Editor’s Note: This article was excerpted from The Sweet Corn Nutrient Management Guide, Oregon State University. For more inforomation, go to http://extension.oregonstate.edu/catalog and type “Sweet Corn Nutrient Management Guide” in the search box.
Two approaches have been evaluated inOregonfor postharvest nitrogen (N) testing in sweet corn: stalk nitrate-N and soil nitrate-N testing. Both tests are designed to assess whether the overall plant-available N supply for the crop (from organic inputs, fertilizer, irrigation water, and N mineralized from soil organic matter) was excessive. Of the two end-of-season tests, the postharvest soil nitrate test, which will be discussed in this article, is the most useful and reliable.
Postharvest tests will not indicate whether N supply was deficient or adequate. When building an N monitoring program, start with the presidedress nitrate test (PSNT). In westernOregon, using the PSNT when sweet corn has five or six leaves is an excellent indicator of the need for additional N.
Postharvest testing has the most value where organic inputs supply N because accurate predictions of the timing and amount of plant-available N supply are difficult with these materials.
Determine Excess Nitrogen
The postharvest nitrate test is used to diagnose accumulation of excess N at the end of the season. Excess N accumulates when plant-available N supplied from all sources exceeds crop capacity for N uptake.
Begin by collecting soil (0- to 12-inch depth) immediately following harvest. When rainfall follows harvest and substantial crop residue decomposition occurs, soluble N may move from decomposing leaves into soil. This soluble N may add nitrate-N not present at harvest time, confounding test interpretation.
The effect of N fertilizer application rate on postharvest nitrate-N has been measured in a number of on-farm and experiment station field trials. Based on these trials, when a NO3-N (nitrate-N) test for the top foot of soil is above 50 pounds per acre, N supply from all sources was probably greater than needed for maximum ear yield.
An example of the relationship between N fertilizer rate, crop N uptake, and postharvest soil nitrate is shown in the chart below. Maximum ear yields occurred with 100 to 200 pounds of N per acre in these trials. At N fertilizer rates of 0 to 150 pounds of N per acre, sweet corn was efficient in N uptake, and postharvest nitrate-N was less than 50 pounds N per acre.
At the highest N fertilizer rate (200 pounds per acre), postharvest nitrate-N averaged 75 pounds per acre and had the most variability (range of 43 to 123 pounds per acre). Although a relationship has been shown between N fertilizer rate or N supply and postharvest soil NO3-N, keep in mind that soil NO3-N is linked with irrigation practices. Low NO3-N values at harvest may be a result of excess irrigation.