Tillage Effects on Previously Flooded Soils

Abstract

Excessive rainfall inundated many midwestern agricultural fields in 1993 and resulted in numerous questions regarding the depth and number of tillage operations needed to prepare fields for crop production in 1994. Five on-farm locations were selected to compare effects of shallow, moderate, and deep tillage on soil compaction and crop yield. Bulk density, soil water content (θV), penetration resistance, and yield of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were measured, and net return was computed for the three tillage treatments. Preplant bulk density within the upper 12 in. averaged 1.27, 1.26, and 1.22 g/cu cm for no-till, chisel, and subsoiled treatments, respectively. Penetration resistance averaged 130, 120, and 97 psi for the no-till, chisel, and subsoil treatments, respectively (LSD(0.05) = 7), and showed highly significant differences at depths from 0.5 to 12 in. Post-harvest bulk density was not significantly different, but penetration resistance averaged 378, 341, and 283 psi (LSD(0.05) = 19) for no-till, chisel, and subsoiled areas, respectively. Increased preplant tillage intensity significantly affected some soil physical properties, but corn and soybean yields were not affected. With regard to return on investment and time, we conclude it would be more profitable to limit tillage in fields subjected to excessive rainfall and inundation events like those occurring throughout the Midwest in 1993. Research Question What type of tillage should be used on silty clay loam soils that appear to have been compacted by intensive rainfall? Literature Summary Numerous studies have been conducted to evaluate soil compaction, but very few address compaction thought to be caused by excessive or intensive rainfall. Conclusions of soil compaction studies are also highly variable depending on whether the primary focus was to determine effects on soil properties, wind and water erosion, crop yield, or the economic bottom-line. The lack of information available regarding optimum preplant tillage for soils which received excessive rainfall during a previous growing season prompted this study. Study Description On-farm studies were conducted on silty clay loam soils at five locations within a 20 mi radius in Henry, Des Moines, and Louisa Counties in southeastern Iowa. All five sites received rainfall that was 66 to 85% above normal in 1993 and produced crop yields that were at least 50% below normal. Two replicates (~5 acres each) were established at each location to evaluate shallow, moderate, or deep preplant tillage. Shallow tillage included no-till or very shallow field cultivation if available planter units were not suitable for no-till planting. The moderate and deep tillage treatments consisted of chisel plowing or subsoiling in autumn, followed by field cultivation to prepare a seedbed the next spring. Corn was planted at three sites and soybean at two, with all having a history of being managed in a 2-yr crop rotation. Soil penetration resistance, bulk density, crop yield, and a simple economic analysis comparing income minus costs for machinery operations were used to evaluate the treatments. This study represents a unique condition created by unique weather patterns that can seldom be duplicated. Applied Question Does intensive rainfall result in yield-limiting soil compaction? Very little research information is available to help answer this question. In 1993, rainfall throughout the midwestern USA was 50 to 100% above normal and rainfall-induced soil compaction was perceived to be a major problem. This on-farm study conducted using “plots” that were ~5 acres in size showed that deep tillage significantly reduced soil bulk density prior to planting and cone penetration resistance throughout the growing season. However, corn and soybean yields were not increased. A simple economic analysis was made using measured yield response, costs associated with having the tillage treatments performed by a custom operator, and assuming that (i) secondary tillage would be needed following chisel plowing or subsoiling, (ii) planting cost for all treatments was the same, (iii) grain values were $3/bu for corn and $7/bu for soybean, and (iv) assuming weed control and other input costs were the same for all treatments. Based on those assumptions, the net return for corn was $54/acre less with subsoiling and $42/acre less with chisel plowing than with no-tillage or the least amount of tillage needed to prepare a seedbed and ensure proper operation of available planters. For soybean, moderate tillage (chisel plowing) returned $20/acre less than the shallow or no-tillage treatment. Recommendations Our results indicate that money spent on subsoiling or chisel plowing was essentially wasted on a perceived problem of rain-induced soil compaction. For silty clay loam soils such as those upon which this study was conducted, we recommend using the least amount of tillage necessary to prepare a seedbed for crops grown after a year of intensive and extensive rainfall rather than investing in subsoiling or other forms of deep or moderately deep tillage.