By Val Whitmyre, U. C. Master Gardener
Because soil and compost are interconnected, knowing the characteristics of soil will help you understand why compost is an important garden component.
The three particles that make up soil are clay, silt and sand. Soil composed of roughly equal parts of all three is called loam.
Clay is sticky when moist and holds a shape. Because it is made of tightly packed, microscopic flat slices, water penetrates with difficulty. Once it does penetrate, you have a mud ball that eventually dries as hard as a rock. But clay retains moisture and nutrients long enough to feed tiny roots.
A sand particle is thousands of times larger than a clay particle. It has sharp edges and feels gritty, and water runs right through it. Tiny feeder roots love to cling to gritty things, so sand makes a good planting medium for seedlings.
Silt falls between the other two in size. It is slippery smooth and holds nutrients but lacks structure.
These three particles cling together in tiny clusters called aggregates that are responsible for retaining moisture. The larger spaces between the aggregates allow for drainage.
The other part of soil is organic matter, the billions of microorganisms that are just waiting for new organic matter to digest and excrete. These microorganisms reproduce at an alarming rate. One-quarter teaspoon of soil holds an estimated one billion microorganisms, from bacteria and fungi to a part-bacteria, part-fungi half-breed called actinomycetes. These last are the silvery ribbons you see in the soil, and they contribute the sweet woodsy smell.
Compost is nature's way of recycling, returning nutrients to the soil and turning dead material into food for the microorganisms. Composting—the natural decomposition of organic matter—has been happening since there has been soil. The end product is humus, the dark, rich, crumbly material beneath our feet in dense mountain forests. When we hike forest trails, we inhale the fresh, woodsy aroma of humus.
In Latin, compost means "to bring together," and that is how we use it. We mix it into clay soil to aerate it, giving the microorganisms room to be activated. It binds sand and silt together so that water can keep plant roots moist and nurtured.
To make compost, you need nitrogen, carbon, oxygen and water. We composters refer to nitrogen-rich material as "greens" and carbon-rich material as "browns." Lawn clippings and vegetable matter would count as greens; dried leaves would count as browns. The more variety of materials you use, the more varied the nutrients returned to the soil.
To make "hot" compost that decomposes quickly, chop your materials into small pieces. The smaller the pieces, the faster the compost happens. Aim for pieces between 1/8 inch and 1-1/2 inches in size. Mix equal parts of greens and browns, then put them in a bin with kitchen scraps in the middle. Water until the contents feel as moist as a wrung-out sponge.
Turn the pile about every three days with a four-tine pitchfork. Work from the outside in, like mixing a cake. If it dries out, add more water. After a few days, you should feel heat in the center. The tiny organisms respire as they work, generating heat just as we do. This heat is responsible for reducing the pile quickly.
A one-cubic-yard pile (three feet by three feet by three feet) is a good backyard size. Every time you mix the pile, the temperature will drop, but mixing gives the microorganisms more new material to digest. When they have no more to digest, they will produce no more heat, and the compost is said to be finished. It will look like dark, rich soil with a crumbly texture. It should have a fresh, rich smell or no smell at all.
Using this rapid method, the compost should be ready in a few weeks. If you occasionally add material to a pile or don't turn it very often, the breakdown may take a few months. If you don't turn it all, the pile will still eventually decompose but much more slowly. Use the method that fits your lifestyle.
Meat, dairy products and pet feces should not go into the pile. They attract flies and rodents and promote diseases. For a healthy compost pile, don't add corn cobs or large pieces of wood, diseased plants, weeds gone to seed, colored newsprint, charcoal, mud, sand or gravel. Never spray chemicals into the pile.
Locate your pile away from a fence in dappled shade with easy water access. When your compost is finished, use a 3/8-inch screen large enough to fit over a wheelbarrow to screen out large particles.
Compost is not a fertilizer, but it is the great enabler. It balances the soil, strengthens roots, increases flower and fruit production, reduces water run-off and erosion, eliminates the need for petrochemical fertilizers and attracts beneficial creatures to the garden.