Heavy soils are not physically heavier than other soils, but feel heavier when worked. Their stickiness makes them resist the spade or tiller. These soils, which have a high proportion of clay, are difficult to work and can be extremely hard to till. When clay soils are too wet, they’re sticky and compact easily; if too dry, tilling them is like tilling pottery. When overwatered, heavy soils may contain little air. Soil pores may be too small and too few, so they absorb water slowly and puddle easily.
Managing Clay Soil
The trick to working clay soil is to select just the right moment as it dries out. In many clay soils, this moment comes about 3 days after a rain or irrigation. For a period, the clay can be turned over and the clods broken easily with a spade or garden fork. The heavier the soil, the shorter this window of opportunity, so be alert. Test the soil by digging it. If it is not too sticky or hard to dig, and the clods break when hit with a shovel, it is ready to till. If the soil is too dry to dig, water it thoroughly and wait until it dries to the right point.
Heavy soil holds more water than lighter soils, but absorbs it more slowly. Compared to other soils, heavy soils should be watered less frequently, but for longer periods, and the water should be applied more slowly.
All heavily-planted land, as most gardens are, loses water at about the same rate, depending on factors such as heat, humidity, and wind. Over the course of a week, you need to put about as much water on heavy soil to replace the water that was lost as you do on sandy soil. But heavy soil holds a great deal more water than lighter soil. You can think of the soil as a reservoir for water. Heavy soil has a bigger reservoir. It takes longer to fill it, but longer to empty it, too.
Heavy soil absorbs water slowly, so put it on slowly. Drip irrigation and soaker hoses work very well in heavy soils. The soil conducts water sideways better than lighter soil, so each emitter covers a broader area.
Select sprinkler heads that apply water slowly, such as impact-head sprinklers. If this isn’t practical, irrigate until the water begins to run off, then turn off the sprinklers for an hour or so to let the water soak in before continuing watering. Many irrigation timers have “cycle” settings to allow 2 or 3 cycles of watering.
Another reason to water less frequently is that the water forces air out of the soil. Because the water moves so slowly through the soil, air moves in slowly, and it’s easy to suffocate plant roots by keeping the soil too wet to allow air to enter. Allow the soil to dry to the point where it is just slightly moist before watering again. This gives air time to enter the soil and allows roots to breathe.
Besides being a large reservoir for water, heavy soil is also a large reservoir for plant nutrients. Nutrients adhere to clay particles firmly enough to keep from being washed away by water, but loosely enough that the plant roots have access to them when they need them. Gardens in heavy soil can be fed less often, with larger amounts of fertilizer, than gardens in light soil.
Improving Heavy Soil
Porosity in clay soils depends on the structure of the soil rather than on its texture. Soils with a coarse texture, such as sandy soils, have large pores because of their texture. Porosity in clay soils depends on the way the particles aggregate—the soil structure.
Unlike texture, structure can be changed and improved. It is improved by the action of bacteria in the soil, which form humus. Humus is a gummy, relatively stable organic residue of bacterial activity. Humus bonds with clay particles to increase their cohesiveness, forming crumbs of soil. Heavy soil that is rich in humus is not sticky, but is crumbly, loose, easy to work, and soft to the touch. It holds large amounts of air, as well as water and plant nutrients, and drains quickly.
Soil life—and humus production—is increased by the addition of organic matter to soil. A layer of organic matter 2 to 6 inches thick should be spread on the soil surface and tilled into the soil each time the soil is worked. Use well-rotted manure, compost, leaf mold, or other locally available organic material, such as grape pomace, cottonseed meal, or composted sawdust. Any organic matter that decomposes improves heavy soil.
Humus builds up slowly and decomposes slowly. As more organic matter is added each year, the structure of the soil improves. Each addition of organic matter influences the soil for as much as 10 years, so the improvement accumulates for years.
Add organic matter each time you till the soil or plant anything. In addition, add organic mulch under shrubs and trees and around perennial flowers. Earthworms will dig it into the soil for you in these areas where you can’t till.
Organic material can be added in other ways, also. If it’s possible for the area to remain out of production for a season or two, plant a green manure crop and till it into the soil. Then let the soil sit for a few weeks to let the green manure decompose. While it’s idle, cover it with hay, straw, or another type of mulch to prevent erosion.
Although most plants require good drainage, some are adapted to heavy soils and thrive in them. They often have shallow root systems that get oxygen even in waterlogged soils. Select these plants to be sure of getting plants that are adapted to your soil as well as your climate. Look for “bog plants” or “pond-edge plants” in nurseries and plant catalogs.
Adding Sand to Heavy Soil
As you add sand to clay soil, the soil gets heavier and heavier until the mix is about 80% sand. Until the soil is mostly sand, the clay fills up the pores between sand particles, keeping them from acting like sand. In other words, the often-repeated advice to add sand to lighten clay soil is bad advice. Sand makes clay soil heavier, not lighter.