Graduate Thesis Or Dissertation
 

The survival of rhizobia on seeds of Trifolium subterraneum L

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  • Field and laboratory tests were carried out to investigate the survival of the root-nodule bacterium, Rhizobium trifolii, when inoculated onto seeds of the pasture legume subterranean clover, (Trifolium subterraneum L.), in order to improve the nodulation and establishment of this plant in acid soils. Field experiments were carried out in four soils in western Oregon, ranging from pH 5.4 to 4.9, and showed that approaching 100% nodulation could be obtained either by broadcasting two tons of lime per acre over the seedbed prior to sowing with banded superphosphate, or by sowing the inoculated seeds directly in contact with a previously prepared 1:1 mixture of lime and single superphosphate, applied at the rate of 600 or 800 lbs. of mixture per acre. These treatments subsequently produced the highest dry matter yields. Uninoculated treatments, and inoculated treatments sown either without fertilizer, or with superphosphate alone, did not have effective nodulation and did not establish satisfactory stands. Laboratory studies were conducted to investigate the survival of rhizobia in pelleted seeds. Tests using serial dilution plate counting methods showed that there existed a wide variation in survival rates in different adhesives and coating materials used for pelleting. Attempts to pellet seeds with adhesives and coating materials using a yeast-mannitol broth suspension inoculum were all unsuccessful. Warberg manometric respiration studies showed that rhizobial survival was prolonged by using an inoculum suspended in a sterile peat base. This was confirmed in a plate count test. A test in which peat culture inoculum was used measured rhizobial survival in seeds pelleted with 40% gum arabic solution and a range of coating materials. It was found that Lake Oswego lime was far superior to the other limes and coating materials examined, giving a survival of more than 1,000 bacteria per seed 8 days after inoculation. Although a satisfactory method of pelleting was obtained, it was felt that more laboratory research into pelleting materials, followed by extensive field trials, would be required before the exact usefulness of pelleted seeds could be estimated.
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