Using soil data to make sense of soils
Farming entails a constant stream of decisions, with incomplete information being the norm. Farming ‘choices are neither riskless nor simple.’ For new farmers, the learning curve is steep in every part of the farm business, of which soil forms just one part.
Angus Somes and Sally Higgins, two years into managing a family cropping and grazing property near Allora Queensland, are keen to learn more about their soil and how best to manage it. They see this as particularly important given recent climatic events, from drought through to flooding.
Regional Soil Coordinator, Cameron Leckie, along with UniSQ Bachelor of Science (Plant Agricultural Science/Environment & Sustainability) student Chanelle Barrett visited Angus and Sally at Springview to collect soil samples earlier in the year, as part of their prize for winning the Hub's 2023 World Soil Day competition. The sites sampled included an area that was inundated during the 2022 flooding as well as an area that had performed well under a previous wheat crop but poorly this season under sorghum.
The results are now back from the laboratory. Combined with observations from the field, the information provides a number of insights.
In most soils, soil organic carbon (SOC) is highest in the topsoil and declines quite significantly with depth. At three of the four sites SOC was at moderate levels throughout the depth of the soil. Likely a consequence of the Vertosol soil type which shrinks and swells upon drying and wetting, resulting in a more even distribution of SOC. Combined with a very high level of Cation Exchange Capacity (the capacity of the soil to store plant nutrients such as calcium, magnesium and potassium), this indicates highly fertile soils.
One interesting observation was the differences in active carbon (which provides the food source for the soil microbial community) between the two samples collected close to Dalrymple Creek. Whilst SOC levels were similar across the four depths sampled, the active carbon at the site that was inundated in 2022 was less than half of that at the site that was not subject to inundation: potentially a consequence of a microbial post flood feeding frenzy depleting the active carbon.
Whilst nutrient levels were generally adequate or better, there was one nutrient that was consistently low both between sites and across the four depths sampled at each site. Sulfur.
Plants use sulfur to synthesise amino acids, produce chlorophyll, for plant immunity and managing abiotic stresses.
Sulfur levels were well below the recommended critical value and maybe limiting maximum crop production. Given the heavy rainfall of the last few years, there is the potential that the mobile sulfate ion (the plant available form of sulfur) has leached below the crop rooting zone.
One of the sites was deliberately selected for diagnostic purposes. Could we answer the question; why was this area not performing?
The results certainly provide some indicators as to why this may be the case. These indicators included low levels of SOC in the topsoil and extremely low levels in the subsoil, low levels of active carbon, subsoil alkalinity which could be limiting nutrient uptake, magnesic subsoils (where magnesium dominates calcium), inadequate soil phosphorus, sulfur and manganese reserves as well as the lowest overall level of soil microbial health.
What the data couldn’t answer is why was this site so different to the soil at the other sites. The difference could well be a result of historical erosion that has exposed the less fertile subsoil. Angus and Sally are now armed with the information they need to make a decision on whether they should invest in rejuvenating this area or accepting that this area will be less productive than the remainder of their cropping country.
As part of the SQNNSW Hub’s commitment to encouraging farming practices that improve soil health, the Hub will be running a World Soils Day Competition once again in 2024. Competition details.
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