A soybean field in West Carroll Parish with a poor growing area was investigated earlier this month. The visual observation showed mainly weeds growing in the suspect space surrounded by healthy soybean plants (Figure 1). There was prior knowledge of heavy equipment and supplies previously sitting in the weak area. A penetrometer (Figure 2) was used to compare the compaction in the two different growing areas. Approximately 175 psi was required to push through 2 inches of soil where healthy plants were growing, but between 250 and 300 psi was required in the poorly covered area.
With a high penetrometer reading and the knowledge of heavy equipment and supplies stored on wet soils, it was determined that soil compaction at 2 inches was causing the poor growth. Soil compaction can occur with heavy pressure on soils, especially in saturated conditions. Silt loam soils are typically prone to compaction; however, clay soils can also have compaction even though they crack open during droughty conditions. Compacted soils will typically have a reduction in pore space and higher bulk density. These conditions can impede plant root growth leading to a restriction of water, oxygen and nutrient availability. A reduction of nitrogen-fixing nodule formation can also occur in compacted soils. Ultimately, compacted soils can lead to a reduction in yield.
If visual observations of reduced crop growth such as nutrient deficiency or wilting occurs, a penetrometer should be used to determine if compaction is an issue. The penetrometer should be inserted into the top of the plant bed or row when the soil moisture is at field capacity (or when there would be efficient moisture for planting). As the penetrometer resistance reading increases, the root’s ability to penetrate the soil will decrease. The USDA has previously reported that roots typically will not penetrate soils with a penetrometer reading of 300 psi.
Field operations during saturated conditions should be limited to prevent soil compaction. If there is a compaction zone, deep vertical tillage is the most direct method to quickly alleviate the compaction layer. Over time, winter cover crops with deep or tap roots can be used to loosen a compacted soil. The cover crops can improve the soil structure and increase organic matter and may help prevent compaction layers from occurring.
Figure 1. A soybean field with an area of weak plants due to soil compaction.
Figure 2. A penetrometer is used to measure the severity of soil compaction.