The strict definition of a soil amendment is anything added and mixed into the soil. There are many types of soil amendments that can be added, and each is used for its own reason. Some improve the tillage of the soil while others improve the nutrient content. In each of those categories, some things are better than others.
Soil Amendments for Tillage and Microorganism Growth
Tilth is the ability of the soil to be tilled and ultimately to allow seeds to germinate and roots to spread. Certain soil amendments are added to enhance this aspect of soil. Examples include compost and manures. Depending upon the composition of the compost and the age and type of manure, these soil amendments can also add nutritional value. But their greatest asset is the organic matter they add to the soil which nurtures microorganisms, which in turn convert nutrients already in the soil to a kind more readily usable by plants.
Fertilizing Soil Amendments
Fertilizer by legal definition must supply a minimum amount of specific minerals. The basic minerals found in most fertilizers are nitrogen, phosphorus and potash. The percentages of each are designated on the package in a series of three numbers such as 10-10-10 and are listed in the same order as given here. These three add the basic mineral requirements of most plant life, in varying degrees, depending on needs of the soil and the plants to be grown there. Many fertilizers contain other trace elements as well.
Inorganic Soil Amendments
Inorganic soil amendments are those which did not originate as living things. Examples include vermiculite, perlite, sand and pea gravel. These are added when the need exists to change the amount of water that the soil is able to hold or drain. Vermiculite and perlite are added to soils that need better water-retention capabilities. Sand and pea gravel are added to soil that retains too much water.
Organic Soil Amendments
Organic soil amendments come from something that either is or was alive. Examples include wood chips, hay or straw, grass clippings, wood ash and many others. Improving the organic matter in soil can take a number of years of careful maintenance but the desired results of improved soil drainage and aeration are worth the time spent. A general rule of thumb is that your soil's organic matter content should be 3 percent or more. A soil test from your local extension office can help you determine yours.
Factors in Choosing Soil Amendments
Soil amendments should not be applied without first knowing the soil's composition and the plants that are to be grown in it. A sample test from your local extension office can help get this started. Consider how long the amendment will last in the soil, the soil texture, the soil salinity and the potential sensitivity the desired plants may have to it, and the pH of the amendment. Many plants are salt or pH sensitive and can be harmed by adding the wrong type of amendment.