SOME F>iCTORS AFFECTING THE SURVIVAL OF RHIZOBIUM

Survival of Rhizobium trifolii on white clover seed before sowing is adversely affected by both drying and a water-soluble toxin which di f fuses f rom the seed coat dur ing the inocula t ion process . Survival of rhizobia is increased by removal of the toxin by seed washing or the suppress ion of i t s inhib i tory ef fec t by t rea tment of seed wi th phenol ic adsorbents . Surv iva l o f rh izobia in t roduced in to tbe soil on seed before the rhizosphere is capable of supporting growth and multiplication may be affected by the toxic diffusate and competition and/or antagonism from naturally occurring populat ions of soi l micro-organisms, Select ion of effect ive s t ra ins of rhizobia on criteria such as survival and competition abilities, as well as on the ability to nodulate and fix nitrogen, is c.onsidered to be extremely important. I N T R O D U C T I O N SUCCESSFLJL EST.~BLISHMENT of Trifolium rrpens requires the presence of sufficient numbers of an eflective strain of Rhizobium trifolii to promote nodulation. Three distinct phases are involved: (1) Inoculant quality and survival of rhizobia in culture. ” (2) Survival of rhizobia on seed before sowing. (3) Survival and multiplication on or near seed before nodulation. Some of the factors influencing the survival of R, trifolii during these three phases are considered in this paper. INOCULANT QUALITY AND SURVIVAL OF BACTERIA IN CULTURE tnoculant quality refers to the number of cells of an effective Rhizobium strain in an inoculant which are viable and hence determines the number of rhizobia applied to the seed at the time of inoculation. Of the factors which affect the quality of peat inoculants, incubation and storage temperature, moisture content of the peat and sterility of the peat used for inoculant preparation are considered to be the most important (Roughley and Vincent, 1967).