The industry that manufactures cultures of rhizobia for seed and soil inoculation generates approximately $20 million of sales annually and is increasing with new and improved inoculants. The delivery of inoculants is explained based on University of Minnesota information.
Inoculants range from simple tube cultures sufficient for small quantities of exotic seed, to large-scale, fermenter-grown cultures mixed with peat or other carrier material, and used in the commercial inoculation of the large areas planted to soybean, bean, peanut and clover.
The large-scale production of inoculants per batch per vat will produce in excess of 1,000 million highly effective rhizobia per gram of product.
Once the inoculant culture has been produced, a number of different materials can be used to keep these organisms viable until ready for use by the farmer. Most of these "carrier" materials allow rhizobial survival for periods of at least six months. Again, as research has progressed, some companies have made exceptional strides in carriers and rhizobial survival. Characteristics of a good inoculant carrier are
- High water-holding capacity
- Non-toxic to Rhizobium
- Available, inexpensive and easily processed
- Sterilizable by autoclaving (pressurized steam) or radiation
- Adheres well to seed
- Good buffering capacity
The most commonly used carrier is peat, but since peat is not universally available, compost, bagasse (derived from the milling of sugar cane), coal, polyacrylamide, vegetable oils, clays and clay granules, liquid preparations and soil have all been used successfully.
No listing of physical or chemical properties can fully explain why some peats make suitable inoculant carriers and others do not. Peat sterilized by irradiation or electron acceleration is preferred to carriers that are not sterile, Roughley and Vincent (1967) noted that sterile carriers generally support a higher population of rhizobia, and have longer shelf lives.
A problem being addressed recently is that while sterile preparations may enhance rhizobial survival in storage, such inoculants may still undergo deterioration in ability to nodulate and fix N2.