Legume Inoculants and Quality Control

Rhizobial inoculants have been used successfully in world agriculture for about 100 years. About 20 million ha crop and pasture legumes are inoculated in the world each year, although that figure could be increased if high-quality inoculants were available to all farmers. The characteristics of a high-quality inoculant relate to the properties of the carrier, the infective (nodulating) and effective (N2 fixing) attributes of the rhizobial strain and the numbers of the strain and other microorganisms (contaminants) that are present in the inoculant. In countries with strict standards for inoculants such as Australia, the carrier (most commonly peat) must contain >109 rhizobia/g and <106 contaminants/g. Other countries demand that inoculants deliver high numbers of rhizobia to the inoculated seed, e.g. 106/seed for soybean in France. Research in a number of countries has shown that the most effective inoculants are produced using a sterile carrier and are relatively fresh, i.e. <6 months old. Rhizobial inoculants lose efficacy with age. The involvement of private-sector institutions in manufacturing and marketing and public-sector in quality control and R&D appears to be the most successful model for the production and use of inoculants. The future of the inoculant industry, and its potential benefits for world agriculture, depends on improving inoculant quality, both numerically and in terms of strain effectiveness. New technologies may lead to improved inoculants in industrialised countries but the fact remains that, in many countries, the 30and 40-year old technology has yet to be properly mastered. LEGUME inoculation with rhizobia and bradyrhizobia is a long-established and successful practice, especially with particular crops in the more technicallyadvanced countries. It is difficult to access accurate figures on the global use of legume inoculants. The figures available indicate that about 2000 t inoculant is produced annually worth US$50 million, and sufficient to inoculate 20 million ha of legumes. By far, the largest producer of inoculant is the U.S. with annual production of about 1000 t (Singleton et al. 1997). Early attempts at inoculation were rudimentary, such as moving soil from fields growing well nodulated legumes to legume-free fields. The next step came late last century with the commercial use of pure cultures of rhizobia for inoculation (Fred et al. 1932). Since that time, the production and distribution of legume inoculants have become established industries in individual countries throughout the world. Having said that, few farmers in the developing countries of Asia have had access to high quality inoculants or have used inoculants as a normal part of their legume culture practices. In some countries, farmers fertilise their legume crops with N, thereby losing some of the economic benefit of the legume, i.e. inputs of fixed N into the farming system. In other countries, productivity and profitability may be reduced through N deficiency. In a third group, inoculants are readily available. Countries in this group include Thailand (Kongngoen et al. 1997; Boonkerd these Proceedings), Bangladesh (Sattar et al. 1997) and Myanmar (Thein and Hein 1997). The Need to Inoculate Although rhizobia seem to be as widely distributed as the legumes themselves, many soils used for legume cultivation do not contain adequate numbers of highly effective rhizobia. They may be devoid of the rhizobia, they may contain low numbers of effective strains or they may contain high numbers of ineffective or partially effective strains. The question 1 NSW Agriculture, Centre for Crop Improvement, Tamworth, NSW 2340, Australia 2 NSW Agriculture, Horticultural Research and Advisory Station, Gosford, NSW 2250, Australia