Adding Organic Matter

You can think of organic matter simply as materials that were once living. Grass clippings, peat moss, leaves, and wood chips are all examples of organic matter. Compost is organic matter that has decomposed, and is the best material to add to garden soil. (Adding fresh, or undecomposed, organic matter to soil can cause temporary nutrient deficiencies as soil microorganisms break down the materials.)

Organic matter provides food for soil life, including microscopic life like bacteria and fungi, and visible life like earthworms and insects. Why would you want to feed soil life? Because although a few microbes and insects can harm plants, by far most soil life is beneficial. Earthworms aerate the soil and their castings are rich in nutrients. Also, many bacteria and fungi break down organic matter and release the nutrients in forms plants can use. Finally, various beneficial microbes compete with harmful plant pathogens, keeping their numbers in check.

Some soil test results will report the percentage of organic matter in the test sample; a good number to shoot for is 5%. But organic matter isn’t static; by its nature it changes over time as soil microorganisms break down complex molecules. A generous addition of organic matter at planting time will get your garden off to a good start; subsequent annual applications of compost and organic mulches will replenish the supply.

The best (and cheapest) organic matter is compost that you’ve made yourself, using grass clippings, vegetable scraps, leaves, and other garden debris. Some municipalities collect leaves and yard waste, compost it, then bag it up and distribute it.