Good Garden Soil
In most cases, a combination of the 3 types of minerals (loam) gives the best growing conditions. This is because the combination of small, medium and (relatively) large particles provide for uneven cavities which allow for a good combination of aeration, drainage and nutrient holding capability. Some plants do better with relatively more clay in the soil, but most plants in general prefer a loam or sandy loam soil.
Living Soil.
What truly distinguishes soil from "dirt", however, is the presence of numerous living bacteria, fungi, worms, and insects which inhabit the soil. Many of these organisms help maintain fertility of the soil by decomposing and recycling plant and animal residues. Soils rich in these residues, "Organic Matter", are commonly termed "living soil".
During the decomposition process, fungi binds small particles into larger aggregates which further improve the structure, aeration and water infiltration (see drawing above). Organic matter can help reshape the large cavities in sandy soils and thus aid in the retention of water and fertilizer nutrients. In clay soils, the sticky particles are separated by organic matter allow roots passages through the soil. Organic Matter is also capable of holding fertilizer nutrients in a (chelated) form which is available for the roots.
Changing the Structure of Soil.
The structure of our garden soils are typically determined by the general area in which we reside, e.g. the sandy soils of Florida or red clay of the Southwest. In some instances, builders will have purchased "top soil" but unfortunately it is very seldom high quality loam.
Trying to change your soil profile into loam by adding clay, sand, or silt can be difficult, and expensive, and it does not solve the goal of achieving "living soil". For example, adding sand to clay soil can sometimes "set" the clay into concrete-like structures unless substantial amounts of sand are added. This happens when the small clay particles pack tightly around the larger sand particles which then become similar to steel rebar in concrete.
Instead, the addition of organic matter is the simplest and most effective way to improve any type of soil for almost any type of garden purpose. In a sense you can consider it the "universal ingredient" which is used to transform dirt into living garden soil irrespective the profile of your existing soil. No guessing or soil testing is necessary when you add
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