How Adaptive Management Using Field Records and Corn Stalk Nitrate Results Informs Farmers about Nitrogen Availability From Manure.

Nitrogen recommendations for corn where manure is applied are commonly made by subtracting a N credit for the manure from a total N recommendation calculated using the yield goal concept or the N Rate Calculator. Nitrogen availability from manure is difficult to estimate, and farmers are often thought to discount or ignore the N credits due to the uncertainty of the estimates. We used field records and cornstalk nitrate results from greater than 2000 field-years to evaluate the accuracy N credits from manure applications made by farmers to corn fields in Lancaster County Pennsylvania. The results show that N credits typically used by soil testing laboratories were poorly related to corn stalk nitrate N concentrations and amounts of manure reported by farmers. The results also revealed that when manure was applied for the growing season when the corn stalk nitrate was collected or when a field had a long-term history of manure applications there was a two fold increase in the probability of a field testing in a higher corn stalk nitrate category. The amount of N applied by the farmers as manure or fertilizer compared with the amount recommended by various soils testing laboratories indicated that the farmers are making substantially different estimates of N availability. The results of the corn stalk nitrate test suggest that neither the farmers nor the soil testing laboratories were capable of accurately estimating N availability from the manure. These data indicate the need to use corn stalk nitrate results, field-by-field records, which include previous crop, manure history, manure applications, and tillage for a number of years to make accurate estimates of N availability from manure.