Mechanics of Soil Compaction in Pastures

Soil compaction can be a big problem on any kind of farmland, whether it’s cropland or pasture. Soil compaction makes it harder for plant roots to penetrate the soil, reduces water infiltration and air exchange, limits the ability of plants to use some nutrients, and increases the chances of water erosion. Traffic of any kind, mechanical or from livestock, can cause compaction.

Illustration courtesy of UMN extension
Illustration courtesy of UMN extension

To understand compaction, we should start by thinking about soil structure. In healthy soils, the mineral soil particles (sand, silt, and clay) and organic matter are grouped together into small bodies or packets called aggregates. Each of these aggregates acts sort of like a little piggy bank where moisture, nutrients, and humus can be stored and protected. Around each of these aggregates, there is a void—a pore or channel. These channels are the pathways for water, air, plant roots, and soil organisms to move through the soil.

When soils are compacted, the first thing that happens is that the aggregates are pushed together, eliminating much of the pore space. As compaction continues, the aggregates can change shape. Surface soils should ideally have structure that is rounded (granular) or in fine rounded blocks. When these soils become compacted, this granular or blocky structure gets flattened out and changes into a type of structure called “platy.” Platy structure looks just like it sounds—the aggregates are wider than they are high and are arranged horizontally. This type of structure makes it hard for roots, air, water, and microbes move through the soil. With less pore space in compacted soil, it’s also harder for plant roots and soil organisms to move through the soil. If compaction continues, the aggregates can actually be destroyed.

DSC_0028-LWhen we think about soil compaction, most people probably think of big tractors and other heavy implements. Compaction from hoof traffic is also very common in pastures. This type of compaction is usually just in the upper several inches of the soil, and it’s very easy to identify. Take a shovel and dig in a few areas of the pasture to see what kind of structure there is. If you see granular or blocky structure (this is often described as looking like the interior of a loaf of homemade bread), all is well. If you see platy structure or no structure, you have a compaction problem.

You can sometimes help alleviate shallow compaction using an aerator. In more extreme situations, you might need to use more intense tillage like a rotavator or a ripper. On some soils, it’s hard to avoid compaction altogether, but that’s always the best goal. Freezing and thawing help to open the soil back up after it’s compacted, but that doesn’t restore structure. Only vigorous plant growth and the corresponding biological activity that follows it will restore damaged structure.

To help avoid compaction, keep mechanical and hoof traffic off of wet soils as much as possible. Having a vigorous pasture sward with lots of vegetative density and leaving enough residual vegetation at the end of each grazing event will help cushion the surface of the soil. Use short grazing periods and allow pastures to regrow to the proper stage before you re-graze a paddock. Applying a low rate of gypsum (200-300 lb/a) to pastures each year can help aggregate the soil and restore structure. Finally, monitor your soil fertility to keep plants growing as vigorously as possible. Good plant growth, earthworm activity, and other soil biological processes can restore good soil structure over time.

One thought on “Mechanics of Soil Compaction in Pastures

  1. Good article. I would like to see a story on compaction from over irrigation and how to better manage available water content.

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