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September 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 63 G R A P E G R O W I N G izers in organic vineyards, to make up for nutrients removed at harvest. What is organic matter in soil? Organic matter in soil ranges from fresh, unchanged organic materials through to highly decomposed organic materials. Light and electron micrographs of soil show that organic matter consists of soil biota (living organisms such as micro- organisms, fauna and roots), plant frag- ments, polysaccharides and amorphous materials. Solvents are used to extract proteins, fats, lignin, waxes, resin and humic materials from soil. Where does organic matter come from? Most organic matter is most concentrated in the surface soil, where most of the roots are, and comes mainly from plants and soil biota. Normal, healthy roots con- tinually release organic compounds into soil that feed the biota. However, other plant residues, such as leaves and shoots, provide most of the natural organic residues added to soil in vineyards. Organic mulch or manures added by the vineyard manager also con- tribute to organic matter in soil. Soil biota and decomposition of organic matter Soil biota in the vineyard include roots A lthough organic matter is important in soil, viticultur- ists usually do not think much about it. They are too busy man- aging their crops and vines, controlling pests and diseases, irrigating, organiz- ing labor and doing other things. Plus, many viticulturists think that supplying organic matter to soil is too costly. Organic matter makes up only a small part of a soil (usually less than 5%), yet it plays a significant role in the vineyard. When the vineyard manager builds up organic matter in the soil, he or she contributes more in the long- term to well-structured, fertile soil and grapevines than any other resource — far greater than the 2% to 5% organic matter that the soil test suggests. Here, we show why organic matter is important, what it is, where it comes from and how the vineyard manager can build it up in soil. Why is organic matter important? Organic matter in soil affects the growth and production of grapevines chemi- cally, biologically and physically in the following ways: • Provides nutrients to the grapevines and cover crops, • Provides nutrients to the many micro- organisms and fauna in soil, • As decomposable substrates, provides energy and carbon for their own tissue for the many microorganisms and fauna in soil, • Helps to buffer soil against rapid changes in chemistry, such as changes in salinity and availability of inorganic nutrients, • Helps to buffer soil against big swings in pH when chemicals are added to the soil, • Improves the structure (including the water-holding capacity) of soil; in turn, this affects the supply of water and air to roots, and the removal of toxic gases from soil, • Organic residues act as slow-release fertilizers. The nutrients attached to the humus are not easily leached down to the groundwater away from the roots. However, the vineyard manager still needs to add inorganic fertilizer in con- ventional vineyards, and organic fertil- Judith Tisdall and Bas van den Ende BY U N D E R S TA N D I N G SOIL ORGANIC MATTER Ungrazed natural range pasture soils: Knights Valley, Sonoma County. Well established Bunch grasses with deep roots. Much organic matter on and near the Suther Loam soil surface. Bisordi Ranch soils: Russian River Valley, Sonoma County. Previously in vineyard, dense cover crop tilled in, Yolo Loam soil high in incorporated organic matter. New vineyard development is under way. PHOTOS BY HECTOR BEDOLLA