In addition to enhancing water-holding capacity, high organic matter (carbon) performs many other functions in soil, including the maintenance of soil structure allowing water, air and nutrient availability.
Another very important requirement by plants is nitrogen. Did you know that of the atmosphere approximately 78% is nitrogen but that it isn’t directly available to plants? It can only be accessed via microbes, the microbial bridge that we spoke of in Farming Secrets Digest #14 (www.farmingsecrets.com/fs)
These microbes are in the form of bacteria – nitrogen-fixing bacteria – which derive most of their energy from liquid carbon fixed during photosynthesis from CO2 in the atmosphere. These bacteria can be free-living in the rhizosphere, confined to nodules on plant roots or existing as endophytes in leaves or stems. When these microbes are present and able to function, there is no need to add synthetic nitrogen.
A commonly used product is urea which certainly helps crops along and farmers often see a rapid response. However what happens in the soil when the nitrogen is added? This extra nitrogen sends a signal to the plant to stop supplying liquid carbon to the microbes thus reducing the activity of the bacteria and mycorrhizal fungi. This then means that the microbes which would take the nitrogen from the atmosphere and supply it for free slow down! So when a farmer buys nitrogen and applies it to his paddocks, he unknowingly is destabilizing the plant-soil symbiotic relationship. Causing
- soil acidity,
- loss of nitrogen to the atmosphere and
- leaching to aquifers, rivers and streams.
If plants are mycorrhizal, they actually don’t require nitrogen in a mineralised form, that is, in the form of nitrate or ammonium. In fact it takes energy to have to convert it to glutamate. However when mycorrhizal fungi use organic nitrogen the nitrogen loop is closed.
Microbially balanced soils also reduce the abundance of ‘weedy’ species such as annual ryegrass, capeweed, mustard weed and thistles. The germination of these species is stimulated by the ready availability of nitrate nitrogen.
It is well known that mycorrhizal fungi access and transport nutrients in exchange for carbon from the host plant (Killham 1994, Leake et al. 2004). What is less well known is that in seasonally dry, variable, or unpredictable environments (that is, most of Australia), mycorrhizal fungi play an extremely important role in providing much needed moisture to the plants.
# Inspired by Walter Jehne’s presentation at the recent OAA conference: Future Farming: Productive, Competitive and Sustainable & Dr. Christine Jones