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Fluid Journal : Fluid Journal 2008-2009
Dr. John G. Clapp !"#$%&' !"#$%&'()*+,-&'(*"(./0(1'+%2(10,.%'%30,(1*+-2&.%*- !"#$%! !"##$%&&' ()#*$+,$-)*$./$01123$455 In the 1940s, dry fertilizer materials available at that time were evaluated for changes that occurred in the soil solution osmotic pressure upon application. In 1943, Rader et al. reported salt index values for 45 dry fertilizer materials based on the osmotic pressure of the soil solution when applied to Norfolk sand. This method involved mixing fertilizer materials with air-dried soil and then spraying with water to bring the moisture content to 75 percent of its moisture equivalent. After five days, the soil solution was removed and evaluated for conductivity and freezing point. The resulting freezing point values were then converted to osmotic pressure by tables developed for vegetable saps. A salt index value was then expressed relative to the increase in osmotic pressure as compared with that obtained with the same weight of sodium nitrate. During this time, three nitrogen (N) containing solutions were evaluated, but they could not be urea-ammonium nitrate solution (UAN) since the N content ranged from 37 to 40.8 percent. A laboratory method was later published by W.L . Jackson in 1958 where salt index of a fertilizer was measured by electrical conductance, rather than by osmotic pressure, relative to sodium nitrate. However, this method generally results in significantly higher salt index readings than the original method and data derived from this laboratory method did not correlate well with earlier soil-applied applications. Fluid fertilizers such as UAN, ammonium polyphosphate (APP), ammonium thiosulfate (ATS), potassium thiosulfate (KTS), calcium nitrate (CN9) and others were not available until after the original study. Data from these materials have been added to data from the original study in the fertilizer salt index reference tables being used today. Recent studies Method comparison. In 2004, Murray and Clapp compared several potassium (K) sources for salt index values, as determined by the Jackson method, with the original data published by Rader. As noted in Table 1, salt index values from the two methods do not directly correlate. Some minor differences are noted as a result of differences in the K2O concentration because Rader used chemically pure material for K2SO4 and KNO3. In this study, a !"#$%&'"&()*"+,-&./"0&1"20304&5)-#&607"8 Summary: The original data and definition of salt index come from a time before many of the current fertilizer products, especially fluids after the 1940s, were developed. In recent years, some have adopted a method that measures electrical conductivity (EC) and not the original osmotic pressure approach. A few products may have widely different salt index values, depending on methodology used. Salt index, by itself, does not tell us how much of a given product is safe when applied with the seed. It only provides relative differences among products. Many other factors such as soil temperature, soil moisture, and potential free ammonia formation may all impact germination and/or seedling root development. !"#$#%&'()&*&(&%)()+,%#*#-%(-.(/&'*0#%)+1(2"+)&*+(3&%4(56""+%*(.+"*#'#7+"/8(
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