Soil Structure

Introduction to Soil Composition

Soils are important because they are the most common base for bulk food production. Knowledge of soils is necessary to estimate soil’s suitability for agricultural use.
Generally, Soil is comprised of 4 components
• Water (actually a solution of salts)
• Air and other gases
• Organic Matter (residue from plants, animals, bacteria and other)
• Minerals / Nutrients

Soil Organic Matter (OM) provides essential nutrients to the plant and bonds the soil together.
• Too little OM and plants will not be able to access the necessary nutrients for growth (ex: Stoney Ground or Sandy Desert)
• Too much OM and there is risk of ground water contamination and disease from excess nutrients

Soil structure and types

The particles in soil aggregate in different ways.

• When they aggregate in globular structures, there is room for air and soil solution (water plus nutrients). The soil is structured and in good conditions for plant growth. This conditions depends on many factors, mainly minerals’ composition, OM, Ca, and tillage practices.

• When minerals do not aggregate, soil is unstructured. Causes are many, mainly poor content in OM, poor content in Ca, mineral composition, bad tillage practices bringing to compaction. In this conditions, there is less or no room for water and air. Plants growth is difficult, because nutrients can not be absorbed by roots.

• Farmers can intervene in improving soil structure by adding OM (residue, manure and similar), adding Ca (liming) and choosing the proper tillage system. Changing mineral composition is too much expensive in common farming.

Relationships among soil components

This chart is broadly used for determining what type of soil we are dealing with. It is possible to estimate texture of soil “on the field” (Portland Cement Association. 1962. PCA Soil Primer)

Sand: Individual grains can be seen and felt readily. Squeezed in the hand when dry, this soil will fall apart when the pressure is released. Squeezed when moist, it will form a cast that will hold its shape when the pressure is released but will crumble when touched.

Sandy loam: Consists largely of sand, but has enough silt and clay present to give it a small amount of stability. Individual sand grains can be seen and felt readily. Squeezed in the hand when dry, this soil will fall apart when the pressure is released. Squeezed when moist, it forms a cast that will not only hold its shape when the pressure is released but will withstand careful handling without breaking. The stability of the moist cast differentiates this soil from sand.

Loam: Consists of an even mixture of the different sizes of sand and of silt and clay. It is easily crumbled when dry and has a slightly gritty, yet fairly smooth feel. It is slightly plastic. Squeezed in the hand when dry, it will form a cast that will withstand careful handling. The cast formed of moist soil can be handled freely without breaking.

Silt loam: Consists of a moderate amount of fine grades of sand, a small amount of clay, and a large quantity of silt particles. Lumps in a dry, undisturbed state appear quite cloddy but they can be pulverized readily; the soil then feels soft and floury. When wet, silt loam runs together and puddles. Either dry or moist casts can be handled freely without breaking. When a ball of moist soil is pressed between thumb and finger, it will not press out into a smooth, unbroken ribbon but will have a broken appearance.

Clay loam: A fine-textured soil which breaks into clods or lumps that are hard when dry. When a ball of moist soil is pressed between the thumb and finger, it will form a thin ribbon that will break readily, barely sustaining its own weight. The moist soil is plastic and will form a cast that will withstand considerable handling.

Clay: A fine-textured soil that breaks into very hard clods or lumps when dry, and is plastic and unusually sticky when wet. When a ball of moist soil is pressed between the thumb and finger, it will form a long ribbon.

Concerning texture, agricultural use of soils has limit only for sand and loamy sand soils. Of course, other limits exist depending on factors as slope, permeability, climate, rock presence and others.