From University of Maryland Extension Service
What is organic matter?
Plants and animals that are alive, dead, or in some stage of decomposition. The stuff 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 or compost or soil!
Why is it 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 vegetable 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 break down 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 will 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 vegetable gardener should have a way of recycling organic wastes into compost. (For more information read University of Maryland Extension’s HG 35, Backyard Composting.) Contact your county/city government to see if compost is available at your local landfill.
Yes, you can purchase compost by the bag or cubic yard (pick-up truck load). Examples are available at every garden center, and mushroom compost is especially effective. Homemade or purchased compost can be added any time of year and can be used as a top-dressing or mulch during the growing season.
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..
Manure—animal manures (goat, 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 uncomposted animal manures may contain human pathogens. Manures are considered fully composted when the pile or windrow reaches at least 130º F for three 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 uncomposted. Here’s the USDA’s National Organic Program’s standard for using uncomposted animal manure: Apply no less than 90 days prior to harvest if there’s no contact between crop and soil, (e.g., staked tomatoes) or 120 days prior to harvest if the crop is in contact with soil, (e.g., cabbage). Do not apply uncomposted manure after crops are established. Fall application and incorporation is recommended for home gardeners.
- 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.
- 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 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 one inch each year. It takes eight cubic feet of compost to cover a 100 sq. ft. garden to a depth of one inch. The goal is to have organic matter comprise 25 to 30% of the top eight inches of soil by volume.
© 2015 – University of Maryland Extension Service. All rights reserved. Originally posted here – https://extension.umd.edu/growit/food-gardening-101/step-3-prepare-your-soil