Effect of Rhizobium japonicum Inoculant Rates on Soybean Nodulation in a Tropical Soil1

Abstract Soybean [Glycine max (L.) Merr.] research and production is expanding in tropical areas. Successful soybean nodulation in a soil free of Rhizobium japonicum is dependent totally upon viable rhizobia from the inoculant. This study examined soybean nodulation as influenced by the rate of applied R. japonicum in a tropical soil without indigenous R. japonicum. Rates of R. japonicum were evaluated with soybeans grown in a field experiment on a Puerto Rican Coto clay (Typic Haplorthox, clayey, kaolinitic, isohyperthermic) lacking indigenous R. japonicum. Eight rates of liquid inoculation were added to the seed furrow at planting and supplied number of R. japonicum from log 2.59 cells/cm row with 10 fold increases through log 9.59 cells/cm row. R. japonicum rates of log 2.59 and 3.59 cells/cm row were not successful in establishing nodulation. Nodule numbers increased with increasing rates of applied rhizobia from log 4.59 up to the highest rate of log 9.59 viable cells/cm. Regression analyses indicated linear relationships between the rate of inoculation with rhizobia and the number of tap root nodules, total number of nodules, and nodule weight/plant. An inverse relationship was observed between weight/nodule and total number of nodules. Inoculation levels above log 5.0 rhizobia/cm were necessary to establish effective nodulation in a R. japonicum free tropical soil.