Occurrence of certain physiological groups of soil micro-organisms in the rhizosphere and rhizoplane of watermelon, cucumber, and cowpea.

A field experiment was run for studying the occurrence of ammonifiers, Azotobacter, aerobic cellulose-decomposers, and nitrifiers in rhizosphere soils as well as in rhizoplane samples of watermelon, cucumber, or cowpea at different stages of growth. In unamended soil, all the heterotrophs were generally stimulated in the rhizosphere of each plant, especially the leguminous one, resulting in positive rhizosphere effects. In amended soil, on the other hand, both negative and positive (but lower than in unamended soil) rhizosphere effects were recorded for the heterotrophs. The rhizosphere effects differed according to the type of plant, to the growth phase of each single plant, and to physiological properties of micro-organisms under study. A modified method for removing and treating the rhizoplane samples is suggested. The presence of high number of the heterotrophs and the absence of the autotrophs may support the evidence that roots of the studied plants are colonized with ammonifiers, Azotobacter, and cellulose-decomposers, but not with nitrifiers.