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Accounting for Soil Wetness Prior to Conducting Farm Operations to Minimize Compaction

Accounting for Soil Wetness Prior to Conducting Farm Operations to Minimize Compaction

In the spring many agricultural producers are anxious to get into the field and perform tillage, planting, and chemical applications. However, if field operations are done when the soil is too wet, this can lead to soil compaction. Soil compaction can occur from agricultural equipment, animals, and raindrop impact. Although tillage can be used as a management practice, it also creates a compacted area below the tillage implement.

Soil compaction from agricultural equipment can be long lasting in the soil and lead to reduced yields, crop emergence, root growth, and water and nutrient uptake of plants (Shaheb et al., 2021). Soil compaction also can reduce infiltration, soil water storage, and biological activity, and also increase erosion and destroy soil aggregates (Shaheb et al.,2021). One review says compaction from agricultural equipment can lead to a 10 to 20% reduction in crop yields (Sonderegger & Pfister 2021). Force placed on the soil is dependent upon the weight of the equipment and the area the force is placed over. The larger area a load is applied over reduces the force from the load applied on the soil surface. Figure 1 is an example of what a tractor tire and planter does to a tilled soil. Bulk density is the mass of dry soil in a volume of soil. More information on bulk density can be found in the article, Bulk Density is an Indicator of Soil Health.

Soil Compaction Factors

Many people think that soil is more susceptible to compaction when it is wet. However, this is not actually the case. When soil is dry there is friction between soil particles, which will limit how much compaction occurs when a load is driven on top of the soil (Rahmat and Ismail 2018). Air is easily released from the soil when it is dry, but there are strong bonding forces between soil particles, which limits how much soil can become compacted. As the soil becomes wetter, there is increased lubrication between soil particles, and the soil will compact to a denser level (Rahmat and Ismail 2018). Maximum soil compaction will occur (highest bulk density) when the soil is about 80% of saturated water content (Hillel 1998).

Air will easily leave the soil when it is at an intermediate water content and water reduces the friction between soil particles. When the soil is wetter, water gets in the way, and air and water are not easily pressed out of the soil. This leads to the soil being compressed to a bulk density that is not as high as when it is at intermediate water contents. Engineers are interested in packing the soil to high bulk density values when constructing roadbeds and other building sites (Rahmat and Ismail 2018). Engineers use a test called the Proctor compaction test to determine how compacted a soil can become at different water contents. The water content where the soil becomes the most compacted is the Proctor optimum water content. The highest bulk density that the soil can become compacted to is called the Proctor maximum bulk density. An example of the Proctor compaction test on two soils is shown in Figure 2.

The soil with less organic carbon has a lower water content where it is most compactable and packed to a higher bulk density. Organic carbon strengthens the soil particles’ attraction to each other and reduces how much the soil can become compacted. Soils that are sandy generally have a lower water content and they will become the most compacted. Soils with a higher clay concentration usually have a higher water content where they will become the most compacted. This has to do with the effects of water content on how strong soil particles of different sizes are held together. Well-graded and poorly sorted coarser soils (sandier) will pack to a higher bulk density than soils that have a lot of the same particle sizes and/or are finer textured. Spherical sand particles will not pack to a high bulk density due to cohesion forces holding the particles close to each other.

Soil particles with a variety of shapes and sizes will better fill voids between the different particle sizes when compacted. The stronger attraction of clay particles to each other and the absorption of water into small pores generally limits how much clayey soils can become compacted. The water content where the soil is most susceptible to compaction is usually when the soil is a little drier than field capacity, or where soil changes from a plastic to semisolid state.

Once the soil becomes wetter than the optimum water content, it will not pack to as high of a bulk density. However, the soil is weaker and cannot bear as much weight. When loads are driven on a wet soil, they sink into the soil deeper, because the soil is not as strong. Soil particles do not form as strong of cohesion to each other when soil is wet compared to when it is dry. This will lead to ruts being formed in the field. However, when ruts are formed in the soil, it may not be as badly compacted as a field that is driven on when soil is near the water content where the maximum bulk density occurs. Although sandy soils will pack to a higher bulk density, compaction will have a larger effect on crop yield in finer-textured (clayey) soils. However, some sands can become so packed that physical separation is almost impossible due to friction-locking forces.

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Photo Credit: gettyimages-mvburling

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