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Fluid Journal : Fluid Journal 1996-1998
1 Fluid Journal Spring 1996 The influence of soil fertility management on water quality is a legitimate concern because nutrients intended for crop use are either soluble in water and therefore subject to leaching, or are in equilibrium with the soil-water complex and can be carried off the land in runoff. In either case, water is the medium for movement. That is why water quality management must be considered any time crop nutrient availability and efficiency are discussed. Pesky N Because of the different forms and properties of crop nutrients, each has unique set of management considerations. Of the three crop nutrients (N, P, and K), N is the most evasive and difficult to manage. This is because it exists in both organic and inorganic forms. Conversion between these forms typically involves biological systems either in the soil as microorganisms, or above the ground as part of plant growth. This situation begins to explain why N management is so incredibly complex. The process is further complicated because inorganic N exists as nitrate (NO3-) in one extreme and ammonium (NH4+ )in the other, with several intermediate forms. In the NO3- form (anion), N is soluble in water and moves with water as it percolates through the root zone or flows along the soil surface in runoff. As NH4+ (cation), N is attracted to soil particles and organic matter, which are essentially anions. The exception is volcanic derived soils. Therefore, most NH4+ in runoff is transported with soil particles in runoff. When combined with organic N sources, particle-borne N amounts to 80 to 90 percent of the total N load in agricultural runoff. Similarly, particle-bound P amounts to the same portion of total P in runoff water. Sediment-bound forms of N and P are in equilibrium with soluble forms of both nutrients. The soluble forms of N and P serve as the sources of nutrients for aquatic plants in eutrophic ponds, lakes, streams, and wetlands. Trading off Soluble forms of P are not considered a health hazard, except that even at low concentrations (<1 ppm) they promote eutrophication. In contrast, NO3- is considered a health hazard at concentrations above 10 ppm NO3-N. The concern over high NO3- water (>10 ppm) is somewhat misdirected because humans and livestock also ingest NO3- from sources other than drinking water. For example, many vegetables contain relatively high levels of nitrate that add to the total load of ingested NO3- . To put this concern in perspective, a two-ounce serving of fresh beets contains about 38 mg NO3-N. A person would have to drink over four quarts of water containing 10 ppm NO3-N to get an by Dr. James S. Schepers Precision Farming: One Key To Quality Water Author discusses the elements of site-specific management that ensure maxi- mum nutrient-use efficiency and environmental stewardship. equal dose of NO3- . It seems that emotion frequently dominates when making such comparisons. People don't mind taking a risk if it is their choice (smoking, drinking, etc.). Problems arise when alternatives are beyond our control or we perceive that someone else has imposed a situation upon us without our consent. We may choose not to eat beets, but in the U.S. we feel it is our right to have a supply of safe drinking water while having at the same time no regard for what it takes to produce an abundant and pleasing supply of food. If the truth was known about how inefficient N fertilizer is when used in the production of fruits and vegetables compared to grain crops, it might change our eating habits. The net effect might be the consumption of more legumes that grow quite well without N fertilizer. Problem is, humans don't get much enjoyment out of eating alfalfa or clover until it first has been fed to livestock or poultry. All too often, consumers don't associate the cost of manure handling and efficient use of nutrients with the cost of food and environmental steward-ship. While society should not downplay the importance of maintaining an adequate supply of drinking water with <10 ppm NO3-N, there are other forms of contamination that contribute to the problem we call "blue baby syndrome" (methemoglobinemia). Medical reports that document methemoglobinemia
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