Amendments are substances worked into the soil in order to encourage plant growth by improving the soil. Most do this by improving soil structure, but some change the chemical balance of the soil, or add plant nutrients.
Soils benefit most from the addition of organic matter, such as compost, leaf mold, sawdust, manure, and sewage sludge. As a side benefit, these amendments may contain plant nutrients; in most cases, however, they are added to improve the physical characteristics of the soil. Although some soils are naturally better than others, all benefit from regular additions of organic matter.
Gardening with organic matter is not new. Before chemists learned to make fertilizers and soil scientists discovered chemical means of making soils more hospitable to plant growth, farmers and home gardeners were working animal manures and cover crops into their soils.
Unless a garden site has been well maintained over a number of years, soil amendments will be a help in growing most vegetables, fruits, and ornamental plants. In all gardens, in order to maintain a high quality of plant growth, the soil should be amended regularly with organic matter to maintain and further improve its structure.
To improve garden soil and keep it in good condition, inorganic amendments may also be required. Lime, a mineral containing calcium, is used to sweeten sour soils. That’s another way of saying that it raises the pH by increasing the alkalinity of soils that are too acid. Conversely, sulfur in a variety of forms lowers soil pH, increasing acidity in alkaline soils.
Gypsum, a soft, slightly soluble mineral containing calcium and sulfur, is a valuable amendment for improving sodic soils (those containing sodium). The calcium in gypsum displaces the sodium in the soil and improves the structure and drainage of sodic soils.
Fertilizers as Amendments
Although not thought of as soil amendments, fertilizers should be considered whenever soil improvement is contemplated. In general, fertilizers are soil amendments with more than a few percent of plant nutrients. However, they do more than just supply plant nutrients.
Organic fertilizers like blood meal and fish emulsion add organic matter as well as nutrients to soil. Most fertilizers contain calcium, so tend to raise the pH of the soil. Some, called acid-reaction fertilizers, make the pH lower, or more acid.
Many amendments are also fertilizers. Compost and manure usually contain substantial amounts of nutrients, and may be all the fertilizer you need. For more information about fertilizers, see About Feeding Plants.
Amendments that are deficient in nitrogen not only don’t add nutrients, they can temporarily rob the soil of nutrients while they decompose to humus. This effect is called.