Your Organic Gardening Primer: Lesson 4
Organic matter adds hidden benefits to soil
Addition of organic matter does great things for soil. It works as a slow-release fertilizer and source of essential nutrients. It reduces the density of heavy silt and clay loam soils. It improves soil’s nutrient retention and increases water retention. All of these benefits redound to plant growth.
Retention of nutrients
Adding organic matter to soils increases the retention of nutrients and makes them available to the roots of plants. This process is known as increasing the cation-exchange capacity of soils. You learned in the July 24 column how organic matter releases nutrients slowly through mineralization. In addition to supplying the major elements, compost supplies trace elements such as boron (B), iron (Fe), zinc (Zn), manganese (Mn), sulfur (S) and copper (Cu). These essential trace elements are important to the growth of healthy plants and to the quality of the crops they produce. But they’re not part of commercial fertilizer mixes.
Increasing the cation exchange is especially important in sandy loams or loamy sands. Nutrients leach through these sandy soils quickly. Because sandy soils are well aerated, they do not retain organic matter. So to maintain productivity on sandy soils requires frequent applications compost or animal manure and the use of cover crops.
On sandy loams or loamy sands, use no more compost or manure than six cubic yards per 1,000 square feet for the initial application. On silt or clay loam soils, make that four cubic yards as these soils are better able to retain nutrients than sandy loams or loamy sands. Repeated applications should be one-half or one-quarter.
The addition of organic matter to sandy soils increases water-holding capacity.
The addition of organic matter to heavy silt or clay loam soils increases water infiltration, thus increasing their ability to retain water while at the same time allowing excess water to drain.
Soil Density Reduction
It won’t work to use sand to improve the drainage of heavy silt or clay loam soils. Short of 55 to 60 percent, the addition of sand will only result in making the soil like concrete.
Adding 10 percent compost will increase both the organic matter concentration and the productivity of heavy silt or clay loam. Pine fines are one of the better organic materials to use to lighten heavy soils. Pine fines are a waste product from the manufacturing of pine bark mulches. Because pine fines contain high levels of lignins — a source of organic matter that resists decomposition — pine fines will persist in the soil for a long time.
Another hidden benefit of amending soils with compost is its ability to control soil-borne diseases. Quality compost contains three naturally occurring fungicides and numerous beneficial microorganisms known to control common soil-borne diseases as fusarium, pythium and rhizoctinia. To get this bonus, use recently made compost. As the compost ages, these benefits are gradually lost as the biological activity of the compost decreases.