This study was initiated in 1961 to determine responses of continuous corn grown under flood irrigation to nitrogen, phosphorus, and potassium fertilization. The study is conducted on a Ulysses silt loam soil with an inherently high K content. No yield benefit to K fertilization was observed in 30 years and soil K levels remained high so the K treatment was discontinued in 1992. However, a yield increase from P fertilization has been observed since 1965 and there was concern that the level of P fertilization may not be adequate. So, beginning in 1992, a higher P rate was added to the study which replaced the K treatment.