Nodulation, N2 Fixation, and Seed Yield of Navy Beans as Influenced by Inoculant Rate and Inoculant Carrier

Peat is the most commonly used carrier of Rhizobium sp. (rhizobia) in inoculants. The lack of suitable local peat in many areas of the world leads to interest in other materials as carriers of rhizobia. The purposes of these studies were 1) to compare potential carriers of Rhizobium phaseoli for their ability, under field conditions, to supply viable rhizobia to promote nodulation, N₂ fixation, and increased seed yield of navy beans (Phaseolus vulgaris L.), and 2) to determine the optimum rate of inoculation with Rhizobium phaseoli to promote nodulation, N₂ fixation, and increased seed yield of navy beans in two soils in Minnesota, a loamy sand (sandy, mixed, Udorthentic Haploborall) and a silt loam (fine-silty over sandy, mixed, mesic Typic Hapludoll). In 1978, five inoculation rates (ranging from 1 to 10⁸ cells/ cm of row) of liquid suspensions of R. phaseoli inoculants were applied at planting. In 1979, R. phaseoli in six inoculant carriers, peat, charcoal, vermiculite, corn (Zea mays L.) cobs, peanut (Arachis hypogaea L.) hulls, and liquid medium, was used to inoculate navy beans at rates of 2.5 ✕ 104 and 5 ✕ 10⁶ cells/cm of row. Nodule numbers and weight, acetylene reduction rates, seed yield, and seed N percentage were determined. In 1978, in most parameters measured, inoculation rates of 10⁵—10⁶ cells/ cm were required for a statistically significant increase over the lowest inoculation rate. In 1979 there was no effect of inoculation on a soil with a relatively high population of soil R. phaseoli (10⁴ cells/g soil) and relatively high mineral N content (30 ppm). On a soil with an indigenous population of Ã10 cells/g soil and mineral N content of 8 ppm, peat and charcoal inoculants resulted in the highest seed yield, nodule numbers and weight, and acetylene reduction rates. Peanut hulls and corn cobs gave the poorest responses. These results indicate that peat, charcoal, and vermiculite can be successfully used as carriers of R. phaseoli but peanut hulls and corn cobs are unsatisfactory.