NGA Articles: Organic Matters

Organic Matters (cont)

By: Carrie Chalmers

Nutritive Benefits of Various Organic Residues

Adding different organic residues to the soil produces different results. Succulent green materials such as grass clippings, manure, fresh leaves, cover crops, and weeds contain carbons, but they're also rich in nitrogen. Because they have a low C:N ratio, the soil will absorb them rapidly, then release nutrients. With little decay-resistant material in them to enhance humus production, these soil amendments facilitate decomposition but aren't as instrumental in the humus phase.

In contrast, carbon-containing materials such as straw, sawdust, papers, wood chips, and dried leaves have low nitrogen levels and are high in carbohydrates and lignins (the bonds that connect woody cell walls of plants, making them decay-resistant). These substances are the primary building-blocks of stable humus and long-term nutrient reserves.

Humus: Nutrients in Reserve

A reserve of stable, partially decomposed organic matter (humus) in the soil develops over several years and preserves the soil's physical integrity. Humus is a huge storehouse of solar energy; in your garden, it reduces erosion, increases water retention, and affects soil chemicals. For example, humus particles are negatively charged, attracting positively charged cations like calcium, magnesium, and potassium. Humus increases the surface area available to hold the cations, and it enriches the nutrient pool available to plant roots. Nitrogen is stored in a form that's released slowly, sustaining season-long growth. Soils that have been used for years without careful management generally lack this nutrient reserve, and that's when building stable humus is a high priorty.

Add Materals to Suit Your Soil's Needs

If you usually add food wastes, manure, and green yard waste to your soil, try to add high-carbon residues to your compost pile, or mulch plants with straw. The additives create a balance between fostering biological activity and accumulating humus. You can also quickly improve soil structure by planting a cover crop such as buckwheat, or incorporating grass clippings or manure directly into the soil, rather than adding mature compost. (Fresh manure could burn plants, however, so if you add it, don't plant immediately afterward.) If you want to boost stable organic matter levels, a cover crop will contribute less than manure would.

In his book, Fertile Soil (agAccess, Davis, CA, 1990, $44 including shipping), Robert Parnes demonstrates that incorporating a layer of hay into your garden boosts the soil's organic-matter content more effectively than adding manure, and stable compost contributes more quickly to the level of stable organic matter. Except when you are reclaiming damaged soils or correcting for past imbalances, most garden soils need a balance of fresh and decomposed organic residues to fuel both decomposition stages constantly.

Conditions that affect buildup of organic matter

Tilling, the equivalent of aerating your compost pile, greatly affects the soil's level of stable organic matter. The resulting increased oxygen levels promote biological activity; consequently, organic amendments are necessary to compensate for further decomposition caused by soil cultivation. Excessive wind, heat, rain, and compaction hinder high biological activity. Grass or legume crops and organic mulches protect the soil and accomplish a crucial part of soil management: the creation and conservation of organic matter.


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