Optimum Phosphorus Placement for Reduced Tillage Systems

Row-crop agriculture is under pressure to reduce sediment and nutrient losses to surface water resources by practicing less tillage and more precise application and placement of nutrients. Field studies started in 1997 were conducted through 2002 on very low to low P-testing (3 to 5 ppm Bray P1) and medium to high-P testing (11 to 19 ppm Bray P1) sites of a Nicollet-Webster clay loam complex at Waseca, MN. Four tillage systems (chisel + field cultivate, one-pass field cultivation or disking, strip-till, and no-till were compared in a corn-soybean rotation. Fertilizer P at rates of either 40 or 50 lb P205/A was applied to corn as either a starter with the seed or as a deep-band (4 to 5 inches) below the row and was compared to 80 or 100 lb P205/A applied broadcast and a no P control. The residual effect of the P treatments was measured in the soybean phase of the rotation. On the low P-testing site, tillage affected the 6-yr average corn yields but not the 1998-2002 soybean yields. Starter fertilizer increased corn yield by 49 bu/A/yr across the 6-yr period, while soybean yields were increased by15 bu/A/yr across the 5-yr period due to the residual effects of the starter. A starter fertilizer x tillage interaction was never found for either crop. Corn and soybean yields were generally increased over the 50-lb starter P and deep-band P treatments by the 100-lb broadcast P treatment suggesting that band applications at a half-rate are usually not sufficient in low to very low P-testing soils. On the high P-testing site, tillage affected the 6-yr and 5-yr corn and soybean averages. Highest yields were generally found with the chisel plow system followed by the one-pass and strip-till systems. Lowest yields were usually found with no tillage. Both corn and soybean yields were affected by the P treatments in 2002 (date not shown) because STP had declined to yield-limiting levels in the control plots in this third year of the corn-soybean cycle. Soil test P in the 0to 6-inch samples was greatly influenced by P management strategy whereas both tillage and P management strategy affected STP in the 0to 2-inch layer. In general, STP in the 0to 6-inch layer was greatest with the in-row starter and broadcast treatments. Broadcast P coupled with a shallow, one-pass tillage gave greatest STP in the 0to 2-inch layer. Economic return to fertilizer P averaged almost $100/A/yr on the low P-testing site with virtually no return to fertilizer P on the high P-testing site. On the low P-testing site, returns were greatest with the 100-lb broadcast treatment and lowest for the 50-lb "deep" band treatment. 1/ Soil Scientist and Professor, Southern Research and Outreach Center, Univ. of Minnesota, Waseca, MN 56093-4521.