Phosphorus Placement and Rate in Reduced Tillage Systems.

Producers often question the need for better fertilizer placement methods in strip-tillage systems.  The objective of this study were to evaluate the long term effect of phosphorus (P) placement on corn (Zea mays) and soybean (Glycine max) yield.  The study was conducted at two locations, one under supplemental irrigation and one dry land, with a corn-soybean crop rotation established fall 2005.  Strip-tillage was conducted before corn and soybean was planted without previous tillage.  Treatments included a control, starter only at 10 kg P ha^-1, and two P rates of 20 and 39 kg P ha^-1 with placements broadcast, broadcast with starter, deep band, and deep band with starter, applied before planting corn.  Additional treatments included broadcast 20 kg P ha^-1 to soybeans subsequent to corn applications of broadcast with starter at 20 and 39 kg P ha^-1, totaling 40 and 59 kg P ha^-1 for the two year rotation.  Results show increasing significance of corn and soybean yield over time as soil test P levels were affected by application rates.  Both locations were low soil test P (STP) that should have shown response to fertilizer.  In Ottawa, corn yields were not increased with the addition of starter.  However, in Scandia, yields were significantly increased with the addition of starter with both broadcast and deep band at both the 20 and 39 kg P ha^-1 rates.  The additional broadcast to soybeans was found to increase soybean yield, showing insufficient P rates for the two-year crop rotation.