The Biology of Soil Compaction

Soil Compaction Soil compaction is a common and constant problem on most farms that till the soil. Heavy farm machinery can create persistent subsoil compaction (Hakansson and Reeder, 1994). Johnson et al. (1986) found that compacted soils resulted in: (a) restricted root growth; (b) poor root zone aeration; and (c) poor drainage that results in less soil aeration, less oxygen in the root zone, and more losses of nitrogen from denitrification. Subsoil tillage has been used to alleviate compaction problems. Subsoilers are typically operated at depths of 12 to 18 inches to loosen the soil, alleviate compaction, and increase water infiltration and aeration. Subsoiling usually increases crop yields but the effects may only be temporary as the soil re-compacts due to equipment traffic. Some no-till fields never need to be subsoiled, but in other no-till fields deep tillage has increased yields especially if equipment traffic is random. When subsoiling removes a hard pan, traffic must be controlled or compaction will reoccur. If a hard pan does not exist, equipment traffic generally will create one (Reeder and Westermann, 2006). If the soil is subsoiled when the soil is wet, additional compaction may occur. In a loamy sand, Busscher et al. (2002) found that soil compaction increased with time, and cumulative rainfall accounted for 70 to 90 percent of the re-compaction due to water filtering through the soil and the force of gravity. The fuel, labor, equipment, and time to subsoil makes it an expensive operation. Subsoiling in dry conditions requires even more fuel (Reeder and Westermann, 2006). Two other factors that impact soil compaction are rainfall impact and gravity. In soils that have been tilled, both the velocity of the raindrop impact on bare soil and natural gravity combine to compact soils. Low organic matter levels make the soil more susceptible to soil compaction. Organic residues on the soil surface have been shown to cushion the effects of soil compaction. Surface organic residues have the ability to be compressed but they also retain their shape and structure once the traffic has passed. Like a sponge, the organic matter is compressed and then springs back to its normal shape. However, excessive traffic will break up organic residues, and tillage accelerates the decomposition of organic matter. Organic residues in the soil profile may be even more important than surface organic residues. Organic matter (plant debris and residues) attached to soil particles (especially clay particles) keeps soil particles from compacting. Organic matter binds microaggregates and macroaggregates in the soil. Low organic matter levels make the soil more susceptible to soil compaction (Wortman and Jasa, 2003). In the last hundred years, tillage has decreased soil organic levels by 60%, which means that approximately 40% soil organic carbon stocks are remaining (International Panel on Climate Change, 1996, Lal, 2004). Carbon provides energy for soil microbes, is a storehouse for nutrients, and keeps nutrients recycling within the soil. Humus or old carbon (>1,000 years old) is the most stable carbon and binds micro soil particles together to form microaggregates. Humus is non-water soluble so it stabilizes microaggregates and is not readily consumed by microorganisms. Humus is more resistant to tillage and degradation than active carbon. The Biology of Soil Compaction