Compaction can be measured by a penetrometer, or soil probe, which indicates the resistance to pushing a cone into the ground. Typically, critical values of soil probe resistance that cause the beginning of reduction of root growth range from 150psi to 250psi, and those stopping root growth range from 350psi to 400psi (depending on plant and soil type, together with pore size distribution and moisture content).
What Causes Compaction?
Compaction occurs when soils become denser, lose porosity, increase in strength, and reduce their ability to transfer water. Soil compacts when it is in a weak state and subjected to high loads during harvest, planting, and tillage. Weakness occurs when soil is too wet or too loose. Multiple passes on non-permanent track ways lead to tire compaction over a high proportion of the field. The first pass can cause up to 85% of the total compaction, and random, multiple tillage and trafficking passes can compact up to 70% of the total field surface.
Wet soil compacts more easily than dry soil, filling previously air-filled pores with water. Tractor or combine traffic then smears the soil, disrupting pore continuity and resulting in impeded water movement. This situation is often more serious than compaction in its effect on root growth.
Repeated passes at the same ground pressure and at the same position in the field further exacerbate compaction created (Raper, 2006). Therefore, growers should minimize the number of tillage passes over the soil.
Typical compaction in damp conditions resulting from increased axle loads is shown to the right (Raper, 2006). This clearly illustrates that measurable compaction below the 15-inch deep horizon is virtually impossible to avoid when combines and grain carts are operated on wet soil.
Any form of tillage performed in wet conditions is extremely likely to cause deep-seated compaction. Keeping compaction to reduced depths, however, allows for it to be alleviated by subsoiling.