Well first, let’s start with the basics of soil fertility, particularly regarding soils under cultivation or grazing. In relative terms, soils can be considered the most biodiverse ecosystems on the planet, however this largely goes unseen to the naked eye. In a gram of topsoil, you’ll capture between 4,000-50,000 different species of microbes and up to 100 billion individuals (1), made up of bacteria, fungi, protozoa, nematodes and actinomycetes. Zoom slightly further out and you’ll capture macroinvertebrates such as earthworms, gastropods, mites, beetles, ants etc. As a collective group, these are soil bugs and they exist in the soil as part of a thriving food chain.
Soil bugs have coevolved with plants and animals over millions of years and are inextricably linked to how plants access nutrients and water, and how organic matter is recycled in soils.
Most macro and micro nutrients are unavailable to plants in the soil in a naturally occurring form, and inorganic nutrients applied to soil are commonly lost (to the atmosphere or leached) or converted into forms directly unusable to plants (2). This is especially the case for critical nutrients: nitrogen and phosphorus. Soil bugs are incredibly adept at accessing nutrients unavailable to plants, which they share either directly or indirectly with plants and other bugs. Good soil fertility is contingent on a thriving and diverse community of soil bugs, and as such the amount of carbon in soils is increasingly used as a simple heuristic to assess soil health (3)
Soil carbon comprises only a small percentage (typically 2-10%) of total soil by weight but it is a crucial component as it is the primary food source for soil bugs. Soils account for a significant portion of total carbon storage in the environment, far more so than plants. A eucalypt forest is estimated to contain approximately 140 T of carbon per hectare in above ground biomass (4), whereas a soil with 10% carbon will contain upwards of 400 T of carbon per hectare in only 30 cm of topsoil (for average topsoils that weigh approximately 1.44 T/m3).
Soil carbon broadly exists in two categories:
- Labile and particulate organic carbon - Which is the living and decaying organic matter which can be decomposed/consumed by soil bugs over days to decades and comprises crop residues, manures, and soil bugs; and
- Resistant and recalcitrant organic carbon: which are the organic carbon molecules that are resistant to decay and can remain in the soil in that form for decades to millennia.
Both the labile/particulate and the resistant/recalcitrant forms are essential components of soils. Although plants access carbon through the atmosphere, labile/particulate forms in the soil are the food of soil bugs and store nutrients which soil bugs release and make available for plants.
The resistant and recalcitrant forms help maintain soil structure, increasing the water holding capacity of soils, gas holding capacity, buffering pH, and increase the soils capacity to attract and store essential plant nutrients in topsoils (cation exchange capacity or CEC). Most importantly, resistant and recalcitrant carbon has an incredible high surface area and provides important habitats for soil bugs.
Both labile/particulate and resistant/recalcitrant forms of carbon are being lost from Australian soils at an alarming rate through topsoil erosion, habitat modification, removal of crop residues offsite, tillage, and off-gassing of soil organic carbon as CO2 and methane (5). On the farm, this has concerning implications for crop productivity, the cost of inputs (such as irrigation and fertilisers) to soils to maintain yields, and considering the increasing frequency and severity of drought.
How does biochar work as a soil conditioner and why is this so important for farmers?
Biochar is a carbon rich material that is obtained through the incomplete burning of biomass. Where biomass normally decays or burns under oxygen, and as a result releases much of the carbon content as carbon dioxide or methane, biochar is burnt under low oxygen, which retains the carbon in the material. The carbon that remains is inert and incredibly resistant to decay. Under a microscope, the biochar appears as a complex honeycomb or sponge-like configuration of carbon rich structures. The complex and immense surface area of biochar acts as a veritable labyrinth in which soil bugs can inhabit and because it has a very high surface to volume ratio, the surface is negatively charged so it adsorbs nutrients and water.
The surface area of biochar is so immense that it is estimated that one teaspoon of biochar has the same surface area as a 1 ha football field. For a 1 m2 x 30 cm deep patch of topsoil with of 2% carbon (considered to be low), by increasing carbon in that soil by 1% (to make it 3% total carbon) would require approximately 1,500 teaspoons of biochar. Now imagine the surface area of 1,500 football fields on which countless soil bugs, essential nutrients and gases can adsorb. Additionally, biochar buffers pH and has a liming effect, and can keep soils from becoming critically acidic which, can lock up plant nutrients and lead to low productivity and even toxicity in crops.
Amazingly, it has been estimated that with every 1% increase in soil carbon, up to 144,000 L of extra water can potentially be stored in topsoil, or 14 L per m2 of soil. This effect is most dramatic in sandy soils followed by loam soils. Increased water holding capacity reduces the nutrient leaching and runoff effect, reduces soil erosion, increases soil aeration (stops water-logging), and retains water in topsoils for later uptake by plants.
Australia as a continent has very low percentage of soil carbon (6), that coupled with severe intermittent drought places enormous strain on water supply for farms. Increasing soil carbon is becoming a primary objective for land managers to improve the productivity and resilience of soil and crop systems. The use of biochar as a soil ameliorant has a large and increasing evidence base that indicates myriad environmental and economic benefits from its use.
(1) Raynaud, X. and Nunan, N., 2014. Spatial ecology of bacteria at the microscale in soil. PLoS One, 9(1), p.e87217.
(2) Jacoby, R., Peukert, M., Succurro, A., Koprivova, A. and Kopriva, S., 2017.
The role of soil microorganisms in plant mineral nutrition - current knowledge and future directions. Frontiers in plant science, 8, p.1617.
(3) Gil-Sotres, F., Trasar-Cepeda, C., Leirós, M.C. and Seoane, S., 2005.
Different approaches to evaluating soil quality using biochemical properties. Soil Biology and Biochemistry, 37(5), pp.877-887.
(4) Hero, J.M., Castley, J.G., Butler, S.A. and Lollback, G.W., 2013.
Biomass estimation within an Australian eucalypt forest: meso-scale spatial arrangement and the influence of sampling intensity. Forest Ecology and Management, 310, pp.547-554.
(5) “2011 Australia State of the Environment report”
(6) “Soil Carbon Guide”
1). “What is soil organic carbon?”
2). “Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review”
Reference: Atkinson, C.J., Fitzgerald, J.D. and Hipps, N.A., 2010.
Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review. Plant and soil, 337(1-2), pp.1-18.
3). “Biochar effects on soil biota–a review”
Lehmann, J., Rillig, M.C., Thies, J., Masiello, C.A., Hockaday, W.C. and Crowley, D., 2011.
Biochar effects on soil biota–a review. Soil biology and biochemistry, 43(9), pp.1812-1836.
4). “A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis”
Jeffery, S., Verheijen, F.G., van der Velde, M. and Bastos, A.C., 2011.
A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis. Agriculture, ecosystems & environment, 144(1), pp.175-187.