When applying fertilizer as a “pop-up” or starter near or with the seed, growers have new help in knowing how much is too much.
Band placement of starter fertilizers can be more efficient compared to broadcast applications, especially for phosphorus (P). And with phosphate prices having tripled from just a few years ago, a more efficient P placement is a big plus.
All fertilizers are salts and can damage the germinating seed and resulting seedling. High salt levels can prevent water intake into the seed, preventing germination. Salts can also burn and damage the emerging plant shoot, cotyledons and primary root. In addition, ammonia from urea, 28% N and DAP can inactivate seed enzymes responsible for germination. Thiosulfate can also be toxic to the seed.
In the past, crop advisors used rule-of-thumb guidelines, such as no more than 10 lbs./acre of seed-placed N plus potassium (K) for corn. These guidelines worked fairly well for the few fertilizer and crop combinations for which they were available. However, there are many other factors that can influence seed injury, including the crop planted, row width, width of fertilizer spread, distance between fertilizer and seed, fertilizer type, soil texture, pH and soil moisture.
A review of numerous field studies provided some guidelines, but variability in results reflected the varied environmental conditions of the studies. And there were no studies for many crop/fertilizer combinations. A follow-up SDSU controlled lab experiment covering 16 crops and 16 fertilizers clarifies the effects of various soil and agronomic variables. The combined findings are reflected in a new Excel spreadsheet decision aid to help growers identify safe fertilizer rates for seed-placed fertilizers based on species-specific salt sensitivity, soil type and environments. Dowload the sheet at http://plantsci.sdstate.edu/soiltest/FertSeedDecisionAid.xls.