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64 P R A C T I C A L W I N E R Y & V I N E YA R D September 2015 G R A P E G R O W I N G and beneficial and harmful organisms, ranging from the smallest viruses (0.25 micron) to bacteria (0.3 micron) to acti- nomycetes (0.5 to 2 micron wide) and fungi (less than 15 microns wide) to nematodes to the largest fauna such as earthworms (more than 10 mm wide). Altogether, the soil biota can weigh up to 8.5 tons per acre. Soil organisms use the organic resi- dues for food and energy, and they suc- cessively decompose complex organic molecules and release organic com- pounds that other organisms can use to build into new organic molecules or store in their own bodies. At the same time, the organisms release carbon dioxide, compounds with chemical energy in their bonds, water and inorganic materials such as nitrogen, phosphorus, sulfur and micronutrients that plants and biota can use. There is generally a high turnover of biota in soil, which continues to add organic matter. Fine roots and fungal hyphae do not live long in soil but are fairly quickly replaced by new roots and microorganisms. Amount of organic matter The amount of organic matter in soil depends on: • Amount and quality of organic residues returned to soil, • Number and length of growing roots, • Populations of microorganisms and fauna, • Soil texture, depth and density, • Soil pH, • Temperature, water content and aeration of soil, • Microbial population. Sometimes a soil test shows organic matter as organic carbon. To calculate the percentage of organic matter, multiply the percentage of organic carbon by 1.6. Simple sugars, simple proteins and starch are easily decomposed, whereas hemicellulose, cellulose, fats, waxes and resins are slowly decomposed. Lignin comes from cell walls of plants and some algae, and it is decomposed very slowly. Eventually humic materi- als and complex highly decomposed organic matter are produced and darken the soil. For each soil, there is a limit to the amount of organic matter that can be stored. Organic matter is more quickly decomposed in sandy soils, so they con- Yolo Loam with incorporated cover crop to the 6-inch plow level and soil uniformity down to 12 inches and beyond at Bisordi Ranch, Russian River Valley, Sonoma County. 11 | 12 | 2015 • NAPA PREMIERE PARTNER PLATINUM SPONSORS Agri-Analysis, Central Valley Builders Duarte Nursery, ETS Laboratories connecting vineyard practices to wine quality Register at ROOTSTOCKNAPA.COM High-level, Provocative Seminars with Industry Experts Concept Bar featuring Innovative Technology Wine Trials & Blending Sessions Exclusive Viticulture & Enology Exhibition