By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio’s Soybean Farmers and their check-off.
When is the best time to plant soybeans? That is a very common question this time of year. The answer is quite simple…it depends. A lot of discussion and research is being conducted into which crop to plant first between corn and soybeans, but the question still remains, “when?”. Ultimately, the goal of planting early is to reach canopy closure earlier and maximize the amount of sunlight that can be captured by the plants throughout the season.
When planting soybeans, the goal is to put them in a position to maximize their genetic potential. In the past, the rule of thumb was to plant soybeans when the soil temperature was 50 degrees Fahrenheit at a depth of 1 – 1 ½ inches and into moisture. If the soybean is planted into dry soil, then nothing will happen until it receives adequate moisture. According to work done at the University of Wisconsin-Madison, a soybean will take in the water (imbibe)and germinate when the seed moisture content is at 20%. Once the seed coat splits, the radical will continue to grow if the seed remains above 20% moisture. If the seed imbibes water, but then dries down and the seed coat does not split and the radical has not emerged, then germination will be affected to varying degrees depending on how long the seed had initially imbibed water. If the radicle has emerged and the seed moisture drops below 10% then the plant will likely not survive.
Soil temperature and moisture depend greatly on soil type, tillage practices, and drainage. A dry sandy soil that is tilled will warm up quickly on a sunny spring day compared to a heavier wet clay soil. The dry sandy soil will also cool quickly in the evening. A moist soil has a higher heat carrying capacity. It takes longer to warm a heavier soil type; however, the heavier soil also retains the heat longer. A heavier clay soil will retain moisture longer than a sandy soil. A No-till or reduced tillage field in the spring, or a field with cover crops may be cooler and retain moisture longer depending on the growth stage of a cover crop. Soil temperatures are greatly impacted by surface cover and shading the sunlight.
The weather forecast after the soybeans are planted should be considered. If there is adequate soil moisture and warm temperatures in the forecast, then early planting of soybeans should be successful. If the forecast is for cold rains after planting and the seed imbibes cold water, the germination can be affected negatively. According to Iowa State University, imbibitional chilling occurs when there is a drastic change in the water temperature during imbibition. This occurs when soybean seed imbibes very cold water, under 45 degrees in the first 6-24 hours after planting. A cold rain or snow could cause this to occur.
Soybean seeding rates do not generally play a role in the decision of when to plant. A higher seeding rate is recommended for later planting dates. For a crop planted before May 20th in narrow rows, a final population of 100,000 to 120,000 plants per acre are generally adequate for maximum yield. Final harvest stands are typically 60-80% of the seeding rate. Soybeans may not emerge uniformly, and if some earlier emerging beans have issues with frost or freeze, the later emerging seeds can fill in. Soybeans will naturally compensate for thin stands. Stands can be thin due to poor germination or emergence challenges, frost or freeze damage after emergence, disease and insect pressure, and slug feeding.
In general Dr. Laura Lindsey, OSU Extension Soybean Specialist, still recommends planting soybeans early, as long as the ground is fit, and soil temperatures are conducive. “Early planting is still important. We learned in our trials when the stand was reduced by a frost and looked really bad, if the population is 50,000 plants per acre or more, then it was best to leave it. Just be sure you have adequate weed control.”