Spotlight on Soil

Author: National Gardening Association Editors

Have you hugged your soil today? Although many factors contribute to a thriving garden, any seasoned gardener will stress the importance of good soil. In addition to anchoring roots, soil provides life-sustaining water and nutrients. Plants in poor soils will struggle to grow, even if optimal water and light are available. In contrast, plants in good soils will grow to their fullest potential and experience fewer problems with insects and disease.

Soil is composed of minerals and organic matter. Sand, silt, and clay are the mineral particles derived from rock broken down over thousands of years by climatic and environmental conditions (rain, glaciers, wind, rivers, animals, etc). The largest, coarsest mineral particles are sand. These particles are 2.00–0.05 mm in diameter and feel gritty in your fingers. Silt particles are 0.05–0.002 mm and feel similar to flour. Clay particles are extremely fine — smaller than 0.002 mm — feel sticky in your fingers when wet, and clump to the point that you can’t see an individual particle without a microscope. Organic matter is the decayed remains of once-living plants and animals. Good plant growth and development depends on the mineral and nutrient content of soil, as well as its structure.

Soil is teaming with life, including microorganisms like bacteria and fungi (billions in a single teaspoon!) and larger animals such as worms and sowbugs. Many of these underground inhabitants feed on remains of plants and animals, breaking down their tissues. In the process, they create pore space and release nutrients that plants need and the cycle begins again.

Pore space — the arrangement of soil particles in relationship to each other — is an important component of soil structure. In an optimal situation about 50 percent of the volume of the soil would be pore space, with half of that filled with water and half filled with air. The other 50 percent would be sand, silt, clay, and organic matter. Roots need air as much as they need water; plants can actually suffocate or drown if they are completely submerged in water for extended periods of time. 

The proportion of these different-sized particles affects the amount of air, water, and nutrients available to plants, and how the soil “behaves.” The smaller the soil particles, the more they stick together when wet. Thus clay soils can be sticky and difficult to work. Having fewer air spaces, they drain poorly and roots may suffer from a lack of oxygen, but clay soils can be rich in minerals. In contrast, sandy soils can drain water too quickly and be low in nutrients, but they are easier to work. Adding organic material can offset many of the problems associated with either extreme.

While there’s no such thing as a perfect soil, particular plants grow best in particular soils. In general, common garden plants prefer loam — soils with a balance of different-sized mineral particles (approximately 40 percent sand, 40 percent silt, and 20 percent clay) and ample organic matter and pore space, but some common plants grow better in sandy conditions, while others are well adapted to clay soils. 

To learn more about soils visit:
Soil Science Education Home Page

Getting Started with Soils

Soil Sleuths

Digging Deeper with Soils