LONG-TERM EFFECTS OF SUBSURFACE DRAIN SPACING ON SOIL PHYSICAL AND CHEMICAL PROPERTIES
Subsurface tile drainage is a commonly used practice to lower the water table in poorly drained soils, and is often done to improve soil conditions for agricultural operations. Tile drainage has been shown to increase cash crop yield, allow for more timely field operations, and reduce erosion. However, few studies have evaluated the potential long-term changes in soil physical and chemical properties as a result of subsurface tile drainage. This study was conducted on a naturally poorly drained Clermont silt loam soil located at the Southeast Purdue Ag Center near Butlerville Indiana. The intent of this study was to characterize possible evolution of soil physical and chemical properties after 35 years of subsurface drainage. The field site was established in the spring of 1983 with tile drains installed in 2 blocks with tile spacings of 5, 10, 20, and 40m, with the 40-m spacing used as the undrained control. Soil samples were collected in May of 2018 to a depth of 1 meter and were analyzed for carbon and nitrogen content, aggregate stability, and fertility at depth increments of 0-5, 5-15, 15-30, 30-50, 50-75 and 75-100cm. In-field measurements were also taken in May of 2018 for vane shear resistance and in May of 2019 for cone penetration resistance. Total carbon content was found to be significantly higher in the 5-m tile spacing than the 40-m tile spacing in the 0-5cm and 5-15cm depths, with the 10-m and 20-m tile spacings being intermediate. Conversely, in the 75-100cm depth the inverse trend was observed, where the 40-m tile spacing was found to have significantly greater carbon content than narrower tile spacings. Trends observed with carbon stocks per depth increment closely followed trends observed with carbon content at the same depth. However, no significant differences were observed among treatments with the summation of carbon stocks to the 1-m depth. Tile spacing did not have a significant effect on aggregate stability at any depth. The soil fertility data showed some indication of the potential translocation of soil calcium from the soil surface to lower depths in the soil profile resulting in significantly higher soil pH in the 5-m tile spacing than the 40-m tile spacing in all depths below 30cm. No consistent differences related to treatment were found with the cone penetrometer or vane shear penetrometer measurements. After 35 years of drainage history, tile drain spacing did not have a significant effect on total carbon stocks to the 1-m depth, but rather seems to have had a significant effect on the vertical distribution of soil carbon content throughout the soil profile.