Plant growth promotion is a complex phenomenon that often cannot be attributed to a single mechanism, as most PGP microbes are capable of multiple mechanisms and in some cases their PGP effect may only occur through interactions with different microbes. It is important to note that any microbial agent added to the rhizosphere would have to interact not only with the plant but also with any other organism sharing the same ecological niche. To be successful the inoculant has to maintain a critical population mass in the soil and have the right conditions to exert its beneficial activity.

Despite the challenges, a growing variety of microorganisms with properties that can be exploited in plant growth promotion are being discovered and tested under field conditions, with increasing number of cases reporting success. Reductions in the uses of chemical fertilisers and pesticides as well as improved yield are the main direct benefits reported from use of inoculants. Indirect benefits are reduced build up of toxins in agricultural soils and reduced environmental pollution with agricultural runoff. Success is often associated with using a combination of inoculants with complementary beneficial traits, e.g. biofertilisers that increase nutrient availability in the proximity of the roots together with a mycorrhizal fungus that enhances the root system and assists the plant to absorb the nutrients. It is not surprising that often indigenous microbes are proving to be the most effective, as such microbes would suit the environmental conditions in the cropping system they are intended for. Nevertheless, indigenous microbes would still have to out compete other microbes for resources and, in the case of biocontrol agent, suppress pathogens.

The Australian GRDC has been investing in a Soil Biology research since 1992. Between 2002 and 2008 they have invested approx AUD 8,000,000 on the Soil Biology initiative, which they report was successful in generating economical, environmental and social benefits. Economical benefits were attributed to investment areas related to inoculants, disease control and nutrient balance. Environmental benefits were attributed to a reduced use of fertilisers and reduced energy input. The GRDC announced their next generation soil biology plan to run between 2009 and 2014. One of the main focuses of the plan is the Beneficial Microbes Program, which aims to isolate, test and commercialise microbial bio-inoculants for the grain industry. One of their rationales for such an investment is the readiness of the Australian

farmers to reduce on-farm chemical inputs and replace them with biological alternatives in order to reduce costs, increase sustainability and maintain healthy soils and ecosystems (GRDC Five years Business plan, 2009-2014).

The cotton industry, via the CRDC, would benefit from maintaining its program

on soil biology and, from reasons outlined above, extend research into isolation of cotton specific beneficial microbes, from different cotton growing regions in Australia. Other than isolating microbial agents for augmentation, further research should be directed into cropping practices that enhance existing and introduced beneficial microbes, such as controlling the amount of chemical inputs, which might otherwise suppress the activity of beneficials and increase the activity of detrimental organisms.

There is a great scope for collaborations between the different Research and Development Corporations in Australian agriculture and worldwide with regard

to the development of technology for e.g. screening for and identification of

microbes with beneficial traits; assessment of the benefit to the plant; testing

strains for commercialisation; designing best formulations for inoculant delivery; detecting and assessing the performance of inoculants in the soil; analysing soil microbial communities and the effects of the soil inoculation on soil health; and the studying of general effects of cropping practices on specific microbial communities. In general, it can be said that similar groups of beneficial microbes seem to be involved in promoting the growth of different plants. Examples for this are bacteria from the Bacillus, Azospirillum, Pseudomonas groups and mycorrhizal fungi. Nevertheless, there is enough evidence to suggest that particular microbial species, or even strains, benefit specific plants under defined conditions. Therefore, beside collaborative efforts, regional efforts need to be maintained to produce inoculants specific to the crop, agronomic practices, soil type and other environmental conditions.