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The oxidizing power of rice roots was observed in narrow transparent root boxes containing different media. Plants precultivated in nutrient solution were embedded in semisolid agar medium to observe oxidation of ferrous iron cations and leuco methylene blue as well as solubilization of ferrous sulfide. In the presence of ferrous sulfate reddish brown coloration due to formation of ferric oxide/hydroxide was observed around the roots and on the root surface during one day of incubation. When agar medium blackened by ferrous sulfide was used, the root zone became transparent. Within a few hours leuco methylene blue was oxidized to methylene blue on and near the roots. Furthermore, seedlings were grown in agar medium containing ferrous sulfide inoculated with soil filtrate. Besides diffuse ferric iron precipitation, iron was also deposited on spherically shaped structures in the rhizosphere and near the agar surface as well as in slimy layers appearing on the root surface. The spherical structures and slimy layers were obviously bacterial colonies extending with time. As the roots grew old, parts of them turned black. In the rhizosphere, black spots occurred resembling colonies of sulfate‐reducing bacteria. Rice was also grown in sand supplemented with nutrients and iron sulfide. While root growth was straight in agar, it was twisted in the sand medium. Again, heavy ferric iron deposition occurred on the root surface. On older root parts the lateral roots became blackish. The results suggest participation of bacteria in ferric iron deposition in the rhizosphere of rice.

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A comparison was made of 15 different techniques which are used in assessing soil microbial populations and/or biomasses. These include direct observations (fungal standing crop, fluorescein diacetate active mycelia, acridine orange stained bacteria), cultural methods (bacterial plate counts), physiological methods (total microbial, bacterial and fungal biomasses, O2‐uptake), soil enzyme analyses (dehydrogenase, catalase, alkaline and acid phosphatase, protease, amylase), and ATP‐analyses. The various techniques were applied to six soils known to have different microbial characteristics. The results are discussed with respect to the convertability of counts and measurements into microbial biomasses, the variability of the techniques, the correlations within comparable groups of methods, and the practical limitations in application of individual methods to different soils.

The effect of soil moisture content and K nutrition on denitrification measured by the acetylene inhibition method in short‐term assays was studied in pot experiments with wheat on a low‐nutrient sandy soil. Increasing the soil moisture from 60 to 80 or 100% water holding capacity (whc) increased denitrification which was further increased when the soil moisture was raised to 100% whc 24 h before the assay. This effect was not observed with unplanted pots. Denitrification increased with progressing plant age, provided sufficient nitrate was present. N2O production was particularly high after removal of the shoots as decaying root matter improved conditions for denitrification. Optimal plant growth reduced denitrification through more rapid nitrate uptake and lower soil moisture, whereas poor plant growth, induced by K deficiency, restricted NO3 uptake and sustained high soil moisture causing higher N2O production. In a separate experiment it was demonstrated that denitrification occurs mainly in the immediate vicinity of the roots.