Long-term tillage and cropping systems affect bulk density and penetration resistance of soil cropped to dryland wheat and grain sorghum

Stubble mulch tillage (SMT) and no-tillage (NT) are well adapted for dryland crops in the US Great Plains. Long-term use of NT, however, may impair soil physical conditions and crop yields, and, by inference, soil quality and production sustainability. We determined effects of using SMT and NT in several cropping systems for dryland winter wheat (Triticum aestivum L.) and grain sorghum (Sorghum bicolor (L.) Moench) production on soil bulk density (BD), penetration resistance (PR), and water content (WC). We determined these in 1994 in plots of a tillage method and cropping system study started in 1982 on Pullman (Torrertic Paleustoll) clay loam at Bushland, TX, USA. Due to the nature of the study, a common statistical analysis of the data was not appropriate, but eight separate analyses were possible. Besides tillage method and cropping system, these allowed comparisons due to rotation phase, land condition (level or non-level), and crop grown. Soil BD and PR always increased with depth and WC often increased. The tillagexdepth interaction effect also was significant. Soil BD and PR were lower in the tillage layer (0-10 cm depth) in SMT than in NT plots, but no definite trends for BD were evident below 10 cm. Based on regression analyses, PR with SMT was related to BD and WC of the entire profile and most depth increments. With NT, PR was related to profile BD and WC, but only to WC for individual depths. These results indicate some strength factor largely independent of BD and affected by WC strongly influences PR of NT soil. Because NT does not disturb the soil, we concluded that stable biopores created by soil organisms and root channels reduced the effects of BD differences among NT plots and that NT soils developed a rigid structure independent of BD. Reports of improved trafficability on NT soils support this conclusion. Results of this study and previously reported crop yields suggest long-term use of NT will not impair the quality and production sustainability of this and similar soils under dryland cropping conditions.