"To be a successful farmer one must first know the nature of the soil."
— Xenophon, the Greek historian, writing around 400 BC
Throughout the history of agriculture, good soil has been recognized as the basis of successful food production. Without good soil, carefully tended and replenished, crops will struggle. For that reason, a program of soil improvement is an integral part of sustainable farming and gardening.
Soil quality can be described by two factors: tilth and fertility. Tilth refers to the physical condition of the soil, and how well it allows for essential plant processes including seed germination, root growth, water infiltration and drainage, and root aeration. Fertility refers to the nutrients that are held in the soil and available to plants.
The ideal vegetable soil is deep, friable, well-drained, and with adequate available nutrients to support optimum plant growth. A few lucky growers have those conditions on their land, but most of us have less-than-perfect soil that requires some work to get it into shape and keep it healthy. The work will pay off in the future with crops that have higher yields, fewer pest and disease problems, stronger drought resistance, and, in some cases, better flavor.
To preserve good tilth, it's essential to avoid compaction. Growing areas should never be driven on with a truck or tractor. Tillage should be minimized because, although it may seem to create a fluffier soil texture, it actually breaks down the soil aggregates that are important to the long-term health of the soil. Soil should be kept covered with crops, mulches, or cover crops to prevent compaction and erosion in heavy rains.
Another strategy for improving both tilth and fertility is to increase soil organic matter (SOM) by adding compost or manure and by growing cover crops and tilling them under. SOM comprises only a small portion of soil from less than 1% to about 10% in the best of conditions. But organic matter contains almost all of the nitrogen and a large amount of the phosphorous in soil, as soil organisms decompose organic matter and convert it to plant nutrients. Organic matter also contributes to good tilth, as it improves water-holding capacity, drainage, and soil structure.
SOM can be measured by a soil test, which also provides information about nutrient levels and recommendations for improving fertility. Soil tests are available from most Cooperative Extension Service offices or from commercial soil labs, which can be found online. It's important to know that organic matter increases very slowly over time.
"A soil with 3% organic matter might only increase to 4% after a decade or more of soil building," writes Marianne Sarrantonio, a sustainable agriculture researcher at the University of Maine, in the book Managing Cover Crops Profitably. "The benefits of increased organic matter, however, are likely to be apparent long before increased quantities are detectable. Some, such as enhanced aggregation, water infiltration rates and nutrient release, will be apparent the first season; others may take several years to become noticeable."
Cover crops improve soil in many ways besides increasing organic matter. For example, leguminous cover crops such as clovers and soybeans fix nitrogen, moving it from the air into the soil. Cover crops also catch nutrients before they can leach out of the soil. Many cover crops have deep tap roots that break up compacted soil layers. Some cover crops, especially the grasses, have masses of fine roots that improve soil texture. All cover crops protect the soil from erosion and compaction. With only a few exceptions (such as providing a fallow period in a field to break up weed problems), soil should always be covered with either a cash crop or a cover crop.
One of the best resources for learning about soil improvement through cover crops is the book Managing Cover Crops Profitably, published by the Sustainable Agriculture Research and Education program. It can be viewed online. Printed copies of the book are available from Johnny's for $17.95. Visit Johnny's bookstore.
The University of Massachusetts has an excellent series of articles on soil and nutrient management for vegetable growers. Learn more.
ATTRA, the National Sustainable Agriculture Information Service, has numerous publications about soil management.
Lynn Byczynski is the editor of Growing for Market and the publisher of The Hoophouse Handbook.