Effect of Long-term Nitrogen, Phosphorus and Potassium Fertilization of Irrigated Corn and Grain Sorghum

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.


Justification
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.


Objectives
Determine responses of continuous corn and grain sorghum grown under flood irrigation to nitrogen, phosphorus, and potassium fertilization.


Methodology
Initial fertilizer treatments in 1961 to corn and grain sorghum in adjacent fields were N rates of 0, 40, 80, 120, 160, and 200 lb N/a without P and K; with 40 lb P₂O₅/a and zero K; and with 40 lb P₂O₅/a and 40 lb K₂O/a. In 1992, the treatments for the corn study were changed with the K variable being replaced by a higher rate of P (80 lb P₂O₅/a). All fertilizers were broadcast by hand in the spring and incorporated prior to planting. The soil is a Ulysses silt loam. The corn hybrid was Pioneer 3379 (1992-94), Pioneer (3225 (1995-97), and Pioneer 3395IR (1998) planted at 32,000 seeds/a in late April or early May. Sorghum (Mycogen TE Y-75 from 1992-1996, Pioneer 8414 in 1997, and Pioneer 8505 in 1998) was planted in late May or early June. Both studies were furrow irrigated to minimize water stress. The center 2 rows of all plots were machine harvested after physiological maturity. Grain yields were adjusted to 15.5% moisture for corn and 12.5% for sorghum.