Soil particles

Particle sizes

Sand

The largest of the fine soil materials since anything larger is considered a rock fragment. Like marbles, these particles don’t fit tightly together, leading to plenty of space for air and water to move through. Normally, this means good drainage and higher gas exchange for roots. Don’t assume that a sandy soil will be dry though, as even a sandy soil at the water table will have water-filled pores. Think of a beach when the tide is high; the sand is wet or under water, but when the tide goes out, the sand can quickly drain.

Silt

These are medium-sized particles, that generally feel soft and like flour. While they are considered more erodible and low strength for building purposes, they are generally favorable for plant growth. A higher percentage of water held in this soil is available for plants, and these are normally younger soils with weatherable minerals to provide some fertility for plant growth. Erosion can be a significant challenge to manage in a high-silt soil.

Clay

These are the smallest particles, and generally feel ‘sticky’ to the touch. The surface area per gram is significantly higher than sand, leading to more ability to interact with other things in the soil, such as water. Think of clay more like sheets of paper in a book; there is a lot of surface area in a given weight or volume and it would take a long time to dry out or move water through. While they hold significant amounts of water, not all is available for plant uptake. Timing field operations, providing aeration, and improving drainage can all be challenging aspects in a clay soil.

Textural Class

Textural classes group soils with similar sand, silt, and clay amounts into categories that help with management decisions.

While we may say ‘clay’ as a particle, a ‘clay texture’ requires over 40% of the soil to be in the clay-sized particle range. Generally, the most important word for management is last, with modifiers added to the front.

For example, consider a sand, loamy sand, and a sandy loam. Following the axis at the bottom of the triangle, we see that a sand needs at least 85% sand, whereas a loamy sand needs 70% and a sandy loam could have as low as 45% sand if it also has low clay. We would therefore expect the management challenges associated with the sand-sized particle, like low water holding capacity, to be most limiting in a sand, followed by a loamy sand, and then a sandy loam.