Graduate Thesis Or Dissertation
 

Influence of soil acidity upon nodulation and growth characteristics of Trifolium subterraneum L.

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  • Previous research had identified four serogroups of Rhizobium trifolii indigenous to Abiqua soil (fine, mixed, mesic, Cumulic Ultic Haploxerolls). Nodulation of subterranean clover (Trifolium, subterraneum L.) by two of the serogroups, 6 and 36, was influenced by the application of CaC0₃ to the soil. The studies described in this thesis showed that liming the soil with either CaC0₃, Ca(OH)₂, MgO, or K₂C0₃ increased significantly (P =0.05) the percentage nodule occupancy by serogroup 36 whereas the percentage nodule occupancy by serogroup 6 was decreased by liming, but only significantly (P = 0.05) from applications of CaC0₃ or Ca(OH)₂. Application of KH₂P0₄ (25 mg P kg⁻¹ soil), which did not change soil pH, nor increase P uptake by subclover plants, also increased significantly (P =0.05) the percentage nodule occupancy by serogroup 36. Application of KH₂P0₄ in combination with Ca(OH)₂ produced the same increase in nodule occupancy by serogroup 36 as when the two materials were applied individually. In contrast, although KH₂P0₄ addition had no influence on percentage nodule occupancy by serogroup 6, a combination of Ca(0H)₂ and KH₂P0₄ prevented the negative influence of Ca(OH)₂ when the latter was applied alone. Soil population of serogroup 36 consistently and in the majority of cases, significantly (P= 0.05) outnumbered serogroup 6 regardless of soil treatment or the outcome of nodulation. Soil chemical and plant analyses provided no evidence that liming was increasing the availability and subsequent uptake of soil P by the subclover plant. Liming did, however, result in significant transformation (30 to 50 mg P kg⁻¹ soil) of inorganic P from the apatite and residual P fractions into a NaOH-extractable organic P fraction. Further studies under superior plant growth conditions provided evidence for differential behavior of the indigenous serogroups. Monobasic potassium phosphate, but not Ca(H₂P0₄)₂ or Mg(H₂P0₄)₂, increased significantly (P = 0.05) the percentage of nodules occupied by serogroup 6. All phosphate sources and KCI were found to stimulate the transformation of undifferentiated cells of serogroup 6 into bacteroid form. No such differential effects were observed on serogroup 36. Differences between the two serogroups in the distribution of cell size classes were observed in soil populations. A greater proportion (x =49%) of the population of serogroup 36 passed through a 0.4 μm pore size filter than of the serogroup 6 population (x = 14%) when soil was analysed prior to planting.
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