Soil Organic Carbon and Climate Change. Let’s Just Do It.
Global increases in soil organic carbon are needed. Here are some rules of thumb from farmers.
Sustainable soil carbon sequestration needs to be increased globally, as a major practice towards our goal of climate change mitigation. Agricultural croplands and pasturelands have the ability to store much carbon, especially where these soils have been depleted of their soil organic carbon. Soil carbon sequestration methods on farms will need to be adapted to differing soils and conditions, but some general methods or rules of thumb are suggested here.
The past decade has seen our carbon emissions, after accounting for carbon sinks in oceans and land, to be 4.9 Gt Carbon per year. The technically feasible potential of agricultural land to fix organic carbon is between 0.79 and 1.54 Gt Carbon per year. This is a sizable amount of carbon dioxide which can be taken out of the atmosphere with a change in farming practices.
Three international soil organic carbon initiatives have been launched in recent years by the UNFCCC, the 4p1000 initiative at the 2015 COP21 in Paris, so named because a 0.4% increase in global soil carbon would equate to the annual growth in atmospheric CO2 emissions. The Koronivia agricultural joint workshops at the 2017 Fiji COP23 held in Bonn, and the 2019 FAO RECSOIL program aimed at soil re-carbonization (Amelung et al., 2020).
These are impressive and important initiatives, and are essential in planning and measuring the work needed to increase our global soil organic carbon.
But what are the practices that farmers are using on their paddocks? What on-ground works have changed?
I have found the farmers I’ve worked with over the years to be well informed about environmental matters and familiar with the expense of not working with nature and natural cycles and the productivity gains of increased soil carbon. So, we can look to the farmers to see what they are doing to increase soil organic carbon.
In summary we have to increase carbon inputs to the soil through: practice minimum tillage, plant perennial pastures where possible, use rotational grazing, maintain soil cover, maintain soil fertility, make use of locally available manure sources and monitor soil organic carbon.
Minimum tillage or no tillage is practiced, as ploughing or turning the soil will increase the rate of decomposition of soil organic matter, by increased aeration, and hence reduce the amount of soil organic carbon. Turning the soil allows for drying and loss of soil moisture. The soil is also exposed to wind and water erosion.
Minimum tillage if used with perennial crops and pastures should not need additional herbicide application. Farmers have used straight tines on their ploughs for seeding, if needed, which don’t turn the soil.
With the best of intentions sometimes actions fail. I was driving home after a successful day in the field arranging revegetation contracts with a couple of farmers, when I passed farmer Bill’s place and he was out in the paddock, ploughing!
Bill is one of the progressive farmers, has been planting windbreaks with local species for years, leans more to organic methods, has fenced off all of his watercourse riparian zones for erosion control and practices no tillage!
I pulled up the ute on the side of the road, hopped the fence and walked across the paddock to Bill. Bill stopped the tractor and shut it down.
“G’day Pete” he said.
“G’day Bill” I said.
With farmers you don’t need many words.
“You’re ploughing?” I said sort of pointing at the paddock.
“Yeah, I know,” said Bill.
“What about no tillage?”
“Yeah, I know,” said Bill again “But you know, sometimes you just gotta get away from the wife.”
Perennial pastures where possible, work in well with no tillage as they don’t need to be planted each year and the associated ploughing that goes with the planting. Perennial plants will provide a more constant carbon source and are generally deep-rooted, making use of deeper soil moisture and nutrients. Inter cropping of perennials with annuals is possible by seeding using straight plough tines.
Use of rotational grazing, that is, moving the stock from one area or paddock to the next, this allows for regrowth and will store more carbon. Short rotational grazing cycles, as little as one or two days grazing with a much longer rest period, will mimic natural grazing animal movement, providing manure while reducing the impact of preferential grazing of the more palatable species.
Rotational grazing compared to continuous stocking requires a lot more fencing, gates and water points and of course the moving of the herd or flock on a regular basis.
When maintaining pastures use the ‘2 2 5 rule’, keep your pasture plant height between 2 to 5 inches (Oregon State University, 2021) by the use of rotational grazing . The pasture grasses grow best, have a higher protein content and are more palatable between these heights.
Another practice is to maintain and conserve soil cover with plants or mulch to protect from wind and water erosion, this is best if it is complete cover but 70% cover is generally set as a minimum. At least 70% cover often applies to standing stubble still rooted into the soil.
The previously mentioned rotational grazing is used to limit grazing impact and bare patches in pasture paddocks, and the earlier sale and culling of herds may be necessary in times of drought, in order to maintain 70% cover.
The use of fallow in annual cropping is not practiced as much as it was, because of the exposure of bare soil to erosion.
At Landcare meetings with farmers, and the mature age of many of those farmers, a standing joke has been that many of the meeting participants are not maintaining at least 70% cover in the top paddock! Referring to a bald patch on the head!
Maintain soil fertility with organic fertilizers if needed, and the encouragement of soil fungi and bacteria for greater uptake of nutrients. Use chemical fertilizers if needed for a specific nutrient deficiency.
Make use of locally available manure sources, compost or other organic amendments such as ‘grape marc’ in grape vine growing areas, the left-over organic material after the grapes have been pressed. These will improve soil fertility but are only moving carbon from one place to another, hence green manures need to be grown on site, together with enhanced crop production, for overall increased soil organic carbon.
Monitor the soil organic carbon to ensure that any change of management practice is indeed increasing the soil organic carbon. The taking of soil samples can be tedious and then testing is expensive but does give a good baseline for the soil carbon improvement work. Laboratory test results will be required if carbon credit schemes are ever introduced.
In the field the darker colour of the soil and a faster water infiltration rate are good indicators of improved soil structure and hence soil organic carbon. When making observations be aware of the difference between soil organic matter and soil organic carbon, the latter being the decomposed portion of the overall soil organic matter. The Youtube ‘Measuring soil organic carbon on smallholder farms’ is a 10-minute video that gives a good explanation of soil sampling and laboratory testing, see reference below.
Achieving high levels of soil organic carbon is the basis of soil fertility as it helps to maintain farm production by promoting soil fungi, bacteria and other microorganic life, by raising fertility, increasing water infiltration and water holding capacity, and resisting soil erosion, and of course sequestering carbon.
Amelung, W., D. Bossio, W. de Vries, I. Kögel-Knabner, J. Lehmann, R. Amundson, R. Bol et al. (2020) “Towards a global-scale soil climate mitigation strategy.” Nature Communications 11, no. 1 1–10.
How should we manage our soils to increase soil carbon? (2013) How should we manage our soils to increase soil carbon? (ccmaknowledgebase.vic.gov.au)
Measuring soil organic carbon on smallholder farms (2017, May 4) Measuring soil organic carbon on smallholder farms — Bing video Online Multimedia.
Oregon State University (2021) Pasture and Grazing Management. How Plants Grow Most Efficiently. Pasture and Grazing Management | OSU Extension Service (oregonstate.edu))