ACTIVITIES OF RHIZOSPHERE MICROORGANISMS AS AFFECTED BY APPLICATION OF ORGANIC AMENDMENTS IN A CALCAREOUS LOAMY SOIL .1. CARBON ASSIMILATION

The activities of soil microorganisms in the rhizosphere (R) and nonrh izosphere (S) regions of alfalfa and wheal plants, growing in calcareo us loamy soil of Saudi Arabia, as influenced by application of organic amendments such as sludge and wheat straw, were studied in pot experi ments for 7 weeks. The study showed that counts of aerobic heterotroph ic bacteria in the rhizosphere soil of both alfalfa and wheat plants s ignificantly increased due to application of organic amendments accomp anied by a high decomposition rate for the carbonaceous materials. App lication of sludge to the soil enhanced the rhizosphere microorganisms during all experimental periods as compared with those treated with s traw. Straw manure clearly enhanced the aerobic cellulase decomposing bacteria, especially in the rhizosphere region. The total microbial co unts in the rhizosphere of alfalfa were generally higher than those of the wheat plants in the first and second weeks and at the end of the period especially when sludge was applied as an amendment. The densiti es of the the total fungi, actinomycetes, and sporeformers were slight ly affected by the rhizosphere soil, and the R/S ratios rarely exceede d 2.0. Soil treatment with either sludge or straw significantly increa sed the organic carbon and, consequently, the organic matter of the so il, especially in the rhizosphere region, with relatively higher value s for those amended with straw. A high carbon-to-nitrogen ratio of the amendment to the soil was associated with slow decay of the organic m atter Amendment of loamy soil with sludge at the common rare (2%) enha nced the microbial flora with no harmful effects to plant growth or so il fertility and can be used safely on agricultural land.