How soil type affects management
How soil type affects management
Water movement. Because soil particles are solid, water obviously cannot move through them. Instead, it must move around them. Water’s movement in and through soil depends on the arrangement and size of the soil’s pore spaces—the spaces between soil particles. Due to the random way soil particles pack together, pore spaces vary in size. Some are large and some are small. A “typical” soil may be about 50 percent pore space—25 percent small pore space and 25 percent large pore space. The proportion of soil occupied by pore space is its porosity and varies a great deal among soil types. When water drains through a soil, most of its movement is through large pores. Coarse-textured soils have more large pore spaces than finely textured soils, and air normally fills these large pores. Larger pores are much better at conducting both air and water through the soil and that’s why sandy soils have excellent drainage and aeration. The rate at which water can flow through a soil is called hydraulic conductivity. Coarser, sandy soils, with their larger pore sizes, have higher hydraulic conductivity than fine, clay soils, which tend to be lower in oxygen and retain more water.
Small pores more often contain water, rather than air. Because clay soils have more small pores and greater water retention, they also are more prone to saturation due to heavy rainfall or poorly drained conditions. When water completely occupies all the pore space in soil, the soil is saturated. Saturated soils, as we mentioned, are lacking in oxygen and therefore make poor environments for root growth.
However, clay soils have their benefits as well. For example, they hold more available water and nutrients, so plants can last longer between irrigations and fertilizer applications. Sandy soils hold much less water and nutrients, so plants growing in them are more prone to drought and nutrient deficiency. One reason loamy soils are valuable is that they hold more water than sand, but they do not have the drainage problems of clays.
Infiltration rate describes how fast water enters the soil surface. Infiltration is similar to hydraulic conductivity and largely dependent on it. Whenever you apply water to the soil surface at a higher rate than the infiltration rate, you will have puddling or runoff.
Because clay soils often require short, light doses of water to avoid runoff or puddling (and possess low hydraulic conductivity), they are more susceptible to salt buildup. This happens because each time you apply water, you also apply a small amount of dissolved salt along with it. In well-drained soils, you easily can apply enough water so that some of it drains, or leaches, completely through the root zone. This water takes some of the dissolved salt with it, thus reducing the amount to which plant roots are exposed. However, when infiltration rates limit you to small doses of water, you cannot apply enough to leach any out of the root zone. Thus, while water leaves the soil by evapotranspiration, the salt stays behind and slowly accumulates to toxic levels as additional irrigation water brings more. This also illustrates why water quality is an important issue.
Compaction and density. An aspect we have not touched on yet is soil-particle shape. Particles smaller than sand tend to be flattened and plate-like. This tendency is very strong with clay particles, and this has important implications. Clay particles, being flat, can stack tightly together, virtually eliminating any pore spaces between particles. In other words, porosity decreases. This is true with silts as well and is what happens when soils compact and why compacted soils conduct little water or air. Further, root growth is reduced because pore spaces through which small roots grow do not exist in compacted soil. Moist soil is more prone to compaction because when ample water is present in the soil, the particles can slip and slide past one another, making repositioning into a more compact state easier.
Clay particles also can seal, for the same reason. The flattened particles can all be oriented in the same positions—flat—and form a barrier through which water and air cannot penetrate. That’s why it’s important to score glazed surfaces—such as those created by tree-spades in planting holes—to disrupt this barrier and allow water and air penetration.
Sand tends not to compact because, unlike clays, sand particles are not flat. They cannot “stack” in a way that reduces pore space. That is why sand is the preferred medium for high-traffic turf such as golf greens and athletic fields—turf growing on sand is not as prone to the damage that compaction causes.
At this point, we should mention pans. Pans are impermeable layers present below the surface of some soils at varying depth. Hard pans are rock-like while clay pans are softer. Most pans occur naturally, but some cultural practices can create them. For example, repeated core aeration at the same depth can create a pan layer of highly compacted soil just below the depth of tine penetration.
Pans all cause serious drainage problems in landscapes. They prevent water from draining and so create perched water tables. This not only saturates soil, it also causes salt buildup because salt cannot leach out of the soil. Even if you’re able to manage irrigation well enough to prevent these problems, pans still effectively create a “bottom” to the soil, which may be quite shallow. This can restrict the rooting depth of trees and shrubs.