Properties of rhizosphere soil associated with herbaceous plant roots analyzed using small-scale protocols

The rhizosphere, which is the region of soil adjacent to plant roots, is affected by the activities of both plant roots and associated microorganisms which cause changes in soil properties including nutrient mineral composition. Accordingly, the actual availability of plant nutrients may not be the same as that estimated on the basis of bulk soil analysis. However, the extent and manner in which the availability of plant nutrients in bulk and rhizosphere soils differ remain unclear. Therefore, the present study defined the rhizosphere as the soil adhered to plant roots, established a set of small-scale protocols for analyzing the nutrient minerals of small soil samples, and then characterized the rhizosphere soil of sorghum, Sorghum bicolor (L.) Moench. The mineral contents of the bulk and rhizosphere soil differed significantly, with nutrient contents generally greater in the rhizosphere, and particularly remarkable accumulation was observed in regards to ammonium ion and exchangeable potassium concentrations. Such accumulation might be due, in part, to the greater per weight surface areas of rhizosphere soil particles, but other mechanisms, including the accumulation of organic matter, could also be involved.