Choose Wisely When Making Soil Fertility Decisions

With high input costs and declining crop prices, farmers need to manage crop production expenses.

With record high input costs and falling crop prices this fall, farmers want to manage their crop production expenses. Because fertilizer accounts for a significant share of the total cost of producing a corn or soybean crop, many farmers are considering holding off on fertilizer purchases in hope that fertilizer prices will fall. Other farmers are planning to cut back on the amount of fertilizer they apply per acre. There are reports of fertilizer prices declining a little in recent weeks, but they still remain a lot higher than last year.

Agronomy experts recognize the challenge of deciding how much fertilizer to buy and apply, but say the best course of action varies with each situation.

Antonio Mallarino, an 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."

Make a withdrawal from soil fertility bank

Moe Russell of Russell Consulting at Panora, Iowa, specializes in risk management and he agrees with the idea of skipping P and K application if your soil already tests high or very high for those nutrients. "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," notes Russell.

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

Tracy Blackmer, director of the Iowa Soybean Association's On-Farm Network, says it's also important to apply nitrogen each year for corn. "Because soil cannot hold nitrogen, it should not be cut," he says. "The form of N, application method and timing farmers choose are big variables. But the bottom line is, you need to apply nitrogen."

You have more options with P, K and lime

Farmers 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 farmer to respond to fertilizer prices.

However, Mallarino cautions, "Many farmers 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 farmers 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 farmers 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 farmers to account for soybean P and K needs when all fertilizer for the rotation is applied once before corn. Cutting back is not worth risking a reduced yield.

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