Evaluation of Soil Fertility in Some Prehistoric Agricultural Terraces in New Mexico

A greenhouse study was conducted to evaluate the fertility of some prehistorically terraced soils in New Mexico that were cultivated sometime between 1000 and 1150 A.D. and then abandoned. The impetus for this study was a finding that these soils still show the effects of cultivation nearly 900 yr after farming ceased. Soil changes, inferred by comparison with nearby uncultivated but otherwise similar soils, were mainly degradative, including accelerated erosion, compaction, and losses of organic matter and nutrients such as N and P. A traditional crop (Chapalote corn, Zea mays L.) and a standard test plant (barley, Hordeum vulgare L.) were grown for 28 d in samples of prehistorically cultivated and uncultivated soil, using a check and up to five fertilizer treatments: NPKS, PKS, NKS, NPS, and NPK. Fertility was assessed with measurements of plant growth and by chemical analyses of soils and whole plants. Results indicated that differences in plant growth and composition were associated with soil differences. For example, corn dry weights were about 45% lower in cultivated soils for the check and PKS treatments, and cultivated soils averaged 39% lower in total N relative to uncultivated soils. However, the opposite trend occurred in the NKS treatment because of lower available P in the uncultivated soil. Manganese uptake was greater in corn grown in cultivated soil. Corn dry weights were not significantly different between soils with the NPKS treatment. The five to ten times increase in plant dry weights in the NPKS versus PKS treatment indicates that N is the main limiting nutrient in the cultivated soil. The strong response of barley to N fertilization and comparison of dry weights to those obtained from similarly tested modern cultivated soils suggest that the soils used for prehistoric agriculture are potentially productive by modern agricultural standards. Journal Paper no. J-12666 of the Iowa Agric. and Home Econ. Exp. Stn.