Iron fertilization In relation to nodulation and nitrogen fixation in French bean (Phaseolus vulgarisL.)

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Abstract
Pot and field experiments were conducted to study the effects of three iron sources, viz., ferrous sulphate, Fe‐EDDHA and Fe‐EDTA on nodulation and nitrogen fixation in french bean (Phaseolus vulgaris L.). This plant, as a general, does neither harbour functional nodules nor enlodge the nitrogen‐fixing symbionts (rhizobia) when grown in and around Varanasi soils where natural availability of iron is 2–4 mg kg‐1 soil. Inorganic iron treatments were given to the soils without any exogenous inoculation with the Rhizobium germ plasm. Iron, in general, at a concentration of 5 to 10 mg kg‐1 soil stimulated the production of functional nodules and subsequently increased the dry weights of plants. Remarkably, the number of functional nodules and their leghaemoglobin content were more in the cases of plants treated with Fe‐EDDHA and Fe‐EDTA (@ 5 mg kg‐1 soil) than in the FeSO4 treated ones. Approachingly similar nodule initiations were recorded with the application of a double dose of FeSO4, i.e., @ 10 mg kg‐1 soil. In addition an enhanced in vivo nitrogen fixation (total nitrogen yield) over the control (untreated) level was noted in both pot and field experiments with the identical Fe treatment(s). On the basis of a critical comparative assessment on the individual effectiveness of the three iron sources in the production of functional nodules in french bean, Fe‐EDDHA was designated to be superior most, followed by Fe‐EDTA and FeSO4 in the decreasing order. The known higher stability constant of Fe‐EDDHA, coupled with its higher mobility in plants, are the probable reasons for its apparently greater supporting effects over those induced by other iron sources used during the present investigation. Obviously, the results indicate that Fe could create a condition conducive to the formation of functional N2‐fixing nodules in P. vulgaris. The differential dose response of the two iron sources were significant specifically @ 5 mg Fe‐compound kg‐1 soil. Hence, fanners may be suggested to make use of the benefits of any of the iron source economic to them.