Broadcast and Deep Band Placement of Phosphorus for Soybeans Managed with Ridge Tillage

ing. The band control received a coulterknife pass without fertilizer. Soybean response to direct P application was evaluated in seven trials (Sites 1 through 7). The treatments were applied in the fall (October or November) after harvesting the previous corn crop and before soils had frozen. At the remaining seven trials (Sites 8 through 14), P was applied 1 year earlier (in the fall prior to the previous corn crop). Soil samples were collected immediately before applying P treatments. Soil samples, comprised of 16 cores each, were collected from a depth of 0 to 6 in. One sample was collected from the ridges and another between the ridges (or valleys). At Sites 1 through 7 (direct fertilization), composite soil samples were collected from each experimental area. At Sites 8 through 14 (residual fertility), separate composite samples were collected from fertilized and unfertilized plots. Samples were analyzed for P with the Bray P-1 test. Aboveground portions of 10 soybean plants were sampled from each plot at the V5 to V6 growth stages. Total P concentrations in the plant tissue were measured and total P uptake calculated, based on dry matter accumulation. Grain yields were corrected to 13% moisture. Soybean Grain Yield. As Table 1 shows, statistically significant grain yield Fourteen trials with soybeans were evaluated in farmer fields managed with ridge-till during 3 years. All fields had been planted to corn the previous year, and the fields had 2 to 7 year histories of ridge tillage. Crop and soil management practices were those used by each farmer, except for P and potassium (K) fertilization. Row spacing was 38 in. except for Site 3, where it was 36 in. Phosphorus rates were 0, 29, and 115 lb P2O5/A, applied as granular triple superphosphate (0-46-0). The highest P rate approximately represented the 2-year rate currently recommended by Iowa State University for the corn-soybean rotation when soil test P (STP) is in the Low interpretation class (9 to 15 parts per million [ppm]), Bray P-1 or Mehlich-3 tests. Placement methods were broadcast and deep bands. The bands were approximately 6 to 8 in. deep and 1 in. wide. Banding equipment placed the fertilizer either through a vertical slit opened from the top of the ridge or through one ridge shoulder. The coulter-knife combinations opened and closed narrow slits (1 to 2 in.) that caused a minimum amount of disturbance of the ridge and placed the band 2 to 3 in. below the planned seeding depth, approximately under the planned seed row. The broadcast control received no fertilizer and the ridges were not disturbed until plantBroadcast and Deep Band Placement of Phosphorus for Soybeans Managed with Ridge Tillage