3 Soil Properties

How soil properties shape function

Soil properties determine how well soil stores and moves water and air, supplies nutrients, and supports roots and soil organisms. These properties interact: texture affects pore size, while structure and organic matter influence how pores are arranged and maintained.

Texture and particle sizes

Soil texture describes the relative proportions of the mineral particles sand, silt, and clay. Sand particles are the largest, silt particles are intermediate, and clay particles are the smallest. A soil’s textural class, such as sandy loam, loam, or clay, is determined by the percentages of these three particle sizes.

Texture influences water movement and storage, aeration, root growth, and nutrient retention. Sandy soils usually have larger spaces between particles, so water drains and air moves through readily; they often store less plant-available water. Clay-rich soils commonly retain more total water and nutrients, but water and air may move through them more slowly. Loam, which contains a mixture of particle sizes, often provides a useful balance. Texture is relatively difficult to change, unlike some properties affected by management.

Structure and aggregates

Soil structure describes how individual particles and organic matter are grouped into aggregates, also called peds. Common structural shapes include granular, blocky, platy, and prismatic. Texture describes particle sizes and their proportions; structure describes how those particles are arranged.

Well-formed, stable aggregates create a network of pores that helps water infiltrate, air circulate, and roots grow. Organic matter, roots, and soil organisms help bind aggregates. Compaction and working soil when it is wet can damage structure and restrict pores. For example, a well-aggregated clay soil may allow better infiltration than a compacted clay soil with the same texture.

Pore spaces and porosity

Porosity is the proportion of a soil’s total volume made up of pore spaces. Pores contain air or water, and their sizes matter as much as their total volume. Larger pores, called macropores, commonly allow air and water to move through soil and drain under gravity. Smaller pores, called micropores, hold water more strongly.

Healthy soil needs pores for both water storage and movement, as well as air for roots and organisms. Compaction can reduce pore space, especially the larger pores.

Water movement and availability

Water enters soil by infiltration. It may then move downward, be stored in pores, be taken up by roots, evaporate, or flow over the surface as runoff. The amount and movement of water depend on texture, structure, porosity, soil depth, and organic matter.

After excess water drains from saturated soil, the remaining water content is called field capacity. The permanent wilting point is the moisture level at which plants cannot extract enough water to recover from wilting. Plant-available water is the water held between field capacity and the permanent wilting point.

A clay soil may hold much water overall, yet some of that water is held too tightly for plants to use. A sandy soil drains quickly and often has a smaller reserve of available water.

pH and nutrient availability

Soil pH measures acidity or alkalinity on a logarithmic scale. A pH below $7$ is acidic, $7$ is neutral, and a pH above $7$ is alkaline. Because the scale is logarithmic, a decrease of one pH unit represents a tenfold increase in hydrogen-ion concentration.

Soil pH affects the chemical forms and availability of nutrients, as well as the activity of soil organisms. Many plants obtain nutrients readily in mildly acidic to near-neutral soils, but preferred pH depends on the plant species. At very low or high pH, some nutrients can become less available while other elements may become excessively available.

Soil tests help identify pH and guide decisions about amendments. The appropriate target depends on the soil and the plants being grown.

Organic matter and soil health

Soil organic matter includes living and dead organisms, plant and animal residues at different stages of decomposition, and substances produced as they break down. Well-decomposed organic matter is often called humus. It may make up only a small part of many mineral soils, yet it has important effects:

• It supplies energy for soil organisms and releases nutrients as it decomposes.

• It helps bind particles into stable aggregates, supporting structure and reducing crusting and erosion.

• It supports infiltration and helps soil retain water and nutrients.

• It contributes to nutrient retention and can help buffer changes in soil pH.

Plant residues on the soil surface can protect soil from raindrop impact, while decomposed organic matter within the soil helps stabilize aggregates. Organic matter changes over time as new residues are added and existing material decomposes or is lost.

How the properties interact

Soils with the same texture can function differently. For example, a clay-rich soil with abundant organic matter and stable aggregates may have more connected pores and better infiltration than a compacted soil of the same texture. The compacted soil may have fewer large pores, poorer aeration, and more runoff.

Texture sets important constraints, while structure, porosity, water, pH, and organic matter together shape how soil functions. Soil is best understood as a connected system rather than as a collection of separate measurements.