Crop Nutrient Guide Updated to Reflect Current Recommendations in Iowa

Research-based recommendations for phosphorus, potassium, micronutrients and limestone

February 27, 2023, 11:26 am | Antonio Mallarino

AMES, Iowa – Newly updated crop nutrient recommendations are available in a recent publication from Iowa State University Extension and Outreach.

Antonio Mallarino, professor in agronomy and extension specialist at Iowa State, announced research-based updates to the extension publication PM 1688 A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, available on the Iowa State University Extension Store and Soil Fertility websites.

The publication provides phosphorus (P), potassium (K), zinc (Zn) and lime application guidelines based on soil testing for the major agronomic crops grown in Iowa.

Iowa crop nutrients.Soil-test interpretations and nutrient application recommendations in this publication always have been based on short and long-term field experiments conducted across Iowa farmers' fields and research farms, from the major soils used for production agriculture in Iowa.

Mallarino said the last update was in 2013, and new recommendations were necessary because of improved crop genotypes that lead to higher yields, and because of changes in farming practices across Iowa.

The updated publication describes new soil-test interpretations and categories of soil testing as well as new suggested nutrient application rates.

“Crops are constantly changing and improving, and so are yields and the ways that Iowans farm,” said Mallarino. “Since 2013, nearly 200 field response trials have been conducted with each nutrient, and the results provide a solid foundation of how much nutrients we should be applying, to each crop and growing practice.”

He added, "However, the basic concept used for developing the new soil-test interpretations and application guidelines remains the same, which is to accomplish long-term profitability and reduced risk of yield loss while improving the sustainability of crop production."

Trials conducted since 2013 included several P or K fertilizer application rates and placement methods at multiple Iowa State research and demonstration farms – in the central, northeast, northcentral, northwest, southeast and southwest regions. The research farms have different soil types and slight weather differences that should be taken into consideration.

“The testing of numerous soil samples has increased our awareness of largely unavoidable, intrinsic variability of soil-test results mainly due to large spatial variability, which should be considered when interpreting test results and making fertilization decisions,” said Mallarino.

Key changes:

  • Boundaries of the interpretation categories were adjusted – mainly by moving upward the boundary for the optimum category to make it wider. Removal-based P and K application is recommended to maintain these desirable soil-test values over time.
  • The suggested P and K fertilization rates for the very low and low interpretation categories needed to be increased to assure fertilization for these categories maximize yield for most growing conditions and gradually buildup soil-test levels.
  • Interpretations for zinc by the Mehlich-3 test were added.

The soil pH and lime management guidelines, which are also included in the publication, were not updated because the recent research has confirmed changes made in 2013.

New physical copies are being printed and will soon be available from the Iowa State University Extension Store.

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