Soils at Prehistoric Agricultural Terracing Sites in New Mexico: III. Phosphorus, Selected Micronutrients, and pH

In this third paper examining long‐term effects of prehistoric cultivation on some New Mexico Mollisols about 900 yr after abandonment, data on total P, inorganic P fractions, Fe, Mn, Zn, Cu, and pH are presented. Cultivation‐induced soil changes, involving losses of total and moderately available P and total Cu but gains in total Mn, were inferred by comparing cultivated soils with nearby, similarly developed uncultivated soils (controls). Comparisons were also made between A horizons and sediment deposits accumulated upslope of recently fallen trees. These deposits are considered analogous to fresh agricultural terrace deposits just prior to cultivation. Total P losses in cultivated soils avg 15, 18, and 18% in the upper A, A, and BAt horizons, respectively, relative to control soils, and about 22% in the A horizon relative to recent sediment deposits. Below the BAt horizon, total P levels in cultivated and control soils converge. Moderately available P is not significantly lower ( p <0.05) in cultivated upper A horizons but is about 40% lower in cultivated complete A horizons. Other inorganic P fractions have not changed much in cultivated soils. While total Fe and Zn levels are unchanged in cultivated soils, Cu is lower by about 20% and Mn is 20% higher in cultivated A horizons. Manganese increase is attributed to sedimentation under reducing conditions accompanying terracing in cultivated areas. There is a trend towards slightly higher pH in cultivated A horizons.