What Is the Difference Between a Soil Amendment & a Fertilizer?

Soil Amendments

• To improve the soil's water-holding capacity, drainage or aeration, organic soil amendments are mixed thoroughly into the soil. Common organic materials include sphagnum peat, wood chips, sawdust, grass clippings, compost, manure, straw, hay and biosolids. Biosolids are produced as byproducts of sewage treatment plants. Chunks of tires, pea gravel, sand, vermiculite, perlite, and gypsum are examples of inorganic soil amendments.

Selection of Soil Amendments

• The choice of the soil amendment depends on a number of factors including the cost, availability, the transportation cost and the benefits to the soil. Soils vary greatly in texture and fertility and the soil amendment must complement the soil. Care is required because some amendments add extra salt, weed seeds and human or plant pathogens to the soil.

Fertilizers

• The minimum amount of three important macronutrients for plants is included on each fertilizer label. The three-number code indicates the amount of nitrogen, phosphorus and potassium provided by the fertilizer. For example, the fertilizer grade for ammonium nitrate is 34-0-0. This means the fertilizer contains 34 percent nitrogen, and no phosphorus or potassium. A wide array of fertilizers provides any of the essential macronutrients such as sulfur or calcium. They also provide micronutrients such as boron, iron, zinc and manganese required by plants.

Types of Fertilizers

• Similarly to amendments, fertilizers are divided into organic and inorganic categories. Organic fertilizers include aged livestock or poultry manure, sewage sludge, bone meal, fish emulsion and blood meal. The chemically manufactured fertilizers include ammonium nitrate, ammonium sulfate and ammonium phosphate. Inorganic fertilizers are designed to be quick or slow release fertilizers. Quick release fertilizers are immediately available to plants while slow release fertilizers release the nutrients over a period of time. Inorganic fertilizers are often purchased as granules or liquids.