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Fluid Journal : Fluid Journal 2002-2004
1 Fluid Journal Winter 2003 R RRRR Summary: Isotope studies have shown that the quantity of soil-applied nitrogen (N) failing to get into a crop can range from 85 percent in the worst case to 38 percent in the best. By contrast, foliar-applied N can be up to seven times more efficient than soil- applied N. Other benefits of foliar- applied N include lower application rates (higher efficiency), plus the relative ease of obtaining timely, uniform applications. A combination of soil-applied and foliar applied N is the best management practice to reduce the potential for nitrates in groundwater. esearch has shattered the belief that only roots absorb nutrients. The use of radioactive and isotopically tagged nutrients has confirmed that plants can be fed through their leaves. Foliar N, in particular, is absorbed through other green tissue and soft woody tissue including stems, buds, blossoms, and fruit. Loss pathways for foliar-applied nutrients are few, but they are many with soil applications. During the past several decades, increased levels of nitrate in the groundwater have occurred in many agricultural areas. Soil applications of N fertilizer, when associated with porous soils and irrigation or with shallow water tables, are linked to increases of groundwater nitrates. By contrast, foliar applications have provided a highly Foliar Fertilization Improves Nutrient Use Efficiency Studies are showing foliar fertilization is more efficient and reduces potential environmental impact by Robert C. Dixon efficient alternative for feeding N to plants. The propensity for nitrate to move below the root zone and eventually to groundwater is being significantly reduced with foliar applied N. Slow-release N in the form of technologically advanced liquid polymethylene urea-methylene diurea is being applied with a high degree of plant safety. Applications are being made at agronomically significant rates at any time in the vegetative growth cycle. Pathways for soil N loss Plant roots usually occupy less than one percent of the soil volume, hence less than one percent of soil-applied N will be directly contacted by growing roots. Pathways for soil-applied N losses include: • Mineralization • Clay particle fixation • Leaching • Microbial immobilization • Denitrification • Volatilization Considerable N can be lost through volatilization and denitrification. Some N is immobilized by microbial activity and clay mineral fixation. These pathways reduce the amount of N available for plant uptake. The efficiency of this uptake can be defined by the ratio of fertilizer N in the crop to fertilizer N applied to the soil. By any measure of efficiency, uptake of soil- applied N is relatively low. Even in best case studies the crop did not recover 38 percent of the soil-applied N. In worst case scenarios, 85 percent of the soil- applied N failed to reach its intended target. How it moves During the growing season, most of the available N is in the nitrate form. The direction and degree of its movement is dependent on the amount of water entering the soil and the rate of its downward movement. Nitrate can move downward at a rate greater than that which is used by the crop. The depth it moves is dependent upon the amount of water entering the soil and the texture of the soil. Studies have shown that the vertical downward movement of nitrate in a sandy soil at field moisture capacity was about 45 inches for 10 inches of water entering the soil. Downward movement was 30 inches in medium-textured soil (loam) and 20 inches in a clay soil for every 10 inches of water entering the soil. All too frequently, soil-applied nitrate nitrogen is positionally unavailable to plants. By comparison, foliar-applied N is always positionally available. Foliar efficiency Foliar-applied nutrients have the benefit of being 4 to 30 times more efficient and there is no risk of
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