By Sjoerd Willem Duiker
As average farm size increases, farm equipment typically becomes bigger and heavier, posing an increasing threat of soil compaction. Compaction is measured as a loss of porosity and an increase of resistance to root penetration. Soil compaction is called the 'silent' thief because its effects are often going unnoticed. However, compaction effects can be long-lasting, especially if the subsoil is compacted.
In an international compaction study in the northern hemisphere, soil was compacted four times with 10-ton axle loads with tires inflated to 40 psi, after which the soil was tilled and farmed with normal cropping practices. The average yield reduction was 15% in the first year, but after 10 years, the yields were still reduced by 3% compared with plots that were not compacted. The latter was assumed to be due to the effects of subsoil compaction. The study showed that the effects of surface compaction are larger, but they can be corrected more easily because of tillage, freeze-thaw and wetting-drying cycles, root proliferation, and greater biological activity. Subsoil effects are smaller but last longer.
Tillage can also cause compaction. If tillage tools are run year after year at the same depth, it is possible to cause a plow pan that has low porosity and high penetration resistance. While tillage tools like the moldboard plow are most renowned for causing plow pans, reduced tillage tools such as chisel plows, disks or coulters (e.g., vertical tillage) can also produce them if used yearly and run at the same depth. No-till soils tend to support the weight of equipment better because the matrix of the soil is firm, while the soil has high macroporosity because of biological activity. Nonetheless, compaction effects can be significant because there is no alleviation besides natural factors. In a Penn State study in long-term no-till, soil was compacted once in the spring with 10-ton axle load trucks with 90-100 psi tire pressure. Corn yields were reduced by 27% in a year with a dry summer and 14% in a year with more moisture in the summer, showing that yields will be reduced more in dry years due to restricted root growth. If wider tires were used on the trucks, inflated to 35 psi, the yield reduction was almost cut in half, showing the importance of increased footprint and reduced contact pressure to reduce compaction.
We discussed the effects of compaction on root growth and yields, but compaction also affects other aspects of production. For example, earthworm numbers were reduced by 95% in an Australian study where soil was annually compacted when wet with 10-ton axle loads. Earthworm tunnel length was also reduced significantly because the earthworms had to spend more energy to dig through the compacted soil. Soil compaction also reduces water infiltration and percolation. After the soil is wet, water doesn’t drain out quickly, and the soil becomes anaerobic, increasing nitrogen losses through denitrification. Therefore, we often see yellow crops in compacted areas of the field. Because of the reduction of infiltration, liquid manure sits at the surface longer, increasing the losses of ammonia and reducing manure's nitrogen value. On the other hand, phosphorus and potassium uptake can also be reduced because of the restricted root system in compacted soil. Thinking about it, we realize we should take compaction seriously.
If you would like to learn more, Penn State is organizing four soil compaction workshops this month – in Centre County (March 19), Potter County (March 21), Lancaster County (March 25), and Lebanon County (March 26).
Source : psu.edu