How Does the Percent Total Pore Space Vary With the Percentage of Clay in the Soil?

Soil Basics

• Soils contain about 45 percent mineral particles. The size of the particles determines the soil's texture. Large particles -- visible to the naked eye -- include gravel and sand while silt and clay particles are so small that you need a microscope to see them. Soils consist of a mixture of sand, silt and clay, and the amount of each determines the soil's texture. When individual particles rest together in your soil, they leave small spaces between them called pores that fill up with air and water. Ideally, soil should contain about 50 percent pore space, and that pore space should include half each of air and water.

Soil Texture and Pore Space

• The size of the pore space varies with the size of the soil particles: Sandy soils have large pore spaces between individual grains of sand while clay soils have tiny pore spaces. However, because clay particles are microscopic and a clay soil contains more particles than a sand or silt soil, a larger percentage of a clay soil is given over to pore spaces than in other soil types. Therefore, in a healthy soil type, higher amounts of clay generally indicate more total pore space.

Soil Structure and Pore Space

• In addition to texture -- which describes the amount of each size of soil particle -- soils also have structure. Soil structure describes how individual particles aggregate or form together into groups. Ideal soil contains about 5 percent organic materials, which in clay soils, cause the clay particles to form into large clumps or aggregates. This is called a granular structure. In a granular structure, as the amount of clay in the soil increases, the total amount of available pore space increases as well. The same holds true, when soils arrange themselves into block structures, although the difference in total pore space for clay or sandy soils is less dramatic. Once soils begin arranging themselves into columns, pore space begins to decrease as you increase the amount of clay. Platy soils -- those with the soils arranged in thin sheets -- lose pore space dramatically as the percentage of clay increases. Pure clay soils with a platy structure have almost no pore space. Platy structure is typical of compacted soil.

Implications

• Pore space affects how easily water moves through the soil, which also affects how quickly nutrients leach from the soil. Clay soils tend to become waterlogged as water has trouble passing through the tiny pore spaces. Organic matter promotes a granular soil structure with larger, more abundant pore spaces that allow water to drain more easily. Organic matter also helps prevent the platy structure typical of damaged, compacted soils. Gardeners with clay soils should incorporate a source of organic matter -- aged manure, compost or mulch -- to improve soil structure and drainage.