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Fluid Journal : Fluid Journal 1999-2001
Table 1. Salt index values of fertilizer materials. ---------------Salt Index------------ Per equal Per unit of wts of materials nutrients* Material and analysis Nitrogen/Sulfur Ammonia, 82%N 47.1 0.572 Ammonium nitrate, 34%N 104.0 3.059 Ammonium sulfate, 21%N, 24%S 68.3 3.252 Ammonium thiosulfate, 12%N, 26%S 90.4 7.533 Urea, 46%N 74.4 1.618 UAN, 28%N (39% a. nitrate, 31% urea) 63.0 2.250 32%N (44% a. nitrate, 35% urea) 71.1 2.221 Phosphorus APP, 10%N, 34%P2O5 20.0 0.455 DAP, 18%N, 46%P2O5 29.2 0.456 MAP, 11%N, 52%P2O5 26.7 0.405 Phosphoric acid, 54%P2O5 1.613a 72%P2O5 1.754a Potassium Monopotassium phosphate, 52%P2O5, 35%K2O 8.4 0.097 Potassium chloride, 62%K2O 120.1 1.936 Potassium sulfate, 50%K2O, 18%S 42.6 0.852 Potassium thiosulfate, 25%K2O, 17%S 68.0 2.720 a Salt index per 100 lbs of H3PO4 *One unit equals 20 lb. Table 2. Calculating salt index of 7-21-7. ---Salt index--- % Nutrient units per unit in for- Material Nutrient lbs/ton N P2O5 K2O (20 lb)a mulation (1) (2) (3) (4) (5) (6) (7) (8) 10-34-010%N, 34%P 2O5 1,235 6.2 21.0 --- 0.455 12.4 UAN 28%N 570.8--- --- 2.250 1.8 KCl62%K2O 226 --- --- 7.0 1.936 13.6 Water 482 --- --- --- --- --- ____ __ __ __ _____ _ __ 2,000 7.0 21.0 7.0 27.8 b a Salt index per unit (20 lb) of plant nutrients, listed Table 1, also called the partial salt index. b 0.79 SI/unit plant nutrient Spring 2001 Fluid Journal 2 Also, some starter components such as urea, UAN, or ammonium thiosulfate can produce free ammonia (NH3) under certain soil conditions. Direct seed contact by NH3 could result in poor germination or seedling death. Selection of the proper starter fertilizer is the way to minimize this occurrence. Fertilizers best suited for seed-row application have 1) low salt index, 2) high water solubility, 3) contain N, P, K and S, with relatively high P content, 4) contain both urea and ammonium- nitrogen, 5) minimize content of compounds that liberate NH3, and 6) use potassium phosphate instead of KCl as the K source. Salt index Salt content is one of the most critical characteristics of fertilizers used for row-seed placement. The SI is a measure of the salt concentration that fertilizer induces in the soil solution. The SI of a material is expressed as the ratio of the increase in osmotic pressure of the salt solution produced by a specific fertilizer to the osmotic pressure of the same weight of NaNO3, which is based on a relative value of 100. Sodium nitrate was chosen as the standard because it was 100 percent water soluble and it was a commonly used nitrogen fertilizer when the SI concept was first proposed in 1943. Higher analysis fertilizers usually have a lower SI because fewer ions of salts are placed in the soil solution per unit of plant nutrient when they dissolve. Note that the N and K materials of commonly used fertilizers (Table 1) have higher SI values than those of P materials. The SI of a mixed formulation containing N, P, and/or K is the sum of the SI values of its components. Although the total SI for a high-analysis NPK mixture may be greater than that for a low-analysis NPK mixture, the SI per unit of plant nutrients may be lower in the high-analyses product. Therefore, the lower fertilizer rate needed to supply the same amount of plant nutrients subjects the germinating seeds to less potentially adverse salt effects. It should be noted that the SI does not predict the exact amount of fertilizer material or a fertilizer formulation that could produce crop injury on a particular soil. However, it does compare one fertilizer formulation with others regarding the osmotic (salt) effects. It also shows which fertilizers (those with a higher SI) will be most likely to cause injury to germinating seeds or seedlings if placed close to or in the seed row. Calculating salt index The SI of a mixed fertilizer (NPKS) is the sum of the SI of each component per unit of plant nutrient times the number of units in that component. See Table 2 for SI calculations of 7-21-7. To calculate SI of any formulation: 1. List the material, grade, and weight for each component in columns 1-3. 2. Determine nutrient units in columns 4-6 by multiplying the weight of each component by its nutrient content and dividing each result by 20. 3. List SI per plant nutrient unit in each component in column 7. 4. Determine the SI due each component by multiplying the sum of the nutrient units in columns 4-6 times the corresponding SI value in column 7.
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