As compaction pushes particles closer together, the soil becomes dense and pore space is lost. Notably, the larger pores are eliminated. Loss of aggregation from compaction is particularly harmful for fine and medium-textured soils that depend on those pores for good infiltration and percolation of water, as well as air exchange with the atmosphere. Although compaction can also damage coarse-textured soils, the impact is less severe. They depend less on aggregation, because the pores between individual particles are sufficiently large to allow good water and air movement.
Compacted soil becomes hard when it dries, as it has many small pores that can hold water under high suction and pull particles tightly together. This can restrict root growth and the activity of soil organisms. Compacted soils typically have greater resistance to penetration at a given soil moisture level than a well structured soil (figure 6.11), which has large pores between aggregates that therefore easily pull apart. The resistance to penetration for a moist, high-quality soil is usually well below the critical level where root growth ceases for most crops—300 pounds per square inch (psi). As the soil dries, its strength increases, but a high-quality soil may not exceed the critical level for most (or all) of the moisture range. A compacted soil, on the other hand, has a very narrow water content range for good root growth. The soil has increased resistance to penetration even in the wet range (the soil is hard). When it dries, a compacted soil hardens quicker than a well structured soil, rapidly becoming so hard that it is well above the critical 300-psi level that restricts root growth.
SOME CROPS MORE SENSITIVE THAN OTHERS
Compaction doesn’t affect all crops to the same extent. An experiment in New York found that direct-seeded cabbage and snap beans were more harmed by compaction than cucumbers, table beets, sweet corn, and transplanted cabbage. Much of the plant damage was caused by the secondary effects of compaction, such as prolonged soil saturation after rain, reduced nutrient availability or uptake, and greater pest problems.
Actively growing roots need large pores with diameters greater than about 0.1 mm, the size of most root tips. Roots must enter the pore and anchor themselves before continuing growth. Compacted soils that have few or no large pores don’t allow plants to be effectively rooted— thus limiting water and nutrient uptake.
What happens when root growth is limited? The root system will probably develop short, thick roots and few fine roots or root hairs (figure 6.6). The few thick roots may be able to find some weak zones in the soil, often by following crooked patterns. These roots have thickened tissue and are not efficient at taking up water and nutrients. In many cases, roots in degraded soils do not grow below the tilled layer into the subsoil (see figure 6.6)—it’s just too dense and hard for them to grow. Deeper root penetration is especially critical under rain-fed agriculture. The limitation on deep root growth by subsoil compaction reduces the volume of soil from which plant roots can extract water and increases the probability of yield losses from drought stress.
There is also a more direct effect on plant growth, beyond the reduced soil volume for roots to explore. A root system that’s up against mechanical barriers sends a hormonal signal to the plant shoot, which then slows down respiration and growth. This plant response appears to be a natural survival mechanism similar to what occurs when plants experience water stress. In fact, because some of the same hormones are involved—and mechanical resistance increases when the soil dries—it is difficult to separate the effects of compaction from those of drought.