Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 1999
Publication Date: October 1, 1999
Citation: Karlen, D.L., Kramer, L.A., James, D.E., Buhler, D.D., Moorman, T.B., Burkart, M.R. 1999. Field-scale watershed evaluations on deep loess soils: I. topography and agronomic practices. Journal of Soil and Water Conservation. 54(4):693-703. Interpretive Summary: Soil and water quality are affected by tillage and cropping practices in numerous but very subtle ways. Traditionally, agricultural evaluations have focused on crop yield, but to understand environmental impact, interactions among soils, plants, animals, landscapes, the atmosphere, and people making the management decisions must be considered. This paper summarizes more than 30 years of data collected on four field-size watersheds in western Iowa. It shows that soil erosion, water, and heat stress were the primary factors causing corn yield differences. The long-term (1964 to 1995) trend shows an average yield increase of 12 to 32 lb/acre/year in 3 of the 4 watersheds. However, within the watershed where pasture/hay land was converted to continuous corn, grain yield gradually decreased at a rate of 38 lb/acre/year for a period of 22 years. Meanwhile, during the same time period, the genetic potential for corn has increased by about 50 lb/acre/year. Our results show that growing continuous corn resulted in greater weed problems and led to a decrease in soil quality. Our recommendations to growers are that future soil and crop management practices on these watersheds should focus on increasing crop diversity to control weeds and reduce the apparent yield penalty associated with continuous corn. Management practices should also strive to develop soil conditions that are resilient and able to buffer periodic, but anticipated, plant water and heat stress.
Technical Abstract: Long-term field-scale watershed studies are needed to quantify productivity and environmental impacts of various farming practices. Our objective was to summarize geologic, topographic, and agronomic data for four deep-loess, field-scale watersheds in western Iowa. The geology underlying these fields was confirmed by deep (20 to 30 m) coring and is illustrated by an Order 1 soil survey draped over a digital elevation map (DEM). The DEM wa also used for landscape analyses. Long-term fertilizer, soil-test, and corn yield records are summarized and weed populations were quantified with seedbank measurements. Long-term grain yield was lowest on the most severely eroded sites and well correlated with heat stress during July and August. Combinations of available water and air temperature provided the best prediction of yield. Average corn grain yield for 1972 through 1995 on Monona, Ida-Dow, and Napier-Kennebec soils was 7.2, 6.7, and 7.9 Mg ha-1, respectively. After more than 20 yr of continuous corn production, the weed community had reached a high density and was dominated by foxtail species. Giant and Yellow foxtails comprised 95% or more of the weed seed in the soil, with the remainder being amaranthus, velvetleaf, and common sunflower. We recommend that future soil and crop management practices on these watersheds should focus on increasing crop diversity to control weeds and reduce the apparent yield penalty associated with continuous corn. The management practices should also strive to develop a soil condition that is resilient and able to buffer periodic, but anticipated plant water and heat stress.