Deficiencies can be costly. McFarland said nitrogen may be the most challenging to manage since it changes and transforms after application and may be lost to soil leaching or into the atmosphere.

“We recommend a split application for wheat,” he said. “That way we make certain we have nitrogen available at peak demand and also minimize the potential for losses from heavy rainfall or other climate factors. Split applications also give us an opportunity to judge crop conditions and potential before we top-dress so we can apply the most effective and most economical rate.”

Rate and timing also depend on crop use. Demand may be different for wheat intended for grain, grazing or as a dual purpose crop. Yield potential and available moisture also affect fertility rate.

McFarland said farmers should look deeper for residual nitrogen, especially if they planted and fertilized a crop last year and drought prevented typical production. “Nitrogen is very soluble and moves in the soil profile, so we recommend farmers sample deeper than the zero to six-inch depth they usually pull from.”

Nitrogen that has moved deeper into the soil profile is still available to the crop. Tests have shown substantial amounts of nitrogen as deep as 48 inches — from 78 to 210 pounds per acre, McFarland said. “At just 24-inch depths, we still find significant amounts of nitrogen available. With the current price of fertilizer, that may represent a significant savings.”

He said farmers who use residual nitrogen can decrease typical application rates by the amount identified and produce equal yields. “But farmers have to measure it to know how to adjust rates.”

Savings could range from $10 to more than $100 per acre, depending on the crop need and the amount of residual nitrogen available. “And that’s just for nitrogen; other nutrients are also found at depth. We recommend farmers consider deep sampling, especially if they put out fertilizer last year and had no crop to utilize it.”

Farmers will have to pull two samples, one from the usual zero to six-inch depth to determine nitrogen, phosphorus, potassium calcium, magnesium, sulfur, micronutrients, and pH. They’ll need to take a second sample from zero to 18 or 24 inches to determine residual nutrient levels with depth.

He also suggested farmers use N-rich strips in season to judge the proper amount of top-dress nitrogen. N-rich strips consist of narrow strips laid out in the wheat field with each strip receiving a different rate of pre-plant fertilizer. Strips are fertilized with increasing rates of nitrogen (for example zero, 40, 80, and 120 pounds per acre) up to the maximum rate for the expected yield.

Producers evaluate each strip to determine between which increments there is no difference in plant growth. They use the last discernible improvement to establish top-dress rate. If the last strip to show a yield advantage is 80 pounds per acre, and the grower applied 40 pounds per acre at planting, he needs to top-dress his field with 40 pounds to bring the total to 80 pounds.