Categorizing Nitrogen Availability From Manured Corn Fields Using Field History and the Cornstalk Nitrate Test.

Previous studies indicate that there is no significant relationship between N availability to corn and amount of manure applied. Making N recommendations for corn (Zea mays L.) for fields with a history of manure applications is complicated by the uncertainty of the amount of N supplied by the soil. The amount of N supplied by the soil is greatly affected by the C:N ratio of previous manure applications, the application rate in the past and the duration of applications, which is information that is difficult to impossible to obtain. This study categorized N availability from corn fields with histories of manure applications using the cornstalk nitrate test. The categories can be used to improve N guidelines for manured fields. Farmers provided their estimate of the rate of manure applied for the upcoming growing season; the type and time of manure application, fall, spring or irrigated in-season; the frequency of previous applications, 2 of 4 yr or 8 of 10 yr; crop rotation; and type, amount, timing and rate of fertilizer N applications. The information was collected from 30 farms and 1000 fields from 2004 to 2008. Ordinal logistic regressions will be used to estimate the odds ratio of a stalk sample to test in a higher stalk test category. The analyses will provide a relative ranking of the effect of manure history on the availability of soil N to corn during the growing season.