Qualitative Studies of Soil Micro-Organisms: Iv. The Rhizosphere in Relation to the Nutritive Requirements of Soil Bacteria

Abstract

Bacteria of the rhizospheres of flax and tobacco were found to possess more complex nutritive requirements than those in the corresponding control soils. The roots of even young seedlings favour the development of those types that are dependent upon a supply of thiamin, biotin, and amino nitrogen for their growth, thus suggesting that the roots may excrete significant amounts of these stimulative substances. The "rhizosphere effect" was more pronounced with susceptible than with resistant varieties of either flax or tobacco. A greater difference was found to exist between the rhizosphere and the control soil where the latter is poor than where it is richly supplied with organic matter, since liberation of growth substances by plant decomposition permits a limited development of the more typically rhizosphere forms, apart from the zone of influence of the growing plant.

The Rhizosphere Effect of Graminaceous Plants in Virgin Soils

“…This evidently holds good for the effect of the hay plants investigated, even in virgin soils. As to the sporeformers, it is reported that they do not multiply extensively in the rhizosphere (Krassilnikov, Kriss, and Litvinov, 1936, Starkey, 1938, Clark, 1939, Lochhead, 1940. The present investigation showed that this group is promoted under certain conditions, and by certain plants, but that the degree of the »rhizosphere effect* upon these bacteria in general is distinctly weaker than on the non-sporogenous bacteria.…”

“…Numerous earlier investigators have called attention to the difficulties occurring in connection with the classification of »autochthonous* nonsporogenous soil bacteria. As shown by Lochhead and his co-workers (West andLochhead, 1940, Lochhead andChase, 1943), the nutritional characteristics of these bacteria may provide a more accurate basis for the classification than the morphological and physiological properties ordinarily examined. A separate investigation of the nutritional features of the strains isolated during the course of the present study is in progress.…”

Studies of Rhizosphere Activity by the Use of Isotopically Labelled Carbon

“…Nevertheless, the existence of microorganisms in the rhizosphere with precise nutritive requirements (9,12,13,24,26), and the preferential stiinulation of particular groups of microorganisms by the diverse techniques einployed (7,9), inevitably impose certain iimitatioils on the accuracy of numbers of microorgallisms as an index of rhizosphere activity. In the hope of supplementing this technique, an atteinpt was made to estimate directly changes in inicrobiological activity by supplying radioactive carbon dioxide to aerial portions of tomato plants growing in natural soil and determining subsequently the water-soluble radioactivity within the soil without dist~irbing the root system.…”

Microbiology of Soil

“…That the rhizosphere flora is affected by the secretions from the roots was established by West (192) and West & Lochhead (193). Thiamine, biotin, and amino acids were secreted and favored the development of those types of micro organisms that had complex nutritive requirements.…”