Inexpensive to produce and easy to use, backyard compost serves primarily as a soil conditioner, whether it’s spread in a layer on the soil surface or is dug in. A garden soil regularly amended with compost is better able to hold air and water, drains more efficiently, and contains a nutrient reserve that plants can draw on. As a result, almost all plants growing in compost-amended soil are healthier and more productive than they would be without compost. The exceptions are native plants that prefer unamended soil and some drought-resistant plants that may be damaged by the addition of such water-retaining material.
This section describes some of the major benefits of using compost. When you add them up, you’ll find that compost is one of the most valuable resources home gardeners have for maintaining a healthy landscape. The joy of digging in rich, workable soil and tending healthy, flourishing plants will more than compensate for the effort you expend in making the compost and distributing it in the landscape.
Improved Soil Structure
When a source of organic matter, such as compost, is worked into the soil, either by people or earthworms, it dramatically improves the soil structure—the arrangement of sand, silt, and clay particles that make up soil.
Soil structure refers to the way soil particles aggragate, or clump together. The addition of humus, or decomposed organic matter, improves the structure by helping clay particles cling together in small crumbs, giving the soil a granulated, crumbly property described as friable.
Clay soil has miniscule soil particles the size of bacteria. Without sufficient structure, they pack together in a mass like potter’s clay. The pores are so small that water drains from it only slowly, and it holds very little air. Humus added to clay soil makes the particles aggragate into crumbs. The pore spaces between these crumbs are large enough that water drains and air enters easily.
Sandy soil has large particles and large pore space between them. As a result, the soil drains quickly, but doesn’t hold enough water. Humus fills the soil pores with its sponge-like mass, retaining water.
Increased Microbial Activity
Even though the decomposition process in compost has slowed down, billions of microorganisms remain in every handful of the finished material. When you incorporate compost into the soil, you add the microorganisms as well. They reinforce the resident microbial population, increasing their activity.
As the microorganisms continue to decompose organic matter in the soil, they contribute to the chemical reactions that benefit plants. They convert nitrogen, phosphorus, potassium, calcium, and other nutrients in organic materials into forms that plants can absorb. Some of the microorganisms are nitrogen-fixing bacteria, which take nitrogen from the air and make it accessible to plants. Other microbes manufacture antibiotics that protect plants from various diseases.
Like other sources of decomposed organic matter, compost contributes valuable nutrients to the soil. If you’re using compost as the mainstay of your fertilizing program, think in terms of adding inches, not a fraction of an inch. Most home composters don’t produce enough finished material to rely on it as their only fertilizer source. Still, whatever can be added provides a valuable reservoir of nutrients that plants can draw on over time.
The nutrient content of a given batch of finished compost is impossible to predict, since it depends on such variables as the raw materials, the carbon-nitrogen ratio of the pile, and whether any amendments are added. For example, compost made from hay contains more nitrogen than compost made from straw, and composted manure contains significantly more nitrogen than either.
Fresh compost that hasn’t been exposed to rain contains nitrogen, phosphorus, potassium, and many of the trace minerals needed by plants. Compost that has been left out in the weather—or even stored covered for more than six months—has lost some nitrogen and potassium, although it still retains most of the other nutrients. About one half of the nutrients in compost are released for plant use during the first year. One half of the remaining nutrients are released during the second year, and so on after that.
Compost also serves as a ready food source for the billions of soil microorganisms that convert the soluble compounds in compost into a form that can be absorbed by plant roots. Many compounds in compost supply the microorganisms with oxygen, which is essential to their reproduction.
In addition, compost contains certain acids, which are formed as by-products of decomposition. These acids help to break down some of the rock particles in the soil, releasing nutrients that would otherwise be unavailable to plants.
Improved Soil Chemistry
Soils exhibit a tremendous range of conditions from nutrient excesses or deficiencies to extreme alkalinity or acidity. Organic matter, such as compost, fosters a soil chemistry that avoids these extremes.
For example, organic matter increases the buffering capacity of soil—its ability to resist a change in pH when acidic or alkaline materials are added. In addition, organic matter bonds to certain micronutrients—such as iron, zinc, copper, and manganese—increasing their availability to plants. In soils low in organic matter, these micronutrients are often tied up. Even though the nutrient is present in the soil, plants may show symptoms of a deficiency, such as yellowing or poor growth.
Moderation of Soil Temperature
Plants are sensitive to the temperature of the soil in which their roots are growing. Regardless of the air temperature, seeds germinate only when the soil temperature warms up to a certain level. For instance, peas germinate when the soil is at least 45 degrees, and no sooner.
The root systems of most plants grow best when the soil temperature is between 65 degrees and 85 degrees When it rises above 85 degrees, most plants simply stop growing; they just sit there until the soil cools. How does this relate to compost? Soil amended with compost typically has a darker color than unamended soil, since most compost is dark brown or black. A dark soil absorbs more heat from the sun than a light soil. Consequently, garden soil amended with generous amounts of compost tends to heat up faster in spring, stimulating plants to start growing sooner in the season.
During hot weather, soil mulched with 1 to 2 inches of compost remains cooler than soil with no organic mulch. The mulch serves as a blanket, protecting the underlying soil from the sun. If soil temperature can be kept below 85 degrees—that point at which plant growth stops—the plants will perform better in the summer heat.
Improved Plant Health
Most plants growing in soil amended with a good source of organic matter, such as compost, will look better and perform better than plants in unamended soil. Here’s a brief look at how organic matter added to the soil ultimately yields sturdier, healthier plants. If you regularly add compost to your soil, you won’t have to water and fertilize as often, and you’ll spend less time struggling with plant problems.
A soil well amended with organic matter fosters a moderate but steady rate of root growth for most plants. Uniform growth is a result of the consistent supply of water, air, and nutrients that the amendment helps supply over time.
A well-amended soil holds moisture, which plants can absorb according to their particular needs. Plants with steady access to water are healthier and more vigorous than plants with a fluctuating water supply.
Research indicates that a soil to which compost is added regularly tends to produce plants with fewer insect and disease problems. Two reasons explain this phenomenon. First, the compost encourages a more active and larger population of beneficial soil microorganisms, which control harmful microorganisms. Second, compost-amended soil fosters healthy plant growth, and healthy plants are better able to resist pests.
When compost is used as a mulch, it helps control the spread of fungus diseases by keeping water from splashing disease spores onto plants. For this reason and because studies indicate that compost-rich soil discourages many fungus diseases, mixing compost right into the soil around plants vulnerable to those diseases can protect the health of your garden.
Some composts, particularly those made from animal manures, tend to suppress harmful root-invading fungi. Research has shown that harmful soil fungi responsible for diseases such as root rots and damping-off are controlled by other soil organisms. The presence of microbe-rich compost ensures that large populations of beneficial bacteria and fungi are present, thereby reducing the likelihood of disease problems.
Other research has revealed that, in many cases, compost made the slow, passive way has better disease-resisting ability than actively managed compost, which is produced more quickly and under higher temperatures. The reason is that high temperatures kill beneficial organisms. If the temperature doesn’t exceed 140 degrees most of the desirable microbes can survive. However, the compost may become sterile if the temperature goes much higher—for example, above 160 degrees. You can rescue a batch of high-temperature compost by mixing it with small amounts of mature compost made at lower temperatures; the resulting material will offer disease resistance.