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Chemical Properties of the Rhizosphere in an Acid Subsoil¹

ABSTRACT

A method of corn seedling culture was developed which allowed growth of the root systems in samples of an acid subsoil (pH 4.7-5.0) for at least 2 weeks after seedling emergence, without adding additional water to the subsoil. This method was used to culture corn inbred and corn hybrid seedlings in samples of acid subsoil in two separate greenhouse studies. Bulk (non-rhizosphere) soil and rhizosphere soil samples obtained in these studies were assayed for pH, KCl-extractable Ca, KCl-extractable Al, and water-extractable Ca. The pH of rhizosphere samples was less than that of the bulk samples for all inbreds and hybrids. This pH decrease was 0.16 in one experiment with inbreds, and 0.11 in another experiment with hybrids. In comparison to the bulk soil, the rhizosphere soil contained 0.16 cmol_c kg^–1 more KCl-extractable Al, 0.13 cmol_c kg^–1 less KCl-extractable Ca, 0.023 cmol_c kg^–1 more water-extractable Ca, and 0.10 cmol_c kg^–1 less total Ca. The difference in the sum of KCl-extractable Al plus Ca between bulk and rhizosphere soils was not statistically significant. The results of both studies are consistent with the hypothesis that Ca uptake by the root depleted the Ca in the rhizosphere, that H ions were released by the root in response to excess cation (vs. anion) uptake and that this excess H⁺ dissolved basic Al compounds, which increased the KCl-extractable Al. The lack of statistically significant differences between rhizosphere and bulk soil parameters among hybrids/inbreds indicate that differential pH changes in the rhizosphere are not the chief mechanism of any differential Al tolerance among these corn inbreds and hybrids.

¹ Contribution from the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. Agronomy Dep. Paper no. 1565. This research was part of the program of the Center for Root Soil Research.

² Graduate Research Assistant and Professor, Dep. of Agronomy, Cornell Univ.

Received for publication January 10, 1986.