How Soils Work

Mineral Component

• Sand is gritty, silt is floury and clay has particles so small you need a strong microscope to see them. Most soils have some of each, but others have so much clay that they're sticky or so much sand that water runs through it too quickly, carrying nutrients and minerals with it in a process called leeching. The spaces between the particles, the pore spaces, allow air, as well as water, to move through the soil. Roots need oxygen, and well-aerated soil is essential for good growth. Constant pressure of feet or equipment can compact soil, especially a clay soil, and reduce its ability to produce a good crop.

Microorganisms and Macroorganisms

• Each teaspoon of soil contains huge numbers of microorganisms, between 100 million and 1 billion bacteria alone, and they contribution to the health of the soil, the release of nutrients and the balance of the processes of decay. Fertile soil is truly alive. There are also larger organisms that contribute to a healthy soil, including earthworms, beetles and a variety of other insects.

Organic Matter and Decay

• The decay of leaves, roots, dead insects and other organic matter that was once alive releases the nutrients in their tissues for reuse by plants. The end product of decay is a dark, sticky material called humus, a truly amazing substance that can hold tremendous amounts of water and bind tiny particles of clay together, increasing pore space. Even partially decayed material opens the soil and makes it lighter. Additions of compost, manure, straw or other organic matter improves every type of soil, from sandy ground that needs to hold water longer to clay that needs better drainage.

pH and Plant Growth

• pH is a measure of the acidity or alkalinity of the soil and often depends on how much lime is in the original material or on the type of native plants that grew in the soil years ago. An acidic pH makes it hard for many plants to get the nutrients they need from the soil, but many others -- blueberries, rhododendrons and potatoes among them -- need an acidic soil to grow well. Ground limestone is often used to raise the pH of acidic soil to nearly neutral, a pH of 7, and sulfur is added to reduce the pH of alkaline soil.