A local carrier for rhizobium inoculants in Kenya

Abstract

This study was carried out to establish whether some locally available materials in Kenya can be used as substitutes for peat in legume inoculant production during the period 1977 to 1980. The materials investigated consisted mainly of agricultural residues. These were filtermud, bagasse, sawdust, coffee husks, coir dust, charcoal dust, forest soil, diatomite, sisal waste, maize cobs, alfalfa and wheat straw as well as wood ash. Among these materials, filtermud, sawdust, bagasse, coirdust and coffee husks were found most promising in terms of availability, organic matter content and water-holding capacity. Further investigations using the five materials listed above showed that filtermud was superior to the other four. The survival rate for Rhizobium phaseoli, in filtermud as determined by the plate count method, was comparable to that in peat, the acclaimed overall best carrier for rhizobia. Filtermud had an initial viable rhizobia count of 1 X 108 /g dry weight inoculant which declined to 1 X 107 /g dry weight inoculant after storage for six months at 26°C. The quality of the filtermud inoculant was high and consistent over a storage period of four months at 26°C. The inoculant maintained a viable count in the range of 108 to 109 rhizobia /g dry weight inoculant . throughout the peL'iod. The surviving rhizobia effectively nodulated legume seedlings as indicated by the plant infection test. Storage of inoculants at 40°C was found to be detrimental to the survival of R. phaseoli. Viable rhizobia count in a filtermud-based inoculant declined from 1.5 X 109 to 0.4 X 109 rhizobia/g dry weight inoculant within 24 hrs. This represented a loss of 73% in rhizobia viability. Within one week of storage at 40°C, the inoculant lost 90% of the initial rhizobia count. Storage of inoculants at temperatures between 4°C and 26°C greatly improved rhizobia survival and mul tiplication. The rhizobia count increased three-fold within the first week when the filtermud inoculant was stored at between 20° and 26°C. Storage of the inoculant at a lower temperature (4°C) slowed down rhizobia multiplication and only a 50% increase over the initial rhizobia population during the same period was recorded. A count of 1.0 X 109 rhizobia/g dry weight inoculant was maintained over a period of one month when the inoculants were stored at 26°C. This compared favourably with the peat inoculant. Storage of peat inoculant at 40°C for 24 hours showed a 55% loss in viability. After one week at the same temperature, the inoculant lost 97% of the initial viable rhizobia count. When the inoculant was stored below 26°C, the rhizobia numbers increased three-fold over the first week. Several stickers were tested for their suitability during. seed inoculation. Use of water, sucrose (10% w:v) and gum-arabic (40% w:v) as stickers resulted in an inoculum load greater than 10 X 103 rhizobia/seed. When Fowler's syrup, molasses and yeast extract mannitol (YEM) broth were used as stickers, a lower inoculum load of 10 X 102 rhizobia/seed was obtained. Seeds inoculated using gum-arabic as the sticker and then stored at either 4°C or 25°C for a period of four weeks recorded the highest survival of rhizobia as compared to the rest of the inoculant stickers. Rhizobia population on seeds inoculated using gum-arabic declined from 10.5 X 103 to 1.1 X 103 /seed over a period of one month at 4°C. Storage of inoculated seeds at 25°C showed a sharp decline from 10.5 X 103 to 3.3 X 101 rhizobia/seed within the same period. Sucrose was not as effective as gum-arabic in supporting rhizobia survival on seed. Inoculated seeds stored at 4°C lost more than 50% of the initial viable count within one week. Thus, viable rhizobia count declined from 9.5 X 103 to 9.2 X 102 rhizobia/seed within the same period. After one month, only 435 and 8 rhizobia/ seed survived at 4°C and 25°C, respectively. Use of Fowler's syrup and molasses resulted in very poor survival of rhizobia on seeds. Rhizobia numbers declined from 2-3 X 103 to 6-7 X 102 within one month when seeds were stored at 4°C. When similarly inoculated seeds were stored at 25°C no rhizobia were recovered after one month. YEM oroth was not suitable as a sticker. It resulted in a low inoculum load (2. 7 X rhizobia/seed) and poorly supported survival of the rhizobia. When inoculated seeds were stored at 4°C, the inoculum load declined to 28 rhizobia/seed within one month. All rhizobia on seeds were lost within two weeks at 25°C. Using water as the sticker resulted in a high initial rhizobia count (10 X 103 ) but afforded poor protection for rhizobia on seeds. The numbers of surviving rhizobia declined to 2.8 X 101 per seed when stored at 4°C for one month. When the seeds were stored at 25°C, only 3 rhizobia /seed could be detected after one month. The gum-arabic-CaC03 pellet supported the highest number of rhizobia, surpassing all other treatments. This was true of inoculated seeds stored at either 4°C or 25°C. Pelleted seeds had a rhizobia count of 3.7 X 103/seed after storage at 4°C for one month while those stored at 25°C had 90 rhizobia per seed after the same period. Inoculation of seeds prior to planting resulted in well nodulated seedlings when seeds were inoculated using either gum-arabic or gum-arabic lime pellet. Good nodulation was observed even when such seeds were stored at temperatures between 4° and 25°C for upto four weeks. Seeds inoculated using sucrose and water and stored at 4°C supported the survival of more than 300 rhizobia/seed upto the fourth and third weeks, respectively. When inoculated seeds were stored at 25°C for more than two weeks in the case of sucrose treated seeds and one week for water treated seeds, they produced poorly nodulated plants. All plants grown from inoculated seeds appeared greener, had higher nodule weight and higher shoot dry weight than the uninoculated control plants. Storage of inoculated seeds at 25°C or above for more than one week was detrimental to multiplication.