101 Questions answered
Setting Realistic Soil Carbon Targets: What’s Possible Based on Soil Type and Rainfall
In terms of carbon sequestration, there is a realistic soil carbon storage potential, which is largely driven by rainfall and soil clay content. Generally, lower clay and rainfall mean a lower carbon storage ability. If you’re on a sand in an area with 300 mm of rain a year, it may be unrealistic to expect much more than 1% soil organic carbon (SOC).
The graph to the right compares the mean SOC% in various agricultural districts in South Australia. Note how as growing season rainfall increases, so does SOC.
Let’s talk numbers
A few years ago, the team at DEW and PIRSA (Schapel et al, 2021) used just over 36,000 soil samples from across the farming regions to develop topsoil (0–10 cm) benchmarks for realistic SOC levels in South Australia. The benchmarks are categorised based on soil texture and land use (pasture, cropping and orchard/vineyard) and are for the Walkley Black OC test method.
Across the state, the general guidelines are:
For example, if your soil under pasture is a sandy loam and the SOC is 4%, you’re already doing a stellar job. There won’t be much scope to increase SOC further without converting to a higher carbon-capturing land use like growing trees. If your loamy sand soil under cropping has 0.6% SOC, there could be some opportunity to improve SOC stocks, provided there is reasonable rainfall.
As carbon storage potential varies considerably with rainfall, there are benchmarks for individual districts: see yours here.
Let’s take the example of the Southern Murray Mallee. Average growing season rainfall (from 1982–2018) is around 235 mm and the mean SOC for the district is 0.75%. Using Figure 33 from the report (extract above), if your soil is a loamy sand and your SOC is 0.5%, there is scope to increase SOC in your soil. Of the 152 loamy sand samples used in this region, the mean SOC% was around 1%. On clay loams, the upper realistic limit is closer to 1.8%.
