PRODUCTION: CARBON
Carbon: Black gold or just a lot of hot air? By Rohan Davies, Australian Growing Solutions
Carbon footprint, carbon emissions, carbon tax; you could be excused for thinking carbon is a bad thing. Whilst excessive amounts of carbon in the atmosphere is causing significant issues for humanity, carbon itself is crucial within soil or growing media. What is carbon? Carbon in potting media is in a continual state of flux – entering and leaving the system. In general terms, photosynthesis converts and captures atmospheric CO2 into plant biomass. This biomass can be converted into the various fractions of Organic Matter (OM) with the assistance of soil microflora – which themselves, in the process of the cycle, will release carbon dioxide back into the atmosphere restarting the cycle. To maintain themselves soil microflora will consume OM sources, thus we would see an ever-declining OM level if inputs are not sufficient to maintain soil microflora populations. There is a lot of confusion around Organic Matter and Organic Carbon. Organic matter (OM) can be defined as all materials of biological origin irrespective of origin or state of decomposition (Baldock & Skjemstad 1999). OM is essentially broken into three pools:
Active Organic Matter (Non-living) >
Identifiable plant, animal and microorganism residues and metabolites (e.g. stubble, dead plant roots, manure, bones)
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Dissolved organic matter (e.g. sugars, amino acids, organic acids)
Stable Organic Matter >
Non-humic biomolecules (sugars, amino acids, proteins, fats, lignin)
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Humic substances (other identifiable components)
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Charcoal
It is difficult to accurately measure the OM content of soil, therefore most soil testing laboratories measure the organic carbon (OC) content and then estimate OM through a conversion factor.
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Active living (2-12%) eg. soil bugs and living plant roots,
Organic Matter (%) = Organic Carbon (%) x 1.75
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Active non-living (8-24%) eg. Breaking down roots and bugs, and,
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Stable (70-90%) e.g. humus.
This conversion factor assumes organic matter contains 57 per cent organic carbon (range is 50-58 per cent - variance caused by the type of organic matter, soil type and soil depth). Conversion factors can be as high as 2.50 per cent.
Active Organic Matter (Living) >
Viruses, bacteria, actinomycetes, fungi
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Protozoa, (e.g. amoebae, flagellates)
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Nematodes
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Micro arthropods (e.g. springtails, mites)
What is Inorganic Carbon? Inorganic carbon is the result of both weathering of rocks and the precipitation of carbon dioxide dissolved in water as carbonate minerals (usually found as carbonates, bicarbonates, and elementary carbon like graphite). This process occurs over thousands of years and as such is not readily influenced by manmade practices.
Why would I want good Organic Matter levels in my potting mix? Organic matter is an important driver for overall soil health. It is an important driver for so many of the requirements for maximum plant productivity, namely;
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GROUNDSWELL MARCH 2021
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Buffering against rapid changes in salinity, sodicity, and pH. Also reducing the overall effects of sodicity and increases the Cation Exchange Capacity (CEC) of growing media
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Stabilising agent for soil aggregates – thereby decreasing propensity for soil erosion and improving water infiltration and holding capacity
