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Ion Gradients and Nitrification Associated with Decomposition of a Plant Material Layer in Soil¹

ABSTRACT

Changes in the pH and in concentrations of ammonia, NO₂^-, NO₃^-, and K⁺ with distance from layers of decomposing leguminous plant material in Red Bay sandy loam soil were measured after 16 days' incubation. High concentrations of ammonia (2.7 meq/100 g) and of K⁺ (1.4 meq/100 g) were found near the plant material layer. Nitrification was inhibited and NO₂^- was found in this zone. Farther away, NO₃^- accumulated. A change in pH from 8.9, near the layer, to 5.6, about 4 cm away, was observed. Treatment with CaCO₃ decreased the pH range and increased nitrification. When the decomposing plant material layer was separated from the soil on one side by an air space, NH₃ diffused across the air space. This resulted in segregation of some of the NH₃ from the cations and other potentially toxic substances and lowered the overall toxicity to nitrification. Besides the toxicity observed, the availability of plant nutrients as well as growth of microorganisms and plant roots could be drastically affected by pH and cation concentration changes found near layers of decomposing plant material.

¹ Contribution from the U. S. Soils Laboratory, Soil and Water Conservation Research Division, ARS, USDA, Beltsville, Md.

² Research Soil Scientists.

Received for publication September 11, 1964. Accepted for publication November 3, 1964.