Research Note: Soil Testing and Fertilizer Management

By Olga S. Walsh, Research Associate Professor, Cropping Systems Agronomist, University of Idaho Parma Research and Extension Center

For the Idaho Wheat Commission Newsletter

Soil testing is the key component of successful and sustainable fertilizer management for wheat. In a survey conducted in 2015-16, 60 percent of wheat growers listed fertilizer as the costliest input in their farming operation. Growers know their fields are not uniform and require changing fertilizer management from field to field and year to year.

“Idaho wheat growers would need a yield increase of 4.5 bushels per acre to cover the cost of an additional 50 pounds of applied nitrogen.” – Dr. Patrick Hatzenbuehler, University of Idaho Agricultural Economist

Dr. Olga Walsh, Cropping Systems Agronomist at the University of Idaho Parma Research and Extension Center, is conducting field trials to quantify the typical residual nitrogen and phosphorus in Idaho fields prior to planting wheat. Soil testing helps growers apply fertility needed for optimum profitability.

In 2019 demonstration plots at Parma, showed the response of wheat to nitrogen and phosphorus fertilizer. Soft white spring wheat (Seahawk) was fertilized with five nitrogen and five phosphorus rates. Results showed that addition of phosphorus fertilizer did not improve any of the measured wheat parameters. In fact, phosphorus application above 50 ppm resulted in a significant yield decline. This becomes important when using manure or compost. Application rates need to consider both nitrogen need and phosphorus content.

An economic study showed nitrogen applied at 150 pounds per acre rate may be enough to optimize yield of soft white spring wheat while balancing yield against fertilizer cost. Addition of nitrogen beyond 150 pounds per acre did not increase yield or grain quality.

According to Dr. Patrick Hatzenbuehler, University of Idaho Agricultural Economist, Idaho wheat growers would need a yield increase of 4.5 bushels per acre to cover the cost of an additional 50 pounds of applied nitrogen. This estimate is based on the current nitrogen fertilizer price of $0.4 per pound, and the national wheat price for market year 2019-2020 of $4.5 per bushel.

Dr. Walsh recommends taking soil samples across all fields prior to planting wheat. Understand the levels of nutrients already in the field’s soil. If using manure or compost, know the Nitrogen and the Phosphorus levels of the product. Use the UI Extension Crop Enterprise Tool  for wheat to find the sweet spot between fertilizer application, yield and cost of inputs, compared to price/bushel at harvest. Highest yield doesn’t necessarily equal profitability at the end of harvest.

Idaho wheat grower’s assessment dollars helped fund the Improving Nutrient Management for Wheat Through Comprehensive Soil and Crop Survey project.

Do you want to learn more? Contact Dr. Olga Walsh, Parma Research and Extension Center, 29603 U of I Lane, Parma ID, 83660, (208) 291-6218 or owalsh@uidaho.edu.