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 living (2-12%) eg. soil bugs and living plant roots, > Active non-living (8-24%) eg. Breaking down roots and bugs, and, > Stable (70-90%) e.g. humus.

Active Organic Matter (Living) > Viruses, bacteria, actinomycetes, fungi > Protozoa, (e.g. amoebae, flagellates) > Nematodes

> Micro arthropods (e.g. springtails, mites) Active Organic Matter (Non-living) > Identifiable plant, animal and microorganism residues and metabolites (e.g. stubble, dead plant roots, manure, bones) > Dissolved organic matter (e.g. sugars, amino acids, organic acids)

Stable Organic Matter > Non-humic biomolecules (sugars, amino acids, proteins, fats, lignin) > Humic substances (other identifiable components) > 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.

Organic Matter (%) = Organic Carbon (%) x 1.75

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.

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;

> 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 > Stabilising agent for soil aggregates – thereby decreasing propensity for soil erosion and improving water infiltration and holding capacity

> Supplying plant nutrients and energy/nutrients for microorganisms > Moderating extreme soil temperatures

Cation Exchange Capacity

Cation Exchange Capacity Is an estimate of the soils ability to attract, retain, and exchange the main cation elements calcium, magnesium, potassium and sodium. OM (in particular the humus fraction) contains a large number of negative charge sites suitable for the attraction and holding of cations. Therefore, the higher the level of OM, in construction of the media, the higher potential the CEC will be.

pH

pH is one of the most important aspects of soil and potting mix productivity through its influence on the availability of basic cations and its influence on the biodiversity and microbial activity. OM, and in particular Humus, has an important effect on the pH by improving the soil’s capacity for buffering – reducing the large swings of pH. This buffering capacity is mainly governed by exchange reactions where the OM’s functional groups (e.g. carboxylic, phenolic) act as sinks for hydrogen and hydroxyl ions. The availability of these different functional groups allows OM to act as a buffer over a wide range of pH values.

Organic matter is vital in potting mix for containerised plants.

Complexation and availability

Organic matter, in particular Humus, works in two ways to achieve a beneficial effect for plant productivity.

> Components of humus form complexes with metal ions such as aluminium and iron, rendering them less available to react with other nutrients like phosphorus, thereby slowing phosphorus fixation. This process also reduces the toxic effects of Aluminium at low pH improving potential productivity. > Humus acts as a reservoir or sink for nitrogen, phosphorus and sulfur when microorganisms use these minerals to form part of their structure. This removes significant amounts from chemical fixation.

Structural stability

Soil structure can be described as the size, shape and arrangement of aggregates of soil (or in our case, potting mix) and the gaps or pockets in between these aggregates. It provides the habitat and thus dictates the biodiversity of many soil organisms living in the media. The abundance and activity of these soil organisms depend on food supply and is therefore closely related to OM levels.

Organic matter and its various fractions can be found in different parts of the soil aggregates with numerous roles like, for instance, acting as binding agents to enhance soil structure

Organic Humate is a rich source of organic carbon that aids in cation exchange capacity, wettability and buffering pH

thus enabling plant roots to create biopores to enhance water infiltration. Furthermore, soil structure plays an important role in determining many other important plant/soil characteristics like water availability and infiltration, aeration (aerobic and anaerobic zones) and wilting and germination of plants.

At AGS there are various forms of carbon that we use in potting media construction. One very useful material we add to potting mix blends is organic humate. This extremely old material is an excellent tool for giving potting mix a concentrated form of organic carbon. Organic humate assists us to obtain many of the beneficial properties detailed in this article.

It is crucial to look upon the root zone of a containerised plant as a living ecosystem and to provide that ecosystem with as many materials as we can that encourage and stimulate the diverse microbial and fungal communities that promote healthy, prolific growth.