Compost and Compost Tea Boost Soil Vitality
by Cindy Salter

Just about every gardener knows something about compost: how rich it looks and smells; how it replenishes the soil with organic matter, which is so essential to plant health; how it is teeming with beneficial microorganisms; how it helps conserve moisture and moderate soil temperature extremes; and how fun it is to make!

Long before the advent of synthetic fertilizers and pesticides, farmers and gardeners relied on compost and other sources of organic matter such as animal manure or cover crops for plant nutrition. Now as then, many organic growers rely on compost as an integral part of their fertility program.

A relatively new twist on the use of compost is compost tea—a liquid extract of compost produced by "brewing" compost in water under controlled environmental conditions. The process consists of suspending compost in a vessel of water (70–75°F), adding a microbial food source, and aerating the solution continuously for up to 24 hours or more.

These conditions represent an ideal environment for extracting and growing beneficial microorganisms and for extracting the nutrients, micronutrients, and other plant-beneficial organic compounds present in compost. The finished compost tea is a rich, dark-brown liquid that has a pleasant earthy smell.

Compost tea should be used soon after it is made, since it is teeming with live microorganisms. Ideally, the tea should be applied within four hours of removing it from the brewer. Any tea that is not used immediately should be kept reasonably cool and out of direct sunlight in an open-top container. Periodic stirring or continued aeration will prolong its life even further.

Compost tea can be applied to the soil as a drench or directly to the plant as a foliar spray. When it is used as a foliar application, it is best to strive for thorough leaf coverage using a fine mist. Foliar applications are best done early morning or pre-dusk to minimize the effects of UV rays.

Although it does contain some nutrients and micronutrients, and may improve a plant's natural ability to resist pests and diseases, compost tea should not be thought of as a fertilizer or pesticide. Compost tea is more accurately described as a soil or foliar inoculant to be used in combination with other good organic gardening practices and inputs. Its inherent value is in providing the microorganisms and organic compounds that support biological nutrient cycling. This kind of "biofertility" is very effective at providing plants the nutrients that they need in the right amount and form, often reducing the need for supplemental inputs of fertilizer.

Compost and compost tea are used throughout the growing season at the Seeds of Change Research Farm. Previously, all compost used on the Farm was purchased from outside sources. In keeping with its goal of sustainability, the Farm has pursued the development of its own compost operation that transforms farm-generated plant residues into high-quality compost for use as a soil amendment on the Farm, thereby reducing dependency on external sources. The Farm's new on-site composting operation is expected to provide most, if not all, of its compost needs.

The Farm also makes its own compost tea on site and uses it in a variety of applications. Compost tea is brewed twice a week (25 gallons per batch) and applied as a soil drench after transplanting in the spring, and then as a foliar spray on vegetable crops several times throughout the summer. "We are less concerned with effects of the tea on foliar diseases, as these are not much of an issue here in New Mexico," says Emily Gatch, Greenhouse and Pathology Coordinator for the Research Farm. "We are primarily interested in the beneficial effects of the tea on soil microbial communities—the humic acids, micronutrients, and growth-promoting compounds, in addition to the beneficial microbes, that improve vigor and yields of crops." Gatch conducted greenhouse trials on tomato transplants and saw obvious benefits from using compost tea in greenhouse production.

To apply the compost tea, the Farm staff have primarily used backpack sprayers, but will begin using a tractor-mounted sprayer for more efficient application this year.

Compost has become a mainstay in organic farming and gardening, and compost tea is rapidly gaining similar status. Together, they represent a vital biological force of nature that is available to all of us in the pursuit of healthy and bountiful soils.

Cindy Salter,
Compost Consultant to Seeds of Change
and Research Director for Growing Solutions

Photo Captions: (1) Kelle Carter, Field Coordinator, brewing a batch of compost tea (2) Finished compost tea in a bucket, ready for application (3) Kelle using a backpack sprayer to foliar feed kale plants (4) Kelle adding water to the tea brewer.