University of Maryland Extension

Add Organic Matter to Your Soil

What is organic matter?

Organic matter includes plants and animals that are alive, dead, or in some stage of decomposition. The material we think of as dead (e.g. brown, dried up leaves) is teeming with microbial life. There may be a billion living microorganisms in a teaspoon of compost or soil!

Why is organic matter important?

Organic matter is the key to improving soil quality which, in turn, leads to healthy, productive plants. It improves the structure of soils that are high in clay or sand so that roots can better grow and take advantage of available water, air, and nutrients.

The concept “feed the soil and the soil will feed your plants” is very important for gardeners. If you feed your soil different types of organic matter on a regular basis you provide food for soil-dwelling organisms. The vast majority of these- bacteria and fungi- cannot be seen without a microscope. They breakdown organic materials, consume each other, and cause the release of nutrients that roots can pick up.

Your soil is improved with every addition of organic matter. You are building up a reservoir of slowly released nutrients that increase your garden’s productivity over time. You may also need to use fertilizers to make sure that your plants have the nutrients they most need (e.g. nitrogen) when they need it. But your reliance on organic or synthetic fertilizers will probably decrease as your organic matter content increases.

What should I use and where do I get it?

Compost- you can make your own from leaves, grass clippings, kitchen scraps, farm manure (no pet waste), and other materials. Every gardener should have a way of recycling organic wastes into compost. For more information read (PDF) HG 35 Backyard Composting. Contact your county/city government to see if compost is available at your local landfill.

You can also purchase compost by the bag or cubic yard (pick-up truck load). Some examples are LeafGro® and other composted yard waste products, and mushroom compost from PA. Home-made or purchased compost can be added any time of year and can be used as a top-dressing or mulch during the growing season.

The risk from minute amounts of pesticides being found in commercially available compost is extremely low. Herbicides are short-lived in soil and compost and rarely show up as a problem. Producers and sellers have their composts tested regularly and should be able to provide result reports.  However, a few long-residual herbicides (aminopyralid and clopyralid) have contaminted manure, compost, hay, and grass clippings in recent years.

Leaves and grass clippings- from your own yard or neighborhood. Shredded leaves are best because they rot faster than whole leaves. Spread them out on top of your garden in the fall and turn them under in the spring.  Grass clippings (no herbicides) can be used as a mulch around vegetable plants or add them to your compost pile.

Cover crops- growing  plants year-round is a great way to store crbon in the soil and support the soil food web. Cover crops reduce soil erosion and recycle nutrients which can reduce the need for fertilizers. They can also be used as a mulch after cutting and are an essential part of no-till faming and gardening.

Manure- animal manures (sheep, cow, horse, chicken) may be available free of charge in your community. They are very good for improving soil quality and add valuable nutrients to the soil.

There are some risks to consider. Fresh manure can burn plant roots, and un-composted animal manures may contain human pathogens. Manures are considered fully composted when a static, aerated pile reaches at least 131ºF for 3 consecutive days. This kills most plant and human diseases, and weed seeds. Most farmers with animals do not actively compost and monitor their manure to this standard. So, it’s best to treat any animal manure you can locate as un-composted. Fall application and incorporation is recommended for home gardeners. 

More manure tips:

  • Composted animal manures have a somewhat higher nutrient content than plant-based compost
  • Light incorporation of manure is desirable to prevent nutrients from washing away
  • Never use dog or cat manures in your vegetable garden
  • Horse manure may contain many weed seeds; be prepared to control weed growth early on
  • Make compost teas from plant-based composts only
  • Wash all produce thoroughly after harvest

Other sources- kitchen scraps buried in holes or trenches in your garden soil; plant roots- cut the tops of plants that have finished producing and leave the roots in place to rot; cover crops, decomposed mulches.

How much organic matter should I add?

Fresh organic materials lose more than half their volume by the time they are fully decomposed. The best option, if available, is to add compost to the soil. After a few years of large additions, you can decrease the amount to 1 in. each year. It takes 8.33 cubic feet of compost to cover a 100 sq. ft. garden to a depth of one inch. For flower and vegetable beds, the goal is to have organic matter comprise 25 to 30% of the top 8 inches of soil by volume.

soil conditioner
Home-made compost in bushel basket, leaves
in wheelbarrow, commercial bag of LeafGro,
bag on right is shredded leaves and grass
clippings picked up with a mulching mower.

See also: Soil for Containers

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