How coarse or fine soil feels depends on the size of the mineral particles. Sand, silt and clay, the major mineral particles, are responsible for the size and number of the soil’s pore spaces. Soil pore space determines the amount of air and oxygen, the drainage rate and capacity to hold nutrients. Sand grains are the largest particles and create large pores. Sandy soils drain quickly and do not hold water and nutrients well. While sand can be seen by the naked eye, silt particles are microscopic and feel velvety and smooth. Silt creates smaller pores in the soil and results in better water retention. Clay particles are the tiniest of all. When moist, they cling together and feel sticky. Clay soils have a tremendous capacity to hold water and nutrients, and soils rich in clay tend to suffer from poor air circulation and slow drainage.
Soils are rarely pure sand, silt or clay but rather a mixture of all three. They’re often grouped into one of 12 textural classes based on the relative proportions of these particles. Sands and loamy sands, for example, are more than 70 percent sand and share the characteristics of sand. Clays, sandy clays and silty clays are more than 40 percent clay and exhibit the characteristics of clay. Loams, the ideal soils celebrated in so much gardening literature, share the attributes of both—good aeration, drainage and moisture and nutrient retention. Most vegetables do best in loamy soil. It is possible to grow a beautiful ornamental garden in any soil type, as long as the plants are adapted to the particular soil conditions.
How readily soil particles cling together to form aggregates, called crumbs or peds, is the measure of its structure. This determines how permeable a soil is, how well it retains moisture and nutrients, and how easily it allows plant roots to penetrate and grow.
Soils that are ideal for plant growth bind together to form loose, granular aggregates about the size of cookie crumbs and retain moisture and nutrients. Organic matter and thriving communities of soil organisms are critical to this good structure. Sandy soils do not bind together and tend to have the consistency of dry cake mix. Clay soils bind tightly and become very hard when dry. Compacted soils have a “platey” structure, with multiple layers of flat, thin peds. Weight bearing down on the soil has caused the larger pores to collapse, restricting the movement of air and water and limiting the growth of plant roots.
Soil pH is a measure of the acidity or alkalinity of the water held in its pores. The pH scale goes from 0 to 14, with 7 representing neutral. From pH 7 to 0 the soil is increasingly acidic, while from 7 to 14 it is increasingly alkaline.
Soil pH affects whether minerals and nutrients will be available to benefit plants. Before a nutrient can be used by a plant, it must be dissolved in the soil solution. Most plants prefer a slightly acidic to neutral soil, with a pH of 6 to 7, because that is the range in which all nutrients are readily available. In strongly acidic soils (pH 5.5 to 4), important nutrients such as phosphorus, potassium, calcium and magnesium are in short supply. The availability of phosphorus, iron, copper, zinc, and manganese is reduced in slightly to moderately alkaline soil (pH 7 to 8).
Soil pH also affects the activity of soil microorganisms. Bacteria that decompose organic matter are hindered in strongly acidic soils. This prevents organic matter from breaking down and ties up nutrients, particularly nitrogen. Strongly acidic or alkaline soils can increase the solubility of some nutrients and minerals to the point that they become toxic to plants. For example, in very alkaline soils, the levels of calcium and magnesium are so high that they impede the availability of phosphorus.