One of the greatest fears of cotton producers is not having enough fertilizer nutrients for their crop — “they’d rather be over on the amount applied than under,” says Jac Varco.
But with today’s high prices, applying excess fertilizer can not only waste money, he says, it can also lead to rank growth, boll rot, difficulty in harvesting, increased need for plant growth regulators, insecticides, defoliants , and other problems.
“What we want to do is try to find a happy medium that will control costs and maximize profitability potential, Varco, professor of plant and soil sciences at Mississippi State University, said at the annual meeting of the Mississippi Agricultural Consultants Association.
“Nitrogen costs this spring could range from $35 per acre on the low end for anhydrous ammonia and closer to $50 per acre on the high end for urea solution, plus application costs. With potash at $600 to 800 a ton, that’s another $30 to $40 per acre.
“So, you can be looking at nearly $100 per acre in fertilizer costs for cotton, and for corn , double that amount at $200.”
Current fertilizer recommendations, Varco says, are developed by taking into account a number of factors: soil properties, soil test nitrogen, potential for nitrogen mineralization, tissue testing, rainfall/irrigation, variety, yield potential, economics (whether for maximum profit or maximum yield), previous crop, tillage, history of rank growth, etc.
“Some of the factors that should be considered are, how much nitrogen is supplied by the soil, how much fertilizer should be applied to maximize profit, which fertilizer source (the choices are becoming fewer and fewer), and fertilizer timing and placement.”
Based on studies for the past five or six years, with varying rates of nitrogen, Varco says, “our most profitable rate has been 80 pounds of nitrogen per acre to average 1,100 pounds of lint, or two-bale per acre cotton. To simplify data from various locations, we can say it equates to 40 pounds of fertilizer nitrogen per bale of lint on a silt loam soil.”
Applying higher nitrogen rates in an attempt to boost yield should be evaluated in the context not only of the higher cost, but the added problems from rank growth and greater cost for insecticides, PGRs, etc., not to mention environmental consequences from excessive rates.
In developing a strategy for nitrogen fertilizer, Varco says, “If prices are high, that tells me I need to pay attention to N rate and get the maximum efficiency from what’s applied — to be sure all the rows are being fertilized, that all the knives are in the ground, their spacing is correct, etc.
“I like to split applications, as kind of an insurance policy. One thing I emphasize is, don’t put nitrogen out when no crop is there. Don’t put it in the middles, but keep it on the beds — it’s more efficient and it’s less likely to be washed away.
“What other production input gives a growth stimulus as much as fertilizer nitrogen, or costs as much? Nothing. So let’s pay more attention to it and have a nitrogen strategy.
“Start with the 80-pound rate and adjust upward or downward from there. Don’t start at 120 pounds nitrogen per acre and adjust downward.”
If soil fertility is low for phosphate and potash, the probability of getting an economic return from fertilizer is very high. If soil fertility is high based on soil testing, adjust rates accordingly; applying a higher rate won’t give a good return on fertilizer dollars.
Varco says university scientists have done “a fair number of studies” with nitrogen loss inhibitors, primarily Agrotain. ‘The two-year average nitrogen recovery was 36 percent for ammonium nitrate compared to near 20 percent for untreated urea. This is why we don’t recommend broadcasting urea on no-till cotton.”
With the urease inhibitor, N recovery from urea was near equal to ammonium nitrate, he says. “It won’t stop volatilization dead in its tracks, but it will slow the loss rate. Whether to use it or not should be evaluated on a case-by-case basis.”
Varco cautions that producers shouldn’t let high potash prices keep them from maintaining adequate potash levels.
“It’s almost as important as nitrogen,” he says, “particularly for no-till cotton. It’s important, too, for soybeans  — one the factor that affects number of beans per pod is potash.”
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