Previous research had identified four serogroups of Rhizobium trifolii indigenous to the acidic Abiqua soil (fine, mixed, mesic Cumulic Ultic Haploxeroll). Nodulation of subterranean clover ( Trifolium subterraneum L.) by two of the serogroups, 6 and 36, was differentially influenced by an application of CaCO 3 which raised the pH of the soil from 5.0 to 6.5. These studies were designed to characterize this phenomenon more comprehensively. Liming the soil with either CaCO 3 , Ca(OH) 2 , MgO, or K 2 CO 3 significantly ( P = 0.05) increased the percent nodule occupancy by serogroup 36, whereas the percent nodule occupancy by serogroup 6 was decreased, but the decrease was significant ( P = 0.05) only after application of either CaCO 3 or Ca(OH) 2 . Application of KH 2 PO 4 (25 mg of P kg of soil −1 ), which did not change soil pH, also significantly ( P = 0.05) increased the percent nodule occupancy by serogroup 36. Application of KH 2 PO 4 in combination with Ca(OH) 2 produced the same increase in nodule occupancy by serogroup 36 as did individual application of the two materials. Soil populations of serogroup 36 consistently, and in the majority of cases significantly ( P = 0.05), outnumbered those of serogroup 6 before planting and after harvest regardless of soil treatment or the outcome of nodulation. Soil chemical and plant analyses provided no evidence that liming was simulating phosphate addition by increasing the availability and subsequent uptake of soil P i by the subclover plants. Liming did, however, result in a significant transformation (30 to 50 mg of P kg of soil −1 ) of P i from the residual soil P i fraction into an NaOH-extractable organic P fraction during the preplant equilibration period.

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