Ideas on organic matter and carbon in cropping situations
During my time growing high-value vegetable crops in intensive production systems, I observed a decline in marketable yields and product quality regardless of the investment in fertiliser inputs. I also witnessed small tractors make way to larger horsepower tractors with power-driven implements alongside a significantly larger spend on synthetic nitrogen that was harming the preservation of our soil carbon stocks. This shift in the way I was operating led me to look in the mirror and ask questions such as, what advice are we getting and how did we get here? I found that the advice I received only focused on the BIG FOUR nutrients being nitrogen, potassium, sulphur, phosphate and that I was using the advice to focus solely on lifting fresh yield in the crops. What made me sit up and take notice was the increasing lack of crop response to our expanding ‘more-on’ approach. More N, P, S and K was no longer increasing yield and in most instances, was impeding our best efforts in producing what we would consider good quality crops.
It was not until I started to educate myself further on topics such as soil organic matter and soil biology that I gained a much deeper understanding on what would have been steering me in the face many years ago. What was evident to me now was the enormously important relationship of maintaining the health of soil biology, the quality of organic matter (OM) and its cycle in the soil. It was only once I started to measure soil organic carbon and factor in its relationship to a healthy soil system that marketable yields increased along with soil quality, which enabled lower nutrient input costs through improved nutrient use efficiencies.
The OM cycle is a biological process that gives us farmers so much, but can be often forgotten in a conventional farming system or worst still, not even considered. So I would like to share some of my understanding with you on how to capture your hard-earned soil organic matter and convert it into stable forms of carbon.
If simplified, the calculation required to increase soil carbon is for land use and management to change so that CO² captured from photosynthesis and inputs of carbon into the soil, are at greater levels to that which is removed or respired.
Here are some points to keep in mind….
Keep it green.
We are all starting to hear the message that we should keep some form of plant life growing and actively photosynthesising. This is important to continue to feed the soil biology with root exudates that contain carbon. It is also essential to keep it green so that some of the CO² from OM breakdown is recaptured and diverted back into the photosynthesis process. Recapturing soil CO² is necessary as in some circumstances 40-90% of carbon in plant residues could be lost as CO² within a short time frame through biological processes.
Maximise water and nutrient use efficiency.
Maximise nutrient and water use efficiencies to grow as much dry matter biomass as possible on, and most notably, in the soil. Keep the soil pH in the optimum range for biological activity and mitigate any soil aluminium or compaction issues that will inhibit root growth.
Good crop nutrition management requires converting organic matter to more stable forms of carbon such as humus, for every tonne of humus built in the soil profile roughly 85kg of nitrogen, 20kg of phosphorus and 15kg of sulphur also needs to be sequestered. One cannot sequester carbon in isolation, and these nutrients are the price to be paid to the carbon bank.
Maximise organic matter retention.
Minimise tillage where possible. If the soil is needing to be cultivated to establish crops, then carry out operations at the correct time to mitigate damage to soil structure.
Introduce long term perennial vegetation crops into the rotation.
Continuous cropping and tillage will degrade soil carbon by oxidising organic matter at high rates. By introducing long rotations such as viticulture or long-term pastures with animals into your enterprise, you not only spread financial risk but also enable the soil to recuperate and go through a period of carbon sequestration. The ability of soil to convert OM to humus is a complicated matrix that involves the soil type and the soils biological activity. Fine clay loam soils under irrigation tend to retain more soil carbon that courses structured arid soils.
Focus on your crop rotations.
A crop rotation that consists of shallow-rooted crops with intensive tillage practices would be considered detrimental in maintaining soil carbon stock and limit converting OM to humus; this is due to the limited root mass that many of these crops have along with an often-low C: N ratio of the crop residues that will be broken down rapidly by soil organisms. It would be desirable to provide the soil with a crop rotation that involves both low C: N ratio residues along with high C: N ratio organic material from large deep-rooted crops.
Green manure crops and other organic matter inputs.
Since there is a limit to how much biomass a soil can produce in a growing season, it may be practical to include other off-farm organic inputs such as manure or compost to offset the carbon removals within the production system. Be cautious in organic matter input choices as they are not all created equal and can lead to a nutrient imbalance that will hinder photosynthesis that will lower total plant biomass production.
Measure your carbon stocks.
Measuring soil carbon does not need to be complicated on your farm. Pick an area that best represents your farming practice and start measuring total organic carbon, liable carbon and microbial biomass as frequently as your budget allows.
Starting with a benchmark of current practice is an excellent place to start. Over time, the more data gathered will give you a good indication of the trend in your soil of how the microbial biomass and nutrient cycling respond to any changes in management.
Testing for total soil carbon and water-soluble carbon in combination gives you a detailed picture of the impact of farm management practices on your soils carbon stocks along with microbial populations. The relationship between these two carbon tests is also vital in creating a clear picture of the soils potential to supply nitrogen sulphur, phosphorus, and other micronutrients' during the growing season.
At Soil Matters, we have the tools and capacities to test for carbon related parameters, and most importantly, we are able to decipher how Co2/carbon in the atmosphere can be stored in your soil ecosystem.