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Fluid Journal : Fluid Journal 1993-1995
1 Fluid Journal Summer 1994 ike phosphorus, potassium is relatively immobile in the soil. It is also just as vital to proper plant growth. Agronomic crops contain about the same amount of K as N, but more K than P. And in many high-yielding crops, K exceeds N content. Multi-roles Unlike N and P, potassium does not form organic compounds in the plant. Its primary function seems to he tied to plant metabolism. It is involved in several plant processes. Potassium is vital to photosynthesis. When K is deficient, photosynthesis declines and the plant's respiration increases. These two conditions--- reduced photosynthesis and increased respiration--- lower the plant's carbohydrate supply. K also: · promotes protein synthesis · is important in the breakdown of carbohydrates, a process that provides energy for plant growth · helps control ionic balance · promotes translocation of heavy metals such as iron · helps control lodging in corn · promotes fruit formation · improves winter hardiness · is involved in the activation of more than 60 enzyme systems that regulate rates of major plant growth reactions. Another important role of K in plant growth is its influence on water-use efficiency. The process of opening and closing of plant leaf pores, called stomates, is regulated by K concentration in cells that surround the stomates. A shortage of K causes the stomates to open only partially and to be FFF Review L Effective P Effective P Effe ctive P Effe ctive P Effective Potassium Management otassium Management otassium Management otassium Management otassium Management Proper fertility vital with this relatively im mobile nutrient. slower in closing. This increases stress from drought. Table 1 shows how adequate K improves yields of corn under three levels of rainfall. Figure 1 shows the difference between no K and K (120 lbs/A) on yields of corn over a four-year period. Potassium has a great impact on crop quality, including: 1) increased kernel weight and kernels per ear in corn, 2) improved oil and protein content in soybeans, 3) better milling and baking quality in wheat, and 4) improved stand and longevity in forages. One problem in forage production is poor fertilizer management, particularly with regard to N and K balance. Growers use N because they know it increases yield, adds green color, and improves protein content. Potassium is less showy and often neglected. The importance of K in disease suppression cannot be overstated. The USDA Yearbook of Agriculture states: "More plant diseases have been retarded by the use of potassium fertilizer than any other substance.'' When K helps a plant resist disease, it doesn't do it as a direct agent of control but by strengthening the natural resistance mechanisms of the plant. It is the key element in reducing: · leaf blight and stalk rot in corn · wilt and damping off in cotton · black spot and stem end rot in potatoes · wildfire in tobacco · leafspot and dollarspot in grasses. Virginia research on soybeans has shown the benefits of K. The incidence of moldy beans has been reduced by as much as 6 bu/A. Yields have increased by as much as 9 bu/A. And substantial savings have been realized through reductions in dockage for poor quality. Spotting symptoms Scorching. Firing along the leaf margins is one of the most common K hunger signs. Firing first appears on older leaves in most plants, especially grasses. Newer leaves will show hunger signs first on some plants---especially in high-yielding cotton in mid to late season. Corn will scorch on the outer edge of the leaf, while the midrib remains green. Scorching on soybeans begins on the outer edge of the leaf--- the edges becoming broken and ragged as the leaf dies. On fruit trees, scorching develops along edges of leaves, which become ragged. Stunting. Potassium deficient plants grow slowly. Stalks are weak, lodging is Table 1. Response of corn to K under three levels of rainfall. Rainfall during Yield Rainfall growing season Low K High K level ___ inches ____ __________ bu/A _________ Low (Indiana) 7.1 91 130 Medium (Indiana) 17.7 148 156 High (Indiana) 25.7 92 140 Low (Ohio) 9.0 122 164 Medium (Ohio) 19.9 151 173
Fluid Journal 1996-1998