Compost is finished when it’s a dark, rich color, crumbles easily, and you can’t pick out any of the original ingredients. It should have a sweet, earthy smell. If it’s too stringy or lumpy, it may need more time. If this is the first time you’ve tried making compost, keep in mind that the amount of time can really vary. It can take anywhere from three to 12 months to produce compost. Decomposition depends on a number of things including temperature, what organic matter you’ve filled your bin with, type of compost bin used, how fine the waste material was chopped, how often you’ve turned it, and more (see The Biology of Composting).
Once you have achieved finished compost, you can add it to the soil any time of year without the fear of burning plants or polluting water. The benefits of using compost are numerous. It builds good soil structure; enables soil to retain nutrients, water, and air; protects against drought; helps maintain a neutral pH, and protects plants from many diseases commonly found in the garden. It also feeds earthworms and other microbial life in the soil. In general, it doesn’t matter what kind of soil you have. All soils can be improved with the addition of compost.
One easy way to apply compost is to mulch with it. Spread the compost in a thick layer on top of exposed soil. Worms and other creatures will help the compost meld with the soil. Mulching is not only an easy way to apply compost but also keeps down weeds and helps your soil retain moisture. Don’t have a garden? You can also use compost when potting indoor plants. Use seven parts soil to three parts compost to two parts sand.
• Compost contains nutrients that your plants need for optimum growth, such as nitrogen, phosphorus, and potassium. And it’s an especially good supplier of micronutrients that are needed in small quantities and are sometimes overlooked by gardeners, such as boron, cobalt, copper, iodine, iron, manganese, molybdenum, and zinc. The more varied the materials used to make the compost, the greater the variety of nutrients your compost will provide. In some situations, you will not even need to fertilize soil enriched with compost.
• Nutrients are released at the rate your plants need them. In early spring, as your plants are slowly starting their growth, the microoganisms in compost are slowly releasing nutrients. As the weather warms up and your plants begin rapid growth, the microorganisms also work faster, releasing more food for your plants. Isn’t nature wonderful?
• The organic matter in compost binds with soil particles (sand, silt, and clay) to form small aggregates, or crumbs. Crumbly soil is said to have good structure, as opposed to sand, which has poor structure because it’s too coarse to form aggregates, or clay, which can act like cement when wet. These aggregates hold water on their surfaces, making it available to your plants as they need it. As aggregates form, more spaces are created for oxygen, an essential for good root growth. At the same time, the soil spaces form channels for excess water to percolate through the soil, improving drainage.
• Increases water-holding capacity of soil. Compost can hold an amount of water equal to 200 percent of its dry weight, compared to 20 percent for a low-humus soil.
• Acts as an inoculant to your soil, adding microorganisms and larger creatures such as earthworms and insects, which are nature’s soil builders. The compost environment is teeming with life, and all soils can benefit from such a rejuvenation.
• Neutralizes various soil toxins and metals, such as cadmium and lead, by bonding with them so they can’t be taken up by plants.
• Acts as a pH buffer so plants are less dependent on a specific soil pH. The earthworms in the compost help in this process, because in passing organic matter through their bodies they modify the pH of the soil. And you can lower the pH of your soil by adding compost made from acidic raw materials, such as oak or beech leaves, sawdust, and pine needles.