Key points

• Organic matter includes plants and animals that are alive, dead, or in some stage of decomposition.
• Organic matter is a major contributor to soil health. Most garden and landscape plants benefit from increases in soil organic matter.
• Soil amendments are materials applied to or mixed into the topsoil to change or change soil properties and improve plant growth. For example, compost improves soil structure and lime increases soil pH. Most soil amendments supply some plant nutrients and some compost and manure products double as fertilizers with a guaranteed nutrient analysis.

Organic matter 

• Soil organic matter (OM) is made up of living, dead, and decomposing plants, small animals, and microorganisms. Materials we think of as dead, like brown, dried-up leaves or banana peels, are teeming with microbial life. There can be a billion microorganisms in a teaspoon of compost or soil!
• Adding organic matter improves soils high in clay or sand.
• Soils high in OM retain more moisture, have a crumbly structure that resists soil compaction, and contain a reservoir of nutrients that are slowly released over time.
• OM improves soil aeration, water drainage, root growth, and biological activity.
• Compost and pine bark fines are good peat moss substitutes (a mined and non-renewable natural resource).
• Most garden and landscape plants perform best when the soil organic matter level is at least 2% (the goal for vegetable and flower beds should be 5%-10%). These soils are loose, easy to prepare for planting seeds and plants and have a large number of earthworms.
• Organic matter is measured by weight, not volume. Most soil testing labs include the organic matter test in their basic soil test.

An excerpt from the article “Organic Matter and Soil Amendments“, courtesy of the University of Maryland. Author: Jon Traunfeld, Director HGIC, Extension Specialist, Fruits and Vegetables