Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted November 1997
Soils that are compacted or high in clay resist root, water, and air penetration and can seriously hinder plant growth. Compacted soils are very dense and lacking in pore space, which lessens water holding capacity and rooting area. Soils high in clay also tend to be dense. Because of the very fine texture and small pore size of clayey soils, water is so tightly held that uptake by plant roots is limited. Due to the lack of large pore spaces, water passes through both soil types very slowly; therefore, both frequently are too wet.
The ability of a growing root tip to penetrate soil is directly dependent on soil strength. Soil strength is controlled jointly by a soil's bulk density and moisture content. Workable, loose soils have bulk densities of 0.8 to 1.3 grams per cubic centimeter (or have 70 to 50 percent pore space), while severely compacted soils commonly have bulk densities in the range of 1.6 to 1.8 (or 40 to 30 percent pore space). Root penetration is greatly retarded when bulk density exceeds 1.4 during dry conditions. The same soil when moist, however, may not impede rooting because soil strength is then decreased.
Bulk density is difficult for gardeners to determine accurately, so analysis of compacted or dense soils must be estimated. You can identify a clay soil (if you can't just by looking at it) by measuring its plasticity. To do this, take a small amount of soil from the area or layer in question and wet it. Roll the sample between your palms or on a flat surface. A clay soil makes a long, thin ribbon or string that doesn't fall apart easily. The longer and more cohesive the "string" is, the more clayey the soil sample. Compacted soils or soil layers are hard to dig through, whether wet or dry. But perhaps a better way to determine compaction is to evaluate plant root growth. Plant roots will stop or be restricted at/in compacted layers, or root growth will be poor, compared to growth in looser soils.
The best way to improve rooting in compacted soils is to increase the porosity by tillage and incorporating organic matter into the soil. Addition of compost and/or other organic amendments into topsoil ensures that the soil is well aggregated (or better structured) and, therefore, contains more large pores, as well as total pore space.
Compacted layers or zones in soils are called pans. Compacted subsoil layers limit total soil volume available for rooting and restrict total water and nutrient availability. These layers also perch water tables (cause water to "back up" instead of draining downward). Sometimes, water tables perched by compacted soil layers last for extended periods of time, causing saturation or waterlogging within the root zone. Deep tillage or ripping is the only practical way to improve subsoil porosity, but may be too expensive for many gardening situations. However, care must be taken to avoid excessive tillage, since that may lead to the destruction of large aggregates. Too much tillage also decreases organic matter content by speeding decomposition.
The structure of the compacted soil zone also strongly influences its effect on plant growth. Artificially compacted zones (traffic pans) often are much denser than natural clay pans and have no breaks or channels, such as are found in naturally occurring pans. Traffic pans must be physically shattered to allow significant rooting. Natural subsoil clay pans, on the other hand, often have structural planes of weakness that allow roots to penetrate to some extent, even though the layer overall is very dense. Subsoil pans, especially naturally occurring ones, are permanent, quite thick, and very difficult to alter.
(Originally published as "Managing Compacted and Heavy (Clayey) Soils," by W. Lee Daniels, Associate Professor, Soil and Environmental Science, Virginia Tech, in The Virginia Gardener Newsletter, Volume 8, Number 6.)