Guideline Index

Chapter 5: Understanding and Managing Soil Biology

5.8 Microbial inoculants and biological amendments

The interest in soil biology has encouraged market responses in the form of organic amendments, biological inoculants and microbial stimulants. Organic amendments in the form of manures and composts have been in use for millennia with the current challenge being integration of these inputs into conventional fertility management planning.

Many of the newer inoculants and stimulants have not been the subject of rigorous, replicated trials in a variety of soil types or climatic zones. As a result, it is difficult for farmers to know if the product is likely to return a financial benefit – even in the long term – or if the product is a waste of money.

If farmers are persuaded by anecdotal claims of product effectiveness and wish to conduct a trial on their farm, there are a few steps to follow to determine if the product delivers on its promise and the investment is worthwhile.

Firstly, there should be a clear understanding of what the product is claimed to do and how its effectiveness should be measured. In establishing a trial, a paddock which is not the best and not the worst performing should be selected. An area for the trial should be designated and its boundaries clearly marked with a GPS or physical markers that will not be impacted by grazing animals. The trial should have an area to test the product and a similar sized adjacent area on the same soil type not treated by the product, as a control.

Soil tests / pasture tests should be taken before any other action is carried out. Most importantly, only that product to be tested should be used on the trial. If additional inputs are used – e.g. lime – it will not be possible to determine if a response is due to the product being tested, the lime, or a combination of the two. Detailed records of the trial including dates, application rates, weather, pasture responses and other observations will be necessary.

Some biological inoculants have been subject to rigorous trials with, for example, rhizobia – the bacteria responsible for nitrogen fixation in legumes – having a fifty year history of research and development, and quality assurance. Similarly, much work has been done on biocontrol agents such Trichoderma spp., a fungus which has demonstrated control of root rots in onions, and work is continuing on new strains of Bacillus spp. for biocontrol of common root diseases.