Crop growth and yield are known to be limited by A horizon thickness and may be related to more abundant and metabolically active roots in A than B horizon soil. The influence of A horizon thickness on root distribution and rhizosphere pH was observed in the field. Soybean [Glycine max (L.) Merr.] were grown on plots of Mexico soil (fine, montmorillonitic, mesic Mollic Endoaqualfs) with A horizon thicknesses of 0, 12.5, 25, and 37.5 cm above Bt horizons. Plant height, root and shoot growth, and rhizosphere pH (pH in soil 0.5 mm from a root surface) were determined 25, 55, and 85 d after planting. Plant height and shoot and root weight decreased significantly as A horizon thicknesses decreased from 37.5 to 0 cm, and these differences were attributed to later emergence of soybean seedlings from the thinner A horizons. Root distribution with depth in the upper 30 cm was independent of A horizon thickness. Rhizosphere pH was 0.07 to 0.65 units lower than that of the bulk soil, depending on soil horizon, root morphology, and plant age. This pH difference (ΔpH) averaged 0.36 in the A horizon and 0.18 in the Bt horizon and was independent of measurement depth within each horizon. As bulk soil pH decreased from 6 to 5, the mean ΔpH for lateral and main roots decreased 0.28, 0.21, and 0.10 units at 25, 55, and 85 d, respectively. The mean ΔpH for lateral roots was 0.34 and for main roots was 0.20. The mean ΔpH for all roots and horizons was 0.41 at 25 d, 0.28 at 55 d, and 0.11 at 85 d. The changes in ΔpH with root age, morphology, and depth suggest that ΔpH may be an indicator of root activity.
Contribution from the Missouri Agric. Exp. Stn. Journal series no. 12133.