Organic Matter and Water‐Stable Aggregate Distribution in Ridge‐Tilled Surface Soil

Soil surface conditions improve with increased organic matter (OM) and water‐stable aggregation (WSA), but surface variability may influence values obtained when soils are sampled to evaluate these factors. My hypothesis was that cultural operations for sorghum [ Sorghum bicolor (L.) Moench] grown on ridges produce zones of differing soil OM and WSA. Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll) was sampled at the 0‐ to 4‐cm depth in furrows, on ridge tops, and on ridge sides before and after planting, after reforming ridges, and after harvest in 1992. Tillage (conventional [CT] and ridge [early (RT‐E) or late (RT‐L) stalk shredding]), position, and crop cycle (sampling) stage affected OM and WSA. Compared with ridges, OM in furrows was greater due to residue accumulations. Mean WSA was greater on ridges with CT than with RT‐E or RT‐L because disk‐bedding to build the CT ridges moved more stable aggregates to ridges than the other treatments. More thorough mixing of residues by CT may have been involved also. Ridge and furrow positions and possibly ridge sides require separate sampling for obtaining representative OM and WSA data from ridge‐tilled soil, especially where ridge tops are cleared at planting. Ridge clearing at planting should be limited to avoid residue reduction and lowered WSA that could lead to soil crusting and impaired seedling emergence.