The mineral side of soil: sand, silt, and clay

The mineral part of soil is composed of varying amounts of sand, silt, and clay. Their characteristics have a big influence on soil behavior and management needs.

Sand

• Of the 3 kinds of mineral particles, sand is the largest in size; about 50 sand particles laid side by side would equal 1 centimeter (125 per inch).

• Sand is mainly quartz (silicon dioxide) and contains few plant nutrients.

• Moderate amounts improve soil drainage, aeration, and filth (workability).

Silt

• It consists mainly of ground up sand particles (quartz), which are often coated with clay.

• It contains few nutrients in itself except those that might be in the clay coating.

• Silt particles are too small to help improve drainage and aeration.

Clay

Clay particles are the smallest of the 3 (about 4000 of them laid side by side would equal 1 cm). Farmers know that clay has a big influence on soil behavior. High clay content usually makes for harder plowing, more compaction, and poorer drainage, but it does assure good water-holding capacity. Aside from this, clays have 3 other important features:

• Source of plant nutrients: Unlike sand and silt, clays are aluminum-silicate minerals that also have varying amounts of plant nutrients such as potassium, calcium, magnesium, and iron, etc. A good part of a soil's native fertility can come from its clay portion.

• Clays have a negative charge: This makes them act like tiny magnets to attract and hold those plant nutrients that have a positive (+) charge like potassium (K+), calcium (Ca++), magnesium (Mg++), and the ammonium form of nitrogen (NH.+). This helps greatly to keep these nutrients from being carried downward beyond the root zone by rainfall or irrigation. (The term leaching is used to describe this type of loss.)

• Tremendous surface area: Each clay particle is really a laminated structure consisting of tiny plates. This lattice arrangement plus small particle size gives clays an amazing amount of surface area for attracting and holding positively-charged nutrients. In fact, one cubic centimeter of clay particles contains about 1-3 square meters of surface area.

All Clay isn't the Same

There are several different types of clay, and most soils contain at least two. Understanding some basics about clay types will help you interpret the soils in your work area. It's important to understand the difference between temperate clays and tropical clays and why both types are found in the tropics.

• Temperate clays: These are 2:1 silicate clays such as montmorillinite and illite that dominate the clay portion of most temperate zone soils but may also be found in the tropics. The 2:1 figure refers to the ratio of silicate to aluminum plates in a clay particle's laminated structure. Soils with a good amount of these temperate clays are very sticky and plastic when wet; some kinds such as montmorillinite shrink and swell readily, forming large cracks upon drying out. They also have a relatively high negative charge (good for holding positively-charged nutrients>.

• Tropical clays: These are 1:1 silicate clays, such as kaolinite, and hydrous oxide clays of iron and aluminum that often make up most of the clay portion of old, well drained soils in the tropics and subtropics, mainly in areas with at least 6 months of rainfall. These clays have lost lots of silicate due to centuries of weathering and leaching. Unlike the 2:1 clays, these "tropical" clays are much less sticky and plastic and are easier to work with, even when clay content is high. However, they usually have much less negative charge and lower natural fertility than temperate clays. Soils whose clay portion is largely "tropical" can usually be identified by their red or yellow colors.