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p r a c t i c a l w i n e r y & v i n e ya r d M a r c H 2 0 1 4 61 g r a p e g r o w i n g Results in 1990 confirmed the hypothe- sis that soil water deficit imposed by the grower 's common irrigation practice reduced the uptake of potassium in both fertilized and non-fertilized grapevines. This was indirect evidence that diffusion of K in this soil was limited by soil water deficit imposed using the grower 's nor- mal irrigation practices. The availability of K is reduced by soil water deficit in vineyards, more so than other nutrients. Therefore, K deficiencies are more likely to appear under extreme water deficit (Table V). Irrigation alone should increase availability and uptake as long as the root system is healthy. A grower who plans to deficit-irrigate vines by withholding water is allowing Potassium (K) is not an integral part of any major plant com- ponent, but it does play a key role in a vast array of physiologi- cal processes vital for plant growth, from protein synthesis to maintenance of plant water balance. Potassium deficiency is characterized by reduced plant growth and a yellowing and/ or burning of leaf edges. Since potassium is mobile in the plant, the symptoms appear on older leaves first. Another indication of potassium defi- ciency is reduced straw or stalk strength, which results in lodg- ing problems, reduced disease resistance and reduced winter hardiness of perennial or winter annual crops. The secondary nutrients—calcium, magnesium and sulfur— play a variety of roles in plants. Excess K in the soil can lead to excess K in plants and can in some cases result in animal nutrition/health problems. Potassium in the soil is mainly in the form of insoluble minerals and as the K+ cation on the soil CEC. (extension.psu.edu/agronomy-guide/cm/sec2/sec23). • Five pounds of potassium are typically removed from the soil in each ton of wine grapes harvested. ("K Nutrition of Winegrapes," 2010 Stu Pettygrove, UC Davis Extension Soils Specialist.) • In a vineyard yielding 5 tons per acre, 25 pounds of potas- sium per year are removed. Most farmers manage their soil nutrition levels by replacing what is removed especially after a series of heavy yielding crop years. (Hector Bedolla, Crop Production Services, 30 years of practice) • A portion of potassium in the soil solution is leached out by rainfall every year (in certain soils). Heavier clay soils will bind potassium onto exchange sites. A farmer will apply soil amendments such as gypsum to release these bound nutrients exchanging calcium or magnesium for potassium, liberating the potassium into the soil solution. ("International Soil Fertility Manual," International Plant Nutritional Institute, Chapter 5). • Development of a soil fertility program should begin with a soil test. Soil testing and plant tissue analysis go hand in hand. A plant may be suffering from "hidden hunger," where no symptoms appear. ("International Soil Fertility Manual," International Plant Nutrition Institute.) • To restore a soil to its optimum nutrient release capacity, it is basic and absolutely necessary to start with a reliable and precise laboratory analysis of the soil. The analysis must define the real causes of limitations. ("Eco-Farm," Charles Walters 127–128.) Disclaimer: When a diagnosis requires confirmation, get an assay, a lab report and compare numbers to university and extension standards. (Hector Bedolla, Crop Production Services)