Leonardo da Vinci once said, “We know more about the movement of celestial bodies than about the soil underfoot.”
The average soil’s composition consists of approximately 50% solids and 50% pore space. The solids consist of about 45% inorganic mineral particles (sand, silt, clay), and 5% organic particles. The pore space is made up of 25% water and 25% air, but varies some based on rainy weather and dry periods.
Soil texture is the percentages of sand, silt, and clay in the matrix. By particle size, sand is the largest, clay is the smallest, and silt is in between. Texture cannot be changed easily, so it is best to work with what is present.
Soil structure has a major influence on water and air movement, root and plant growth, and seedling emergence. Soil structure describes the arrangement of soil solids and the pore space between them. This arrangement is determined by the type of soil particles and how they bind together to form aggregates. Structure can be improved or degraded by soil management. When the proportion of large to small aggregates increases, soil quality generally increases. In soils consisting of mainly small aggregates, filled pores and surface crusts occur. Filled pores lower water and air capacity diminishing the conditions for root growth. Surface crusts prevent infiltration and promote erosion.
Some practices such as poor tillage methods and soil disturbance activities lead to poor soil structure. Pesticide and fungicide usage and soil compaction, in particular during wet conditions, will result in poor aggregate stability. Soil structure can be improved by the addition of organic matter – compost and mulch – and by preventing compaction and soil disturbance. Unfortunately it can take several growing years for significant organic matter gains, while activities that degrade soils can result in rapid, long-term damage.
Within the soil structure are soil nutrients. These include macro- and micronutrients as well as trace elements. The primary macronutrients are nitrogen , phosphorous, and potassium. The three secondary macronutrients are calcium, sulfur, and magnesium. The micronutrients/trace minerals include chlorine, manganese, iron, zinc, and copper. A soil lab analysis, readily available from your local Cooperative Extension, will reveal basic nutrient levels and pH.
Soil pH refers to the acidity or alkalinity of the soil. Lower numbers are more acidic, higher numbers are more alkaline. Most plants grow in a pH of 6.5-7 (slightly acidic to neutral), the range in which soil nutrients are most available. Many plants grow in a lower or higher pH, but the list grows shorter as one gets further away from neutral in either direction.
A major component of soils that is seldom considered is beneficial microorganisms. Healthy forest soils have abundant communities of microbes, but in most urban soils the population is depleted or even absent. Mycorrhizae are one such symbiotic microbe. These fungi reach out into the surrounding soils and extract mineral elements and water for their host plant, then live off the plant's sugars. Plants with such systems are better able to survive in stressful environments. Fungi also help to form aggregates in the soil. The fungi strands enmesh soil particles and form cross-links between them. Another required microbe is beneficial bacteria. It improves plant resistance to many root diseases, helps make essential soil mineral elements available, decomposes organic matter, and improves physical properties of the soil.
In essence, good plants, whether in a yard, park, garden or woodland, require good soils. And good knowledge results in good soils.