Tillage and Fertilizer Placement for the Corn-soybean Rotation

The information presented is part of ongoing research to identify effective conservation tillage systems and phosphorus (P) and potassium (K) fertilizer placements for corn and soybeans. The rate of adoption of the no-till system by Iowa producers has decreased after a marked increase during the late 80s and early 90s. Actually, the area of no-till corn has decreased slightly since 1994. Several aspects could explain this trend but slow early growth in spring, a perception of lower corn yields, less yield stability over time, and concerns about appropriate fertilizer management are major factors. Although the increased residue cover usually improves moisture availability, root growth, and nutrient uptake efficiency at shallow soil layers. However, it may also reduce soil temperature in spring, delay planting, and reduce early growth. Broadcast fertilizer placements are less costly than banded placements but they seem inefficient for no-till fields because fertilizers are not incorporated. Because of the reduced movement of P and K in soils and nutrient cycling with residues, broadcast applications result in P and K stratification and accumulation within the top 2 or 3 inches of soils. The stratification could result in lower total P and K uptake by plants especially when topsoil is dry. Another related aspect is that producers are uncertain about the value of soil testing in conservation tillage because of the large variability and lack of knowledge concerning techniques for collection of samples. A large research project was initiated in 1994 to address these questions with the support of the Iowa Soybean Promotion Board, the Leopold Center for Sustainable Agriculture, Practical Farmers of Iowa, the Iowa State University Outlying Research Centers, and numerous Iowa producers. Ten longterm trials with the corn-soybean rotation were established on five Iowa State University research farms in 1994 and continue to be evaluated. These experiments evaluate chisel-disk, no-till, and fall striptillage (with no-till) in combination with various P and K fertilizer placements. Fifty-five additional short-term trials were established since 1994 at producers' fields managed with no-till and ridge-till. This presentation summarizes results from the tillage-placement trials at the research farms with the cornsoybean rotation and no-till short-term trials at producers' fields.