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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 65 G R A P E G R O W I N G tain less organic matter than clay soils. The deeper the surface soil, the more organic matter it can hold. The amount of organic matter in soil also depends on the balance between organic residues added to soil and the rate at which organisms decompose the residues. Soil pH also affects the supply and decomposition of organic materials in soil. It is best to aim for a pH of about 5.8 to 7.5 so that the plants grow well and add plenty of organic residues to soil. Organic matter and nutrient supply There is a significant correlation between the percentage of organic matter in soil and soil fertility. Viticulturists in Australia aim for 2% to 4% organic matter in soil, depending on the texture of the soil. Less than 1.5% organic matter is considered low and can lead to water-logged soil in low-lying areas and insufficient nutrients available for plants. Nutrients come not only from origi- nal plant residues, but some nutrients become attached to humic materials (highly degraded organic matter in soil), and are slowly released for plants and other soil biota. The humic materials pro- vide both negative and positive charges that can hold and exchange nutrients. Nutrients with positive charges, such as potassium, calcium and magnesium, adhere to the negative charges on the humus, whereas nutrients with negative charges, such as phosphate and sulphate, adhere to positive charges. In both cases, the nutrients remain available to plants and are not leached down beyond the root zone. A soil's cation exchange capacity (CEC) is a measure of the net negative charge of dry soil and therefore is a measure of the amount of positively charged nutrients that can be stored in the soil. Soils with a low CEC have a low net negative charge and do not hold positively charged nutri- ents in the soil as well as soils with a high CEC. Organic matter, with its negative charge, has a very high CEC compared with that of clay particles; this helps to improve soils with low CEC. Organic matter also decreases the fixa- tion of phosphorus and potassium in the soil and forms metal-organic complexes that stabilize the micronutrients that oth- erwise might not be available to plants. When organic residues or cover crops that contain a low ratio of carbon to nitrogen are tilled into soil, microorgan- isms need to use nitrogen that is stored in soil to decompose the residues. The nitrogen is then locked up in the bodies of the microorganism so that grapevines may not have enough sufficient nitro- gen for growth and development until the microorganisms die and nitrogen is released from their bodies. Hence, it is not wise to incorporate organic residues or cover crops into soil. Organic matter and soil stability Organic matter enhances the stability of soil so that the aggregates do not collapse (slake) or erode when wetted quickly. Large stable aggregates (more than 2 mm) are held together by a fine network of roots and fungal hyphae that exude sticky gels or mucilage (including poly- saccharides), which soil particles adhere strongly to, rather like a sticky string bag. This is why it is difficult to wash soil from roots. Fragments of plants and dead microbial cells are also surrounded by sticky gels and become encrusted with clay particles to form stable smaller aggregates. Silt and clay soils with less than 1.5% LOTUS: Breakthrough Pump Over Technology Lotus gently and uniformly distributes juice over the cap in tanks of any size without excessive bruising or misting. Lotus is compact, continuously adjustable, has few moving parts and is easy to install and clean. Three sizes available. Prices start at $650 The next generation of pump over technology is here. Napa Fermentation Supplies 575 Third St. Bldng. A Napa CA 94559 napafermentation@aol.com | 707-255-6372 www.napafermentation.com