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Choose wisely when making soil fertility decisions

Agriculture.com Staff 10/21/2008 @ 10:41am

With record high input costs and falling crop prices, growers want to manage costs. Because fertilizer accounts for a great share of expenses, many are considering holding off on fertilizer purchases in hope of falling prices. Others are planning to cut back on the amount of fertilizer they apply.

Agronomy experts recognize the challenge, but say the best course of action varies with each situation.

Antonio Mallarino, Iowa State University soil fertility specialist, says, "Producers should work closely with their crop adviser to consider their options. If soil tests are in the high and very high categories, 'banked' soil phosphorus (P) and potassium (K) can be used to grow next year's crop."

Moe Russell agrees. Russell, of Russell Consulting, specializes in risk management. "If soil fertility levels test high and farmers are looking for a year to capitalize on the 'bank' of P and K they have in their soil, this may be the year to do that," Russell says in a report from the Iowa Soybean Association (ISA).

It's important to regularly test soil to know what P, K and lime applications are really needed.

Tracy Blackmer, director of the ISA On-Farm Network, says it's also important to add nitrogen each year for corn. "Because soil cannot hold nitrogen, it should not be cut," he says. "The form, application method and timing farmers choose are big variables, but they need nitrogen."

Growers have more flexibility with P, K and lime.

"Soils testing low and very low in P and K should have fertilizer applied, because the expected yield increase is sufficient to pay for the applied nutrient," Mallarino says. "As soil test levels increase, the probability of a yield increase from fertilization decreases, as well as net return on investment."

Brian Kemp is an ISA director and a soybean producer near Sibley, Iowa.

Kemp's fields have been grid sampled. He'll apply P and K according to these tests. In higher testing fields, he plans to reduce rates and use his "banked" soil fertility. Otherwise, he plans to apply maintenance amounts.

Russell says using withdrawal rates based on yield to determine fertilizer needs is a good practice. He believes this maintains fertility levels and enables the producer to respond to fertilizer prices.

However, Mallarino cautions, "Many producers have not been adjusting fertilization rates to match their higher yields. The maintenance rate has increased significantly from yield levels common five or six years ago. If these producers decrease the amount of fertilizer they use, they may fall further behind. Maintaining soil P and K in the optimum ISU test category is still a good idea, especially with safe land tenure."

Another alternative, for growers who apply P and K once before corn in a corn-soybean rotation, is to apply nutrients for one crop this year, then fertilize next year for the second crop. They may save money if fertilizer prices are lower next fall. Mallarino cautions producers to account for soybean P and K needs when all fertilizer for the rotation is applied once before corn.

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