Organic Soil Amendments

Organic matter should be incorporated into the soil of most gardens to create the best environment for plants to grow. To add organic matter to an empty planting bed, put down a layer 2–3 inches thick, then mix it into the soil using a tiller, shovel, or digging fork. Incorporate it into soil at least three weeks before planting, or preferably earlier (up to six weeks before planting). In established planting areas, add organic matter as mulch around plantings each spring, before the rainy season; as it decomposes it will fertilize plants. Organic matter should be decomposed to the point of containing few or no recognizable source materials—in yard waste, that would mean you wouldn’t see any leaves or sticks.

Benefits

Organic matter improves the ability of soil to hold water and nutrients. It also adds nutrients, attracts earthworms, and supports beneficial bacteria and other microbiological activity in the soil. Organic matter is broken down by fungi, bacteria, molds, and creatures such as earthworms, sowbugs, and insect larvae. In the process, nutrients in the organic matter such as nitrogen are gradually changed into materials that your plants can use. The decomposition process occurs best in soil that is moist, warm, well aerated, and at the right pH.

Animal Manures

Composted animal manures are probably the best source of fertilizer and organic matter for the organic gardener, and they are certainly beneficial for gardens that aren't strictly organic. Manures vary in their content of plant nutrients, and are not always completely well-balanced fertilizers, so if you're not gardening organically, it can be a good idea to broadcast a complete garden fertilizer (such as 6-6-6) in addition to the manure. (Organic gardeners may want to substitute ground rock phosphate or raw bone meal.)

Animal manure can come from chickens, cows, pigs, sheep, horses, or rabbits and should always be composted before use in vegetable gardens to reduce risk to food safety. (Keep in mind that these products often contain high levels of phosphorus, which has been shown to cause water pollution, and should be applied carefully.) Never use cat or dog manure or human waste—there is a greater risk of these sources transmitting disease.

To apply manure before planting, use about 25 to 100 pounds per 100 square feet of garden. After planting, side-dress with up to 5 pounds per 100 square feet of row. If your garden has mulch, first rake the mulch away from the plants, and then re-cover the soil once you've added the manure.

Plant Manures

"Plant manure" is a term used to describe any organic matter made from decomposed plants that's used to enrich garden soil.

Fully broken-down compost, or humusCompost

Homemade compost (typically made of kitchen scraps and/or yard waste) is another excellent source of organic matter for garden soils. Compost is made by piling up organic material and allowing it to decompose long enough to yield a rich, humus-like material. Compost usually contains small amounts of nitrogen and potassium, but very little phosphorus; it typically does contain micronutrients.

You can make your own compost or even often get composted yard waste from your municipality. To make your own compost, you need to build or purchase a compost bin. For more information, read our article "Composting for the Home Gardener."

Vermicompost

Vermicomposting is the process of using worms to produce rich compost from kitchen waste. You can construct or buy worm bins in a variety of forms, and the red wiggler worms needed for your bin are easy to come by. You feed the worms vegetable and fruit scraps, coffee grounds, pasta, bread, cereal, tea bags, newspaper, and other paper products. They're efficient eaters and will convert these raw materials into gardening gold in the form of castings and "worm tea," a rich liquid that plants love.

If maintained properly, many worm bins can be kept inside or on a porch with no odor problems. You'll need to keep the habitat dark, cool, and moist (but not wet). Make sure it has air holes and keep it out of the sun—worm bins kept outside can easily cook the worms if not carefully tended.

Cover Crops

Cover crops are plants grown in a particular area in the garden as placeholders. To add to the organic content of soil, they are eventually incorporated into the soil instead of being harvested, making them “green manure.” Like other organic material, cover crops function as a slow-release fertilizer, since they release their nutrients gradually. You can plant cover crops in areas of the garden that you're letting lie fallow for a season, or between gardening seasons.

To be useful as a green manure, a plant should 1) grow rapidly, 2) produce abundant and succulent tops, and 3) grow well in the conditions of the site. The higher the moisture content of the plant, the more rapidly it will break down and contribute its nutrients to the soil. Look for plants that help control nematodes, and never use crops that encourage nematodes.

Good choices include legumes (they are nitrogen fixers and add to the positive microbial activity in the soil), marigold, and cool-season grasses. Till, plow, or spade your green crops into the soil when they have produced a lot of succulent top growth.