Some factors affecting nodule formation on seedlings of leguminous plants
1935
Ludwig, C.A. | Allison, F.E.
1. The effect of the presence of older plants, including alfalfa, soybeans, wheat, and corn, on the nodulation of either alfalfa or soybean seedlings growing in close proximity was studied in sand cultures. Under conditions where the light intensity was not limiting, 33 Positive and 15 negative results were obtained. The percentage of positive results was approximately the same whether the older plants were legumes or non-legumes. This beneficial effect of the older plants did not appear with consistency and in most cases was much smaller than that observed by Thornton in his work with alfalfa. 2. In similar experiments, where older plants were not present, increased nodulation followed additions of sucrose and of a heavy inoculum. Small quantities of available nitrogen were sometimes slightly beneficial but larger quantities greatly depressed nodule formation. 3. Cold water extracts of sand in which alfalfa, corn, and wheat seedlings had been growing, produced no appreciable effect on nodule formation when added to cultures of alfalfa seedlings. 4. A logical explanation of the favorable effect of older plants on nodule formation seems to be the extreme favorableness of the rhizosphere to bacterial growth, this in turn being due in part to the liberation of the essential bacterial growth substance from the roots. 5. The practical importance of this effect under field conditions is probably negligible.
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