EFFECT OF BORON ON NITROGEN FIXATION BY AZOTOBACTER

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Abstract
N fixation in 6 western soils doubled when the soils were treated with borax up to the rate of 160 lbs./acre, but fell off at the 320-lb. rate. Available B values in the untreated soils varied from 0.3 to 0.4 ppm. and at the peak fixation, available soil B varied from 0.8 to 1.2 ppm. These detns. were run on agar plus soil plates, N values being obtained by a modified Kjeldahl procedure after 7-, 14-, and 21-day incubation periods. Soil plaque tests indicated that Azotobacter was present in each soil. Suspensions of A. chroococcum were plated on N-free agar and inoculated into broth treated with rates of boric acid varying from 0 to 320 ppm. On the agar, N fixation was highest on the 7 day plates at the 40-ppm. rate. In broth, fixation was highest in the 20-ppm. cultures at 7 days. The 14- and 21-day cultures in both agar and broth were erratic but there was a common decline for all treatments following the 160 ppm. rate.

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