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April 2016 WINES&VINES 77 PRACTICAL WINERY & VINEYARD GRAPEGROWING E xternal pressures on mineral nutrient use in vineyards come from a couple of directions. On one hand, to remain competitive in the global wine market, grapegrowers need to contain applied mineral nutrient costs even while global fertilizer demand is growing and increasing the cost of some fertilizers. On the other hand, our society increas- ingly expects applied mineral nutrient resources either to stay in vineyards or to leave only with the harvested fruit. These pressures, originating from disparate sources, converge on mineral nutrient use efficiency. Three primary factors influence mineral nutrient efficiency. They are technological, environmental and grapevine factors. Re- cently, the technological aspects of applied nutrients have been a common topic in the agricultural press and at agricultural gather- ings, often using the construct of the "four R's." That is, utilizing the right nutrient source at the right rate and the right timing, and applying it in the right place that is eas- ily accessible to plants. Fortunately, many wine grape growers are technologically competent, having a wide range of nutrient formulations, effective delivery systems and a basic understanding of soils and vine nutrient demand to which fertilizer applications can be synchronized. Optimum mineral nutrient efficiency, how- ever, broadly encompasses other factors. The soil solution lies at the center of vineyard mineral nutrient action: It is the water and dissolved substances that reside in soil pores. Among these substances are plant mineral nutrients. Grapevines, in fact, acquire nearly all of their mineral nutrients from the soil solution. Mineral nutrient concentrations within the soil solution are in a constant state of flux due to many factors. First, water addi- tions dilute them, while water losses concen- trate them. Second, weathering of soil minerals and decomposition of organic mat- ter slowly add mineral nutrients to the soil solution while plants and microorganisms consume them (see illustration). Depending on texture and the general chemical environment (pH), a large portion of dissolved mineral nutrients may be removed from the solution before vines have an oppor- tunity to take them up. These losses are mainly due to precipitation, adsorption and fixation reactions. After heavy rainfall, water percolat- ing below the root zone carries dissolved mineral nutrients away. Last—but not least— irrigations, fertilizations and amendment ap- plications have significant influences that overlay the natural processes listed above. Optimizing mineral nutrient efficiency Given this complex background, what can grapegrowers do to optimize use efficiency of mineral nutrients present in vineyard soils? Let's begin with actions that promote soil solution flow. This task requires little effort early in the growing season, when soils are fully wet from winter rains and vine roots are actively growing. Later, it depends on irriga- tion schedules that maintain fairly continu- ous, moderate moisture in the soil under drip emitters. As soils dry and water flow within soil pores decreases, mineral nutrient movement becomes increasingly confined to random motion within the thin films of water adher- ing to soil particles (i.e., diffusion). Under these conditions, fertilization efficiency is generally low, and to compensate, higher application rates may be needed to elicit vine responses. The impact of soil moisture con- tent on vine nutrition is an important reason to maintain vines on the wetter side of mod- erate water stress (-10 to -13 bars) while practicing regulated deficit irrigation (RDI). Soil solution movement, however, is not solely a function of water content. It also depends on soil permeability to water, which is largely a function of soil porosity. The porosity of vineyard soils is often sub- optimal due to compaction and cation im- Managing Mineral Nutrient Efficiency Beyond the 'four R's' of fertilization By Stan Grant Soil Solution Root Extraction Inorganic Management Inputs Biological Organic Mineral nutrient pools affect the nutrient content of the soil solution. PROGRESSIVE VITICULTURE LLC