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Soil structure and yield

April 17, 2009
From Wayne Schoper Brown/Nicollet Extension Educator

By the time you read this field work will have started, weather permitting, and the 2009 planting season will be under way. We are going into this growing season a little short on moisture. However, this can change in a big hurry as evidenced by last years rain delays that saw a lot of corn planted well into the month of May. We did receive some moisture last fall and we did have some good snowfall over the winter. It looks like we will have some good planting moisture and then it is up to Mother Nature to provide the rest.

Crop yields have improved markedly over the past several years. There is no doubt that improved crop genetics had a lot to do with this. Brad Carlson is an extension educator out of Rice and Steele Counties. He recently wrote an article that talks about these advances in crop genetics that really brought about an improvement in better root systems. Bigger healthier root systems allow the plant to better obtain soil water, making the plants more drought resistant. In addition, better root systems allow the plant to more efficiently extract crop nutrients from the soil. This is especially true of nutrients mineralized from soil organic matter later in the cropping season. This late season "spoon feeding" of nitrogen and other nutrients from soil organic matter can be assigned responsibility form many crop yields that go from looking good in the summer to turning out great by harvest time.

A plant with the potential for excellent roots only goes so far, as the soil must be in condition to allow for the root growth. This means that the soil should have what is colloquially referred to as "tilth" Basic soil morphology discusses the concept of "soil structure", which refers to the size of ped that would beak naturally from an exposed soil profile. The smaller the soil peds (like crumbs), the looser the soil, and the easier for plant roots to penetrate and find water and nutrition. Soils that lack structure altogether are referred to as having "massive" soil structure. More or less this means that there is no structure, and the soil is one large lump. Under ideal conditions plants can grow in soils with massive structure, but when it is too wet or too dry it becomes very difficult for roots to penetrate.

Good soil structure is achieved by soil biological activity and soil organic matter. The activity of bacteria, fungi, molds, algae, and roots from higher order plants forms natural channels in the soil that make up structure. The activity of arthropods, earthworms, and burrowing mammals also contributes. This activity is encouraged through the balanced presence of oxygen, heat and moisture.

Once a soil gets to the state where it would be classified as having massive structure it can be very difficult to remediate. Soil in this state is prone to drought and saturation and wide temperature swings. When the soil becomes wet, the oxygen leaves and root growth (along with other biological activity) ceases. These soils are often very cold early in the growing season, and can take a long time to become conducive to planting. When the soil dries, particles stick together tightly and the soil becomes brick-like and difficult to penetrate. Soil in this state can get very hot in the middle of summer if exposed to the sun, and prevent plant growth.

There are ways in which gradual improvement can be made in these soils, but the best strategy is to avoid them altogether. Water lubricates clay particles so that they can be compacted together. It is for this reason that farmers need to avoid traffic on soils that are too wet. Tillage is also to blame, as working the soils when they are too wet can cause identical conditions, with the compaction coming from the side instead of the top. The best strategy is to stay out of the field until conditions are dry enough to allow good tillage practices.



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