In the past, farmers have generally assumed that nitrogen fertilizers were needed on manured cornfields and questioned the value of using the late-spring soil nitrate test. In this project, Iowa farmers cooperated in studies at 111 sites to calibrate the test on manured cornfields. The value of animal manures as a nitrogen fertilizer for corn, and the possible benefits to corn producers, were also assessed. Results show that guidelines for using the late-spring test in manured cornfields need to be revised to decrease amounts of fertilizer nitrogen recommended. Widespread implementation of the late-spring soil nitrate test and the end-of-season cornstalk nitrate test should bring about more effective use of animal manue as crop fertilizer.