Chapter 1
Why Compost?
About compost—
Not much to be said,
Save:
It makes living
Of the dead.
Composting and organic gardening go together like peanut butter and jelly: They complement each other perfectly. Yet the feature that many gardeners consider to be compost’s prime asset—adding nutrients to fertilize the soil—is actually a fairly minor side dish in compost’s larger menu of benefits.
The surprising truth is that the fertility content of compost is extremely variable, depending on the ingredients used to make it, and fairly low. Nevertheless, compost is the most important ingredient in the organic garden. Compost contains the three main nutrients necessary for healthy plant growth: nitrogen, phosphorus, and potassium. Nitrogen (N) is primarily responsible for green leafy growth. Phosphorus (P) promotes healthy roots along with fruit and flower production. Potassium (K) is required for overall plant vigor. The amounts of nitrogen, phosphorus, and potassium a fertilizer contains is its NPK ratio, which is listed on all containers of fertilizer, both organic and chemical.
In general, compost contains about 1 to 4 percent nitrogen and more or less those same percentages of phosphorus and potassium, creating average nutrient ratios from 0.5-0.5-0.5 to 4-4-4. But compost also contains a wealth of micronutrients that are essential for good plant growth. At first glance, chemical fertilizers found at garden centers—which may have NPK ratings of 10-10-10 or 20-20-20—may seem much better than compost, but compost’s benefits go far beyond simple nutrient content.
Another way to get a sense of the relatively low nutrient content of compost is to compare fertilizer application rates used by conventional and organic farmers. A conventional farmer adds 150 pounds of a selected fertilizer per acre to produce field corn. An organic farmer, on the other hand, adds 10 to 20 tons of compost per acre to achieve similar NPK results in the first year. (It sounds like a lot, but 10 to 20 tons of compost doesn’t even cover the ground.) And while organic farmers and gardeners do get a nutrient benefit from applying compost, the major benefit from those tons of compost comes over time and can’t be compared to the thin dusting of synthetic fertilizers applied to conventional, nonorganic farm fields.
So, if compost isn’t primarily a fertilizer, what exactly is it and why is it so valuable?
1. Compost Promotes Soil Health
Compost is a case of good ingredients producing greater results. Carbon, microbes, and fungi are present in abundance, and the synergistic alchemy of these biological ingredients generates the soil’s vitality and resilience in organic gardens and on organic farms.
Feeding Soil to Feed Plants
Carbon provides food for the microbes and fungi living in the compost itself, but carbon also feeds the microorganisms living in the soil. These well-fed microbes, in turn, speed up nitrogen cycling in the soil, and it is this cycle that helps make essential nitrogen available to plants. As materials decompose in a compost pile or in the soil, microorganisms break down complex nitrogen compounds into the form that can be readily used by plants. (In addition, certain plants—including peas, beans, soybeans, and other legumes—can take advantage of the abundant nitrogen in the air. These plants are especially valuable as cover crops and as material to be incorporated into compost piles.)
While synthetic fertilizers give plants a quick shot of inorganic nitrogen, in comparison, biological nitrogen conversion happens quite slowly. The nitrogen cycle, fueled by compost, releases this essential nutrient gradually as plants need it, and soil that is well fed with compost produces the majority of the nitrogen plants need to thrive. Applying a burst of chemical fertilizer often supplies more nitrogen than the plants can use. When this happens, the unused nitrogen, which is extremely water soluble, washes down through the soil of the root zone into the groundwater, often polluting it. Fertilizer can also simply wash off the soil surface during rainstorms into nearby streams and waterways, resulting in more pollution.
On the other hand, the nitrogen produced by microbes in compost and in the soil is made available to plants more slowly and over a longer period. This more gradual application process allows plants to take up more consistent, smaller amounts of nitrogen throughout their growing cycle. A steady, slower supply permits stronger growth, which results in plants that are more resilient in times of stress.
This is just one example of how composted soil amendments offer advantages over chemical fertilizers.
Welcome Worms!
Improving Soil Structure
Compost is a fantastic soil amendment regardless of soil type, from clay soil to sandy soil and any kind in between. That’s because the tiny fungi supported by the organic matter in compost produce hyphae (threadlike filaments) and carbon-rich exudates (sugars) that bind soil particles together into small clumps known as aggregates. This biological process, which is fed by compost, builds rich, friable (crumbly) soil structure. The formation of aggregates also binds together essential soil ingredients, such as carbon and minerals, and keeps them accessible to plant roots.
In the process, spaces called pores are created among the aggregates. Ideal garden soil has a granular structure, with plenty of large pores to hold air and equally essential small pores to retain water. Having a balance of large and small pores allows a soil to “breathe” and also facilitates water circulation and thus root growth.
Compost can be used to improve all types of soil because of its role in creating aggregates and good soil structure. Clay soil, which is heavy and easily waterlogged, benefits because aggregates that form help to create larger pores that fill with air as water drains out after a rain. Unamended sandy soil, on the other hand, lets water drain away too quickly, so compost helps build structure in it to enable it to hold onto nutrients and moisture.
It’s important to keep in mind that building aggregates is a process. If feeding the soil fungi and other microorganisms that create the aggregates stops, the soil building process slows down or halts altogether. When this happens, soil structure inevitably suffers.
Compost and Erosion
Increasing Moisture Retention
Soils that have been amended with compost also hold more moisture than ones that are low in organic matter. Finished compost is essentially humus (soil rich in organics), and 100 pounds of humus can hold 195 pounds of water. Add that to the fact that compost improves soil structure by creating pores that help hold water, and the result is soil with improved moisture retention. But that isn’t all: In the process, the compost also acts like a sponge, wicking up moisture (and any available nitrogen that’s dissolved in that water) and keeping it in the root zone where plants can continue to absorb it between rainfalls. So in several significant ways, organic compost helps shield crops from both drought and nutrient stress longer and more successfully than synthetic fertilizers do.
2. Compost Protects Plants
Compost’s benefits to plants go beyond soil improvement. Mycorrhiza fungi in compost colonize among plant roots, creating a symbiotic relationship: The fungi take sugars from the plant and, in exchange, cause thin fungal strands called hyphae to extend out from the plant roots. These long, delicate filaments increase the surface area of each root, allowing it to scavenge more water and nutrients from the soil—particularly phosphorus, which aids photosynthesis and in turn promotes robust plants that are more resistant to pests and disease.
Research has shown that amending soil with compost can help protect plants against a host of diseases, including phytophthora root rot, damping-off (which causes seedlings to keel over and die), early blight, gummy stem blight in squash plants, and diseases in turfgrass. Compost also protects plants against root-knot nematode damage.
3. Compost Protects the Earth
The primary reason that composting is good for the environment is because maintaining a compost pile keeps carbon and other nutrients recycling on the property, without excessive loss or gain to the surrounding environment. This eliminates—or at least greatly reduces—the garden’s carbon footprint, which is the amount of damaging greenhouse gases (specifically carbon dioxide) emitted during a given period. Using a compost pile to recycle kitchen and yard waste can benefit the household budget, too, by reducing or eliminating the cost of trash bags and cans and trash pickup fees.
Making compost also eliminates the need to buy expensive soil amendments—and deal with the plastic bags they’re packaged in—and saves the fuel, time, and energy it takes for them to be produced and for the gardener to drive to and purchase them at a garden center or big-box store.
Composting is one of the most basic forms of conservation. In today’s throwaway society, in which eventually nearly everything seems to make its way to a landfill, what better way to practice conservation on a daily basis than to employ a humble compost heap? A large percentage of the waste produced inside the average home is compostable, as are weeds, grass clippings, fallen leaves, and other yard debris. Composting is the ultimate way to recycle all that waste. Not only does composting return materials to the same soil that produced them, it also plays a key role in creating a long-term, sustainable environment for healthy plant growth. All these factors add up to a process that greatly benefits our planet.
