Effects of pH Level on Yields and Compositions of Pearl Millet and Alfalfa in Soils with Differing Degrees of Weathering

Bulk samples of soils representative of widely differing degrees of weathering (Mollisols, Alfisol, Ultisols, and Oxisols) were collected. Aliquots of each were adjusted to different pH levels, fertilized, and cropped successively to one cutting of pearl millet ( Pennisetum glaucum L. Willd.) and three cuttings of alfalfa ( Medicago sative L.). As postulated, the trends in crop response to pH differed with crop and with degree of soil weathering. Pearl millet responded very little to increase in pH, except that with very highly weathered soils yields increased significantly up to pH 5.6 to 6.2 followed by a significant decrease at higher pH. Significant increases in alfalfa yields with increased pH were obtained on all soils. However, yields from some of the soils increased prograssively to the highest pH; others leveled off at an intermediate pH, and still others decreased at the highest pH. The latter generally occurred in the most highly weathered soils. Soil and plant analyses as well as visual symptoms suggested that toxic levels of Mn (all soils) and insufficient Ca and high Al (one soil only) were major causes of low alfalfa yields at low pH. Depressed yields of alfalfa in the highly weathered soils at higher pH levels may be related to low levels of available P. The yield response to soil pH vs. degree of weathering relationship was not as clear cut as postulated, but this may not be so much a shortcoming of the hypothesis as it is a consequence of the difficulty in selecting individual soils representative of the highest levels of soil classification, and of the effect of specific parameters important to crop response‐soil pH relationships which transcend such soil groupings.