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Fluid Journal : Fluid Journal 1999-2001
Spring 2001 Fluid Journal 1 Summary: Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. SI does not predict the exact amount of a fertilizer material or formulation that could produce crop injury on a particular soil, but it does allow comparisons of fluid formulations regarding their potential salt effects. As we all know, placement of some formulations in or near the seed may decrease seed germination or result in seedling injury. Fluid fertilizers containing potassium phosphate as the source of K have lower SI values than those containing KCl. When applied near the seed, fertilizers with lower SI values generally cause fewer problems in seed germination or seedling injury. SI of any fluid formulation can be calculated using the SI values of the most common fertilizer sources. Dealers or growers then can select those formulations with lower SI values that best fit their needs. Banding of nutrients has received much attention over the years. Usually, the fertilizer is placed at a depth greater than that of the seed to allow root interception of the fertilizer band as roots grow outward and downward in the soil. Band vs broadcast Regions showing the greatest improvement in efficiency from banding over broadcasting lie in the northern U.S. and Canada, where colder soil conditions are experienced during spring seeding of row crops and small grains. Higher P rates are generally recommended if growers broadcast instead of band their fertilizers. Dr. John J. Mortvedt Calculating Salt Index Salt content is one of the most critical characteristics of fertilizers that should be considered when fertilizers are applied, especially with seed-row or "in furrow"placement. Banded P tends to be more efficient on very acid soils, highly calcareous soils, and those soils with very low levels of available soil P. Band applications also are usually more efficient when low P application rates are used. Early planting dates, large amounts of crop residues on the soil surface, and soil compaction may subject plants to more stress. Banded nutrients are usually more effective for crops under these stress conditions. Vegetables respond well to banded fertilizers because they require a relatively large percentage of their total nutrients early in their growth period, and their rooting volume in the soil usually is restricted. As extra equipment has been installed on planters over the years, it has become more difficult to have enough room to include the coulters required to open the soil for fertilizer placement below and to the side of the seed row. Some growers have quit applying starters because of this limitation and also because of the weight of openers for very large planters. Others have applied starters directly to the seed furrow, which does not require extra openers. Other considerations. Banding away from the seed row is recommended over seed-row application under most conditions when applying higher nutrient rates, especially N, K, and S. Plants can efficiently use nutrients banded away from the seed row without adversely affecting seed germination or seedling emergence. Recommendations for fertilizer placement in direct seed contact vary with crop. For many years maximum recommendations ranged from 10-20 lbs/A of N + K2O in direct seed contact with corn and sorghum. These applied to formulations using KCl as the K source and would not be accurate if potassium phosphate was used as the source of K instead of KCl. This is because of the lower SI value of potassium phosphate compared with KCl (Table 1). Crop tolerance to increased osmotic pressures (salt content) of the soil solution in the vicinity of the seed varies considerably. For example, wheat is more tolerant of high salt conditions than is grain sorghum, while corn is intermediate. Tolerance of most oil-seed crops (soybeans and cotton) to seed-row application of nutrients is very low, and seed-row application of fertilizer for these crops should be viewed with caution. Fluid fertilizers may produce a lower osmotic pressure in the soil solution than granular products of a similar grade. Fewer problems generally are encountered using fluids as seed-row fertiliz ers when compared to granular, since less soil water is required and salts are mainly dissolved in fluid formulations. Seed-row application This method refers to placement of relatively lower rates of nutrients in direct seed contact, usually for row crops. It also has been called "pop-up" or "in-furrow" application, but "seed- row" is more descriptive. Seed-row placement increases the possibility of early root interception by nutrients. Problems. Major concern of this practice is decreased seed germination or seedling injury caused by high salt concentrations in the soil solutions around germinating seeds.
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