Stick with science when capturing carbon in soil

Building soil organic matter (which generally is roughly over half organic carbon) has always been considered a good thing from an agricultural productivity, environmental and farm profitability perspective. More soil organic matter generally means better rainfall infiltration rate, more porous soil, better soil structure, better soil water storage, high soil fertility and better crop and pasture production.

Accurately measuring soil carbon is important. Soils are often extremely variable, even within a small area, not just in organic matter content but also aspects such as pH, clay content (closely related to cation exchange capacity), and level of various nutrients such as phosphorus and nitrogen. Measurement methods are well-established, can be costly and complex and currently mainly involve collecting and analysing large numbers of soil samples. Future faster analysis technology will hopefully alter this impediment.

Also read: Why more soil carbon equals more effective rainfall

Raising soil carbon mainly focuses on the top 0 to 30cmm of soil with main gains in the top 0 to 10 and 0 to 20cm layer. Perennial plants can build soil organic matter further down the profile, but gains are generally lower at depth.

Organic matter can rise considerably in a year of good plant growth but can drop significantly in a drought, and certainly in a run of dry years like 2017 to 2019. Long term Australian pasture trials, in reasonable to good rainfall environments, have shown average annual organic carbon rises of around 0.3 to 0.5 t/ha with variation year to year often high. Be wary of claims of annual gains of well over 1.0 t/ha per year.

Variability of soil organic matter levels can also vary significantly within one year. Organic matter is a constant variable as at any time some is being broken down by soil organisms (a part of releasing soil nutrients into a plant available form), and some is being added to the soil via new plant material. Bushfires also can lower soil carbon. Soil scientists also remind us that organic matter cannot continue to increase at a given rate indefinitely. As soils improve in organic matter they gradually reach their upper limit with increments each year gradually tailing off.

Environment is also a critical aspect. The wetter the environment the greater the capacity to increase soil carbon given suitable management. Low rainfall environments have much less capacity to increase soil carbon levels. Soil type is also important. Higher clay content soils have greater capacity to increase soil carbon than sandy soils. However we have seen good gains in once regarded very poor soils.

The lower the soil carbon level at starting point, the greater the capacity to increase given suitable management. Some of the major research sites have started from previously long cultivated cropping land and or pastures that probably were often overgrazed and not corrected for soil deficiencies nor had decent legume growth. In other words land already well improved for many years has less capacity for building soil carbon than an area previously not well improved.

Research has found it's hard to build soil carbon in cropping only systems, although zero till combined with stubble retention has come closest and in higher rainfall environments may be able to slowly build it. Pasture phases in a zero till cropping system are often able to increase carbon.

It is also important to carefully asses carbon credit schemes. Some schemes are overseas based and may only be credited to that country's credits, and may have different protocols to Australia. Nutrient deficiencies need to be addressed with products that are able to supply that deficient element in an available form at a sufficient rate. Lime or gypsum to correct acidity or sodicity, may also be part of sound upgrade strategy. It's sound policy to regularly update on advice from leading soil scientists.

Next week: Early choice of winter legumes.

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