How to Sow Soybeans for Dry Beans: Timing, Depth and Spacing

A dry-bean crop is a long crop, so sow on soil temperature

Soybeans harvested as dry beans are the same plant you would pick green, taken all the way to the end instead. That extra distance is the entire difference, and it costs you roughly a month: a green picking comes at 75 to 100 days, while a crop taken to full dry maturity needs 100 to 140 days from sowing plus a run of dry weather at the end. Everything about sowing follows from needing that whole stretch.

The seed still will not tolerate cold ground. It swells to its own weight in water within hours of going in, and if the soil is below 55F (13C) it sits there swollen and rots before a root appears. Wait until soil at sowing depth has held above 60F (15C) for several days and the last frost is behind you. Emergence then takes six to nine days rather than never.

The two requirements pull against each other, and you resolve it by counting backwards rather than by sowing into cold soil. Work out when your first autumn frosts usually arrive and subtract 130 days. If that falls before your soil reaches 60F (15C), sow for green picking instead, or start the row under cloches to lift the soil two weeks early.

Push a soil thermometer in to sowing depth mid-morning, not at midday, and read it on three consecutive days. A single warm afternoon reading has fooled a lot of people into sowing a fortnight early.

Depth, stations and how many seeds

Sow one inch (2.5 cm) deep in ordinary soil and an inch and a half (4 cm) in light sand where the surface dries fast. Deeper than two inches (5 cm) and the seedling exhausts itself pushing its seed leaves up through the crust, which matters more with soybeans than with most crops because the whole seed comes up above ground on its stem rather than staying below.

Sow direct. Soybeans put down a taproot within days and take badly to being moved, so module-raised plants only make sense where the season genuinely will not stretch, and then they must be planted out at the first true leaf. A checked transplant sulks for a fortnight, and on a crop that already needs every warm week you have, a fortnight lost is the crop.

Put two seeds at each station and thin to the stronger one once both have their first true leaf. Soybean seed germinates unevenly and loses viability faster in storage than most legumes, so sowing to a single seed per station leaves gaps you cannot fill later in the season.

1. Do not soak the seed first Soaked soybean seed splits its coat and the seedling comes up damaged or not at all. It takes up all the water it needs from moist soil within a few hours of sowing.
2. Water the drill, then sow into it Run water along the open drill, let it drain, sow, then cover with dry soil. This gets moisture to the seed without capping the surface into a crust.
3. Two seeds per station, an inch down Firm the soil over each station with the flat of your hand so the seed sits in contact with damp soil rather than in an air pocket.
4. Thin at the first true leaf Snip the weaker seedling at ground level rather than pulling it, which would disturb the taproot of the one you are keeping.

Spacing: closer than you would think

A dry-bean crop is planted for total pod number across the block, not for the size of any one plant, so it goes in closer than a green crop. Four to six inches (10 to 15 cm) between plants in rows 18 to 24 inches (45 to 60 cm) apart gives a stand that closes over, shades out weeds, and holds itself upright when the pods fill and the plants get top-heavy.

Closer spacing also produces a plant that branches less and carries its pods higher up the stem, which matters at the other end of the season when you are pulling plants and want the lowest pods clear of the soil. Pods lying on wet ground pick up mold, and moldy pods contaminate the whole batch when you thresh.

Do not go wider to get bigger plants. A widely spaced soybean makes a sprawling, leafy bush, sets its pods low, flops in the first storm and yields less dry bean per square foot than a tight block.

Birds, and the first fortnight

The seed leaves of a soybean come up above the soil holding the two fat halves of the bean, and birds recognize them. Pigeons and jays will walk a row and take every seedling in a morning, and mice will take the sown seed before it ever emerges. This is the single commonest reason a soybean sowing fails, and it is entirely preventable.

Cover the row from the day you sow, not from the day you see damage. Fine mesh or row cover pegged down along both edges is enough, and it comes off once the plants have three or four true leaves and the seed leaves have yellowed and dropped. After that point birds lose interest in the crop until the pods fill.

Slugs take the same seedlings at night in a wet spring. If you find stems chewed through at soil level rather than the leaves stripped, that is slugs and not birds, and the cover will not help. Go out after dark with a torch for three nights running.

1. Peg cover down the day you sow Leave enough slack for the seedlings to lift the row cover as they grow. A cover pulled tight gets pushed off within a week and you will not notice until the row is gone.
2. Check under the cover every few days Look for emergence and for anything that got in underneath. It is much easier to fix a gap in the first week than to re-sow in the third.
3. Uncover at three true leaves By then the plant is no longer worth a bird's effort, and leaving the cover on into flowering restricts the airflow the crop needs.

If you have to re-sow a gap, sow it within ten days. A patch sown three weeks behind the block will still be green when you pull the rest for drying, and you will end up abandoning it.

One sowing, and what to write down

Unlike a green crop, which is worth staggering across a fortnight so the eating is spread out, a dry crop is sown once. Everything ripens together on purpose, because you want to pull the whole block on a single dry day. A second sowing two weeks later simply gives you a block that is not ready when the weather is.

Write down the sowing date, the soil temperature you sowed at, and where in the garden the block went. Soybeans fix their own nitrogen through root nodules and leave the ground in better condition than they found it, so knowing the exact position of this year's block is what lets you put a hungry crop there next year and move the beans on.

Then record the date the first flowers opened and the date you cut the plants. Those two against the sowing date tell you how many days your particular garden takes, which is what makes next year's counting-backwards honest rather than a guess.