homepage logo

Soil fertility versus slim margins

By Staff | Jan 24, 2016

THIS ISU TEST plot shows how nitrogen-starved corn plants appear — light green and stunted — next to well-fertilized plants.



OKOBOJI – With tight profit margins predicted for 2016, Iowa State University Extension specialists are encouraging producers to be mindful of the cost versus results of their usual soil fertility practices.

Speaking at one of the Crop Advantage sessions in Okoboji earlier this month, Antonio Mallarino, an ISU Extension soil fertility specialist, told producers the practices they’ve been utilizing have been good, but in times when profit margins are tight, they need to take a magnifying glass to them and weed out things that are not absolutely necessary.

In a nutshell, Mallarino said

  • Nutrient and lime inputs should be based on maximizing potential economic return.
  • The Maximum Return to Nitrogen recommendation approach used in Iowa allows for an adjustment of suggested nitrogen application rate based on nitrogen and corn prices.
  • Applications of phosphorus, potassium and lime should be based on soil testing, with tight-margin applications based on immediate economic return.
  • Newly-recommended sulfur fertilization practices should be carefully examined.
  • Micronutrient applications should also be examined carefully, Mallarino said, stating Iowa research shows there is little need for micros in corn and soybean production.


Mallarino said nitrogen application is required for corn in most crop rotations because the soil system cannot solely sustain adequate crop-available nitrogen needed for high yields. The exception is first-year corn following established alfalfa.

He told producers the current approach for nitrogen rate guidance provides rates to attain the Maximum Return to Nitrogen – the point of maximum economic payback from nitrogen application.

ISU Extension publication CROP 3073 and ISU’s Corn Nitrogen Rate Calculator provide nitrogen rate guidelines based on field trials, with suggested MRTN rate and suggestions for the most profitable rate range.

“A rate at the lower end of the range could be appropriate with tight margins,” said Mallarino, adding that the maximum net return is influenced by the ratio of per-pound price of N to per bushel price of corn.

“Nitrogen inputs should be managed as best possible (in terms of timing and methods) to ensure high crop use and efficiency, including optimal application of other nutrients – mainly phosphorus, potassium and sulfur.”

P & K

Mallarino said it would not be good management to reduce P & K across the board.

“Soil testing is not a perfect diagnostic tool, but is useful – especially during times of unfavorable price ratios,” he said.

Mallarino said producers should not reduce P or K fertilizer rates in low-testing soils, even with uncertain land tenure because the economic benefits are both “large and likely.”

He said producers should not fertilize high-testing soils.

“Iowa research in fields managed with tillage, no-till or strip-till has shown that switching from broadcast to band application seldom reduces the rate that optimizes yield,” he said. “Only with ridge-tillage is K banding clearly better.”

Mallarino said variable rate fertilization remains a good practice in fields that have significant variation in soil test or yield levels.

Variable rate can be used to target applications to the most deficient field areas and to improve maintenance fertilization by considering yield variability.

“Producers – mostly those with uncertain land tenure – could temporarily reduce recommended removal-based rates to maintain soil tests in the optimum soil-test category, or apply starter only, because yield increases are small and infrequent,” said Mallarino. “This will increase profits in the short-term, but higher rates will be needed in the future.”


Spreading the cost of lime application could be attained by splitting large applications into multiple applications, and by applying lime to acidic soils with pH less than six.

“As with P and K, variable-rate lime is an excellent practice because most Iowa fields have significant variation in pH and lime needs,” said Mallarino.

Sulfur and micros

Mallarino said applications of sulfur should be limited to alfalfa and corn with field/soil conditions most conducive to a positive response, as he said there is no soil sulfur test or plant tissue calibrated in Iowa (except for a tissue test with alfalfa).

“Recent research in about 100 Iowa corn and soybean fields (along with older research) has shown no yield increase from micronutrients, except for isolated corn yield increase from zinc,” said Mallarino, adding that interpretations of other micronutrient soil and tissue tests used in other states do not apply to Iowa.

“Fertilization to correct soybean iron deficiency in calcareous (calcium-rich) soils has not been cost-effective,” Mallarino said.

He added during times of unfavorable grain prices, producers should apply zinc for corn only when called for by soil testing.

Please Enter Your Facebook App ID. Required for FB Comments. Click here for FB Comments Settings page