A soil amendment is any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. The goal is to provide a better environment for roots. Careful soil sampling is essential for an accurate fertilizer or amendment recommendation. A soil sample must reflect the overall or average fertility of a yard or field, so analyses, interpretations and recommendations accurately represent the nutrient or mineral status of the soil. An accurate evaluation will result in more efficient fertilizer use, which can increase yields, reduce costs and potentially reduce environmental pollution. There are at least four factors to consider in selecting a soil amendment: how long the amendment will last in the soil, soil texture, soil salinity and plant sensitivities to salts, and salt content and pH of the amendment. A soil test can determine the salt content, pH and makeup of organic amendments. The quality of bulk organic amendments for landscape uses then can be determined. The amendment you choose depends on the gardening goal:
--If you are trying to improve soil physical properties quickly, choose an amendment that decomposes rapidly.
--If you want a long-lasting improvement to your soil, choose an amendment that decomposes slowly.
--If you want a quick improvement that lasts a long time, choose a combination of amendments.
Here are some general guidelines for soil amendments:
--Soil amendments improve the physical properties of soils.
--Amendments are mixed into the soil; mulches are placed on the soil surface.
--The best soil amendments increase water- and nutrient-holding capacity and improve aeration and water infiltration.
--Wood products can tie up nitrogen in the soil.
--Sphagnum peat is superior to Colorado mountain peat.
--When using biosolids, choose Grade 1 biosolids.
--To do its work, an amendment must be thoroughly mixed into the soil. If it is merely buried, its effectiveness is reduced, and it will interfere with water and air movement and root growth.
There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that is or was alive. Inorganic amendments, on the other hand, are either mined or man-made. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, manure, biosolids, sawdust and wood ash. Inorganic amendments include vermiculite, perlite, tire chunks, pea gravel and sand. Not all of the above are recommended by Colorado State University. These are merely examples. Wood ash, an organic amendment, is high in both pH and salt. It can magnify common Colorado soil problems and should not be used as a soil amendment. Don't add sand to clay soil -- this creates a soil structure similar to concrete. Organic amendments increase soil organic matter content and offer many benefits. Organic matter improves soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments contain plant nutrients and act as organic fertilizers. Organic matter also is an important energy source for bacteria, fungi and earthworms that live in the soil. For more information on soil amendments, see the fact sheet: Choosing a soil amendment.

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