SOYBEAN GROWTH AND RHIZOSPHERE PH AS INFLUENCED BY A HORIZON THICKNESS

Crop growth and yield are known to be limited by A horizon thickness a nd may be related to more abundant and metabolically active roots In A than B horizon soil. The influence of A horizon thickness on root dis tribution and rhizosphere pH was observed in the field. Soybean [Glyci ne max (L.) Merr.] were grown On plots Of Mexico soil (fine, montmoril lonitic, mesic Mollic Endoaqualfs) with A horizon thicknesses of 0, 12 .5, 25, and 37.5 cm above horizons. Plant height, root and shoot growt h, and rhizosphere pH (pH in soil 0.5 mm from a root surface) were det ermined 25, 55, and 85 d after planting. Plant height and shoot and ro ot weight decreased significantly as A horizon thicknesses decreased f rom 37.5 to 0 cm, and these differences were attributed to later emerg ence of soybean seedlings from the thinner A horizons. Root distributi on with depth in the upper 30 cm was independent of A horizon thicknes s. Rhizosphere pH was 0.07 to 0.65 units lower than that of the bulk s oil, depending on soil horizon, root morphology, and plant age. This p H difference (Delta pH) averaged 0.36 in the A horizon and 0.18 in the Pt horizon and was independent of measurement depth within each horiz on. As bulk soil pH decreased from 6 to 5, the mean Delta pH for later al and main roots decreased 0.28, 0.21, and 0.10 units at 25, 55, and 85 d, respectively. The mean Delta pH for lateral roots was 0.34 and f or main roots was 0.20. The mean Delta pH for all roots and horizons w as 0.41 at 25 d, 0.28 at 55 d, and 0.11 at 85 d. The changes in Delta pH with root age, morphology, and depth suggest that Delta pH may be a n indicator of root activity.