May 22, 2009

Pivots are running in some Nebraska soybean fields to combat soil crusting. For those who planted when the soil surface was a bit wet, the side-oriented press wheels may have closed the wet side walls to only a small crack. If these side walls are now hard as a brick (due to high temperature, wind-aided drying), seedlings may suffer. As soybean seedlings emerge and grow, they have sufficient pull force that cotyledons too big to pass through the crack will be left behind.

When this happens, a shoot without the cotyledons is left to serve as a carbon source during the time it takes for the seedling to acquire leaves. Ordinarily, seedlings depend on their cotyledon reserves to support seedling growth until they get sufficient leaf area to become photosynthetically competent (i.e., acquire unifoliolate leaflets and first trifoliolate leaflets).

The old solution to soil crusting was to drive a heavily weighted rotary hoe over the field in high gear to break up the crust. For those with irrigation, now the solution is to turn on the center pivot. However, with hot temperatures and high-speed, windy drying, the soil surface frequently dries back to its same hardness as soon as the pivot arm passes. Running the center pivot at night when temperatures are cooler and the soil will stay wetter longer can help the seedlings muscle their way through a wetted soil crust without losing their cotyledons.

Assessing Problem Stands

It's important to determine your population once soybeans have emerged so you can decide whether any replanting is necessary. Nebraska research has shown that a stand of 90,000 plants/acre will not adversely affect yield (see Look at Soybean Planting Populations, April 24, 2009 CropWatch). However, those low remaining plant densities must be uniformly spaced. (For example, having 50,000 plants/acre here and 130,000 there, which on average is 90,000, does not offer the same overall yield result as a uniform stand of 90,000 plants.)

Jim Specht
Professor, Department of Agronomy and Horticulture