Estimated read time: 4-5 minutes
This archived news story is available only for your personal, non-commercial use. Information in the story may be outdated or superseded by additional information. Reading or replaying the story in its archived form does not constitute a republication of the story.
The term soil amendment refers to any material mixed into a soil. Mulch refers to material placed on the soil surface. By legal definition, soil amendments make no legal claims about nutrient content or other helpful (or harmful) effects it will have on the soil and plant growth. In Utah, the term compost is also unregulated, and can refer to any soil amendment regardless of microorganism activity.
By legal definition, the term fertilizer refers to a soil amendment that guarantees the minimum percentages of nutrients (at least the minimum percentage of nitrogen, phosphate and potash).An organic fertilizer refers to a soil amendment derived from natural sources that guarantees the minimum percentages of nitrogen, phosphate, and potash.
These should not be confused with products approved for use by the USDA National Organic Program. The federal Certified Organic Label, USDA Organic, allows only certain regulated products as listed by the Organic Materials Review Institute (OMRI). For additional information on certified organic soil amendments and fertilizers, visit the Web at www.omri.org.
Many gardeners apply organic soil amendments, such as compost or manure, which most often do not meet the legal requirements as a fertilizer and generally add small quantities of plant nutrients.
Routine applications of organic matter are an essential component of gardening and soil management. Organic matter improves the water and nutrient holding capacity of coarse-textured sandy soil.
On a fine-textured clay soil, the organic matter glues the tiny clay particles into larger chunks or aggregates creating large pore space. This improves water infiltration and drainage, air infiltration (often the most limiting aspect of plant growth), and allows for deeper rooting depths (allowing the plant to tap a larger supply of water and nutrients).
Application rate depends on the product. Excessive salt levels are common with manure, biosolids and compost made with manure or biosolids. On these products do not exceed 1 inch per year (cultivated into the upper 6 to 8 inches of soil) unless the product is know, by soil test, to be low in salts.
Cultivate or hand-turn the organic matter thoroughly into the soil. Never leave it in chunks as this will interfere with root growth and water movement.
Determine the quality of organic amendments by both visual evaluation and lab testing.
Visual Evaluation
Color - dark brown to black.
Odor - earthy, no ammonia smell.
Texture - less than ½ inch particle size; lawn top dressing less than ¼ inch.
Foreign materials - less than 1 percent and smaller than ½ inch size.
Uniformity within the batch.
Consistency between different batches.
Raw materials - concern of heavy metals (biosolids), human pathogens (manure), and salts (manure and biosolids).
Weed seeds - test by germinating some material.
Laboratory Testing C:N ratio - less than 20 to 1; 10-12 to 1 is better. Ash content - This measurement of the mineral portion after the organic matter is burned off will determine if soil was a primary part of the mix.
- 20 to 30 percent common;
- Keep below 50 percent;
- If greater than 50-60 percent it probably contains a lot of soil.
Bulk density - less than 1.0 gm/cc.
pH - 6.0 to 7.8
Salts - acceptable levels depend on use.
- Potting grade: - Potting media amendment: - Top dressing: - Soil amendment in a low salt soil:
Sodium - sodium adsorption ratio less than 13 percent.
Ammonium - less than 1/3 of total nitrogen. If higher, it may not be finished composting.
Heavy metals - A concern with biosolids but regulated by application permits.
Pesticide residue - Rarely a problem since they break down in composting. Long-term residues of chlordane have shown up in some samples.
Pathogens - E-coli and other human pathogens are a potential in manure.
Nutrient content - varies greatly from product to product.
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 manmade.
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.
Perlite and vermiculite are common inorganic amendments used in potting soils and planter mixes. Vermiculite is made from heat expanded silica (mica). It is used to increase pore space and has a high water holding capacity. Perlite is made from heat expanded volcanic rock. It is used to increase pore space and has a low water holding capacity.
Sphagnum peat is a good soil amendment, especially for sandy soils, which will retain more water after sphagnum peat application. Sphagnum peat is generally acid (i.e., low pH) and can help gardeners grow plants that require a more acidic soil.
Sphagnum peat is harvested from bogs in Canada and the northern United States. The bogs can be revegetated after harvest and grow back relatively quickly in this moist environment. In recent years however, harvest rates have become so high that it is raising questions on renewability.
Biosolids are a way to add slow release nutrients and organic matter to soil. They are available from some communities or sewer treatment districts in bulk and from garden stores in bags. Some biosolids are extremely high in salts.
Larry Sagers
Horticultural Specialist
Utah State University Extension Service
Thanksgiving Point Office
© All Rights Reserved
Adapted from Colorado State University Extension Service Fact Sheet Soil Amendments by D. Whiting, C. Wilson, and A. Card1
