This is not the definitive text on beneficial soil bacteria by any means, but will hopefully give the uninitiated a rough overview of their role (and importance) in the soil food web, especially in relation to organic gardening.

Image of bacillus under the microscopeLike most living things Bacteria are stimulated by food and there happens to be an abundance of this in a healthy rhizosphere. As a plant’s roots grow they shed cells in to the root zone, this then becomes a great source of food for the bacteria. This food source can be utilised by both beneficial and ‘bad’ bacteria so it is in our best interest to encourage or add more of the good. By tipping the scales in favour of the beneficial bacteria we can make sure that if there are any vulnerable / damaged sites on the root that they are then occupied by something of our choosing as opposed to something pathogenic. In effect we can “crowd out” pathogens with numbers.
These same principles also apply to the leaf surface, so foliar ‘feeding’ friendly bacteria can be used to control problems here also.

Different species of bacteria perform different and varied roles within the rhizosphere handily for us these bacteria will choose where and when to perform these functions, if there is no role for them they will simply lay in a dormant state until they’re needed – This once again works in our favour, it means that so long as we apply a product with a varied and balanced range of bacteria we can help ensure that all functions are catered for, this might also be in the form of Worm castings or a compost tea.

Image and link to TNC BactorrS13 - A beneficial bacteria product for sale at the nutrient companyThe stand-out benefit of (or indeed reason for) bacteria is their ability to cycle nutrients – I won’t go in to the various roles of the different types of bacteria here but the most often heard is that of the Nitrogen cycle where varies species of bacteria are able to cycle Nitrogen through it’s various forms.
It’s is their ability to “fix” these various nutrients that make beneficial bacteria so crucial in an organic growing environment. Where destructive gardeners can apply these nutrients in soluble plant available forms such as Ammonium Nitrate, organic gardeners rely on the bacteria to process their organic fertiliser inputs for them.

For instance the Nitrogen in Hoof and Horn isn’t in a plant available form, it doesn’t readily dissolve and make itself available to plant roots. We rely on the bacteria to break the bonds and cycle these nutrients until they become “plant available”.
In actual fact this process doesn’t in itself make the nutrients plant available, we rely on further organisms from the soil food web such as protozoa and nematodes etc to consume the bacteria and excrete the nutrients! These processes however do hold the nutrients within the rhizosphere meaning they won’t simply be washed or ‘leached’ away.

Finally, for this post at least, bacteria help to build soil structure. Most bacteria exist within a biofilm, or bacterial slime as you may know it. This is a complex of proteins, carbohydrates and DNA that can act as a means of transportation and protection for the bacteria. It is this biofilm that forms a glue within the rhizosphere giving soil it’s structure and with that the ability to resist compaction, hold (or rather move) water and oxygen.

For hydroponic users it is worth noting that evidence of this slime on roots doesn’t have to be a bad thing, so long as you are confident that you invited it there! Obviously if you haven’t introduced a beneficial bacterial input then this could be a problem. If the slime is unwanted and has an unpleasant odour this is likely a sign of anaerobic, undesirable bacteria. Inspect your practices such as aeration etc and consider pre-emptive inoculation of beneficial bacteria for the reasons discussed earlier in the post.

By Nigel Davenport