pods form at leaf joints and branching multiplies the joints. And it carries those pods lower and more evenly, where you can reach them.

Timing matters. Pinch too early, before four true leaves, and you check a plant that has not built enough root to recover quickly. Pinch late, after flower buds are visible, and you are removing flowering wood for no benefit. The window is broad in the middle of the season but it closes once buds show.

This is not a substitute for correct spacing or for withholding nitrogen. A pinched plant that has been fed still grows into a thicket, just a wider one. Do all three or the pinching does not show.

1. Wait for four or five true leaves Roughly knee high, well established, growing strongly. A plant that is still recovering from thinning or a slug attack should be left to catch up first.
2. Take out the growing tip with your fingers Pinch just above a leaf joint, removing the soft tip and the smallest unfurled leaves. No tools are needed and a clean pinch heals within a day.
3. Do the whole block on the same day Uneven pinching gives you a block that flowers unevenly, which matters at harvest when you want to cut everything at once.
4. Do not pinch once buds are visible After buds show you are cutting off flowering wood. If you missed the window, leave the plants alone and rely on support instead.

Pinching typically delays the start of flowering by only a few days while adding several branches. On a crop whose flowering date is set mostly by day length rather than by growth stage, that trade is heavily in your favor.

Holding up a heavy plant in autumn weather

A black soybean carrying a full load of large pods in early autumn is one of the heaviest things in a vegetable garden, and it is filling those pods exactly when the wind and rain arrive. Plants that go down stay down, their lowest pods sit on wet soil and pick up mold, and moldy pods contaminate the whole batch at threshing. This is the failure to design against.

Earth up first. When plants are eight to ten inches (20 to 25 cm) tall, draw soil up around the stems to three inches (8 cm). The stems root into the drawn-up soil and the anchorage improves through the season. Do it when the soil is slightly damp so it holds its shape.

Then string the rows, and string them higher than you would for an ordinary soybean. Canes at each end of the row with a line down both sides at knee height, a second pair at hip height as the plants pass it, and on an exposed site a third. The plants lean on the strings rather than on each other, which is what lets you keep the wide spacing that the canopy needs.

If a block does go over, stand it up the same day while the stems are still flexible, and firm soil around the bases. Left for a week the plants set into their new shape, the lowest pods have already been lying wet, and you will be discarding them at sorting.

1. Earth up at eight to ten inches (20 to 25 cm) Draw soil from between the rows against the stems with a hoe. The plant roots into it and the anchorage keeps improving as the crop gets heavier.
2. Put the string lines up before you need them Knee height first, hip height as the plants grow past it. Fitting support to a loaded, leaning block in a gale does not work.
3. Skip individual stakes One stake per plant damages the taproot going in and takes an hour per row. Earthing up plus double string lines does the job better.
4. Lift a flattened block the same day Stand the plants back upright and firm soil at the base. Every day they lie down is a day the lowest pods spend on wet ground.

Water: almost none early, a great deal at pod fill

For the first two months this crop wants very little from you. The taproot goes down two to three feet (60 to 90 cm) and chasing water downwards is exactly what you want it doing. Watering a young block heavily keeps the roots shallow, feeds the leafy growth you are trying to restrain and leaves the plant helpless in a dry spell later.

That reverses completely from the moment flower buds show. Black soybeans fill large seed, and filling large seed takes water. Give the equivalent of an inch and a quarter (30 mm) a week from first flower until the pods are plump, delivered as one or two deep soakings rather than a daily sprinkle. A dry fortnight in this window sheds flowers and leaves you with flat pods that are useless dry and unpleasant green.

Because this crop flowers late, that demanding window falls in early autumn, when rainfall is more likely but so are cool, wet, windless spells that suit disease. Water the soil, not the leaves, and water in the morning so anything splashed dries before evening.

Once the pods are full and the leaves start yellowing, stop entirely. A crop being taken to dry beans needs to dry, and late watering keeps it green and pushes the harvest deeper into bad weather. If you are picking green instead, keep watering right up to the day you pick.

Afternoon wilting on a hot day is normal and not a signal. A plant still wilted first thing in the morning is genuinely short of water, and that is the only test worth acting on.

Weeds, airflow and a record worth keeping

Weed in the first six weeks, hoeing shallowly between rows and hand-pulling close to the stems. A black soybean is slower to cover its ground than a tightly sown standard crop, so the weed-free period has to be actively maintained for longer. Skim the top inch (2.5 cm) only; deeper hoeing cuts the surface roots the crop needs at flowering.

Once the canopy closes, stay out of the block except to water and to open pods for inspection. Walking through a flowering, damp block knocks flowers off and moves fungal spores on your sleeves. Never work among wet foliage.

A straw mulch two inches (5 cm) deep between the rows, laid once the soil is thoroughly warm, holds moisture through the late pod fill and keeps the lowest pods off bare soil. Lay it late, not early: you want the ground warm first, and you want nothing rich in nitrogen anywhere near this crop.

Record the date you pinched, the date the first flowers opened, and the height of the plants at flowering. Those three lines, kept for two seasons, are what tell you whether your sowing date and your pinching are actually controlling the size of this crop, or whether you are still growing a hedge and hoping.

1. Hoe shallow, between the rows only The top inch (2.5 cm). Weeding matters for longer here than on a tight block because the canopy takes longer to close.
2. Mulch after the soil is warm Straw two inches (5 cm) deep between rows. It carries the crop through a late pod fill and keeps low pods clean.
3. Never handle wet plants Leaf diseases move on damp foliage and on your clothing, and a late-flowering crop spends its critical weeks in damper weather.
4. Write down pinch date, first flower and height Three dates a season. Together they show whether the plant is being kept to a workable size or is running away into leaf.