This year, there has already been a significant percentage of corn acres planted prior to May 1st. Thus, many growers are faced with trying to decide when to begin their soybean planting. Soil conditions are of primary importance when considering "early planting," while planting by the calendar date and the effect on final yield should be taken into account when planting later in May.

Soil Conditions and Soil Temperature

Soil conditions at and after planting are the primary drivers in stand establishment. Soybean has "delicate" seed, and benefits when planted about 1-1½ inches deep, modestly firmed into the seed furrow, well-covered, and into soils with temperatures at 60° to 70°F. While it is unrealistic to wait until soil temperatures average 70°F, it is important to remember that soil temperatures of 50°F and below can be very deleterious to soybean stand establishment. Soil temperatures at the 2 inch depth as of April 23rd averaged 56 degrees at Morris, 63 at Lamberton and 61 at Waseca U of MN outreach centers. These soil temperatures cycle up and down as a reflection of the air temperature as a result local soil temperatures are still fluctuating.

Remember that soybean seedlings need to pull their cotyledons through and out of the soil and that this feat is helped a great deal by good surface soil conditions. Of course, several inches of rain immediately after planting can quickly turn surface conditions from good to poor, especially in some soils. The lack of oxygen in saturated soils and the formation of a soil crust of even modest strength can almost eliminate soybean emergence. Therefore it is important to know the 5 day weather forecast prior to planting. Early planted soybeans (last week of April and the first week of May) in cool and wet conditions may lead to poor germination and seedling diseases such as pythium. These problems are magnified by extended cold/rainy periods after planting.

Due to these factors, it's probably appropriate for producers to increase soybean seeding rates slightly when planting very early. The University of Minnesota seeding guidelines are determined by soybean maturity group being planted (independent of latitude or row space). For example, Group II maturities should be planted at no more than 170,000 live seeds per acre, Groups I's at 180,000 live seeds, and Group 0's at 190,000 live seeds per acre. Growers need to keep in mind that seedling rates do not establish yield potential, but plant stands do. So, take stand counts in the spring in every field, so that seeding rates may be adjusted in future years.

The use of soybean fungicide seed treatments may be more easily justified if soybeans are seeded at low populations early in the growing season when plant stands are at the greatest risk of being deficient. Seed rots, damping off and seedling blights of soybeans are generally worse under wet conditions. Poorly drained and compacted soils are especially prone to these diseases. Economic returns from fungicide seed treatments most often occur when soybeans are planted under cool, wet conditions, or when these conditions are present directly after planting. Very early planting, wet or poorly drained soils and under minimal tillage are cases where a seed treatment is most likely to pay for itself.

Planting Date and Soybean Yield

Since early-May plantings usually result in Maximum yields, lower yields should be expected for later plantings. The following table lists expected yield reductions for various later plantings.

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