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Adding organic matter is the single most important step homeowners can take to improve soil health. It provides essential plant nutrients, aerates the soil, improves soil structure and tilth and enlarges the community of soil organisms. Organic matter can help solve a host of soil problems. However, all organic matter is not created equal and three questions need to be answered: what kind of organic matter is most effective, how much should be applied and how often?

The three most important sources of garden organic matter are compost, green manure and mulch. Compost easily deserves the “best in class” award. It can be homegrown or commercial. Ideally, it reflects a healthy mix of sources, livestock manure, grass clippings, rotting leaves, pine needles, yard debris and kitchen scraps. Mixed compost is the best source for both macro and micro plant nutrients.

One perennial concern in using properly aged compost involves nitrogen (N). In the process of decomposing organic matter, considerable nitrogen is used up by soil organisms. Some nitrogen evaporates into the air where it is unavailable to plants. Consequently, aged compost is often relatively low in usable nitrogen, while providing healthy amounts of most other essential nutrients. Nitrogen, unfortunately, is the most important plant nutrient and is what botanists refer to as a limiting nutrient. This means that too little nitrogen will limit plant growth regardless of how much of other nutrients is available. While taking advantage of the virtues of compost, homeowners need to ensure that their soil is receiving adequate nitrogen.

Green manure or cover crops refers to planting crops strategically, not for food but to rejuvenate the soil. Green manure protects soil from erosion, helps aerate even heavily compacted soil and can add significant amounts of usable nitrogen.

The use of green manure is widespread in agriculture and is gaining popularity with home gardeners. One method involves planting a crop during the late summer or fall after produce has been harvested, leaving it to overwinter and then turning it over in early spring, prior to spring planting of food crops. The other method involves incorporating green manure in crop rotation, allowing the crop to go through an entire season or two before it is turned over. And, yes, some green vegetative waste can be incorporated into soil before being broken down. However, be careful to avoid adding diseased plant tissue. Disease spores can create later garden problems.

Most often, legumes are used for green manure. Over millions of years, legumes co-evolved with special soil bacteria, forming a symbiotic relationship. The bacteria create nodules on the roots of legumes. These anaerobic bacteria live inside the nodules. The plant takes in nitrogen (N) from the air — in a form the plant can’t use — and moves it down to the root nodules where bacteria transform the nitrogen into ammonium and nitrates, forms the plant can use. This transformation is known as nitrogen fixation. After the plant dies, those nitrogen compounds are released into the soil, building up the soil’s store of usable nitrogen. Popular legumes used for green manure include alfalfa, clover, vetch, peas and soybeans.

Organic mulches are used as a top dressing around trees, shrubs, garden beds, and vegetable gardens. They help suppress weeds, regulate soil temperature (cooling in summer and heating in winter), provide attractive color contrast in a garden or landscape, and slowly break down, adding nutrients to the soil. Popular mulches include wood bark, pine needles, straw, grass clippings and leaf litter.

How much organic matter should you add and how often? If using compost, usually an annual 2-3” application is beneficial on established lawns or beds. In garden areas, 5-8” of compost can be incorporated into the top foot of soil. Fall application is optimal. Some homeowners have had excellent results with several small top dressings of compost (1-2” per application) spread out over the growing season. Gardeners who mulch usually add a 2 to 6” layer in the spring or early summer.

Organic matter is so important. Remember, soil organisms get their fuel from organic matter. As they eat and break it down, they discharge nutrients back into the soil in forms plant roots can use. As more organic matter is put in the soil, more soil organisms will emerge, and more nutrients will become available to plants. Bacterial populations, for example, soar when new organic matter is incorporated into soil and drop rapidly when that matter has been fully processed. A healthy community of soil organisms is essential for healthy soil. Along with breaking down organic matter into available plant nutrients, they aerate the soil and improve the soil’s water and air retention capacity while improving water drainage throughout the garden. As you add organic matter to your landscape, you are helping your plants by helping the entire symbiotic ecosystem of living things within your garden. That is the foundation of healthy gardening.

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Garden Wise is presented by the Magic Valley Master Gardener Association. We will try to answer questions of general interest submitted by the community. Email questions to gardenwise@cableone.net.

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