Earlier this year, Soil Wealth ICP team members Carl Larsen and Camilla Humphries checked on progress at our Tarwin demonstration site in South Gippsland, Victoria, which is hosted by Schreurs & Sons.
The trial aims to maintain soil carbon in a clay loam greenfield site which has been converted from pasture to a vegetable production system rotation – predominantly celery, spinach and leek – using a combination of cover crops, compost and minimum till.
The importance of soil carbon
Soil carbon refers to the measure of carbon contained within soil organic matter, which is around 50% on average. It plays a key role in soil health.
Labile carbon is the carbon most readily available as a carbon and energy source to microorganisms.
While changes in practice may not demonstrate changes in total soil carbon, they may increase labile carbon, making it a better indicator of improved soil quality. Labile carbon is often a good ‘leading indicator’ of soil biological activity.
The labile carbon field test is useful for comparing management practices that influence organic carbon. We undertook a labile carbon test (Figure 1) and compared this to the previous year’s results (Table 1 and Figure 2), which showed two key findings:
- Areas with compost had improved labile carbon significantly from 2024 (Year 2) to 2025 (Year 3), indicating the importance of building soil organic matter over time
- No treatments or the control area in the trial had poor-average labile carbon in 2025 (Year 3), indicating an improvement from last year (2024).
Figure 1: Soil Wealth ICP team members Carl Larsen and Camilla Humphries undertake a labile carbon field test, April 2025.
Table 1: Comparison of labile carbon field test results from 2024 (Year 2) and 2025 (Year 3).
Figure 2: Labile carbon field test results with soil health ‘traffic light’ indicators.
From the field – leek harvest and soil pit update
Watch our video walk-through of the Tarwin demonstration site to hear about the trial design, labile carbon field test results, and see the leek crop prior to harvest.
One of the key observations from the Soil Wealth ICP team was the slight variation in leek crop quality and uniformity of growth between the treatment and control areas.
The leeks in the treatment areas were more uniform and had whiter, harder stems, while in the control there was more variation in growth and some orange discolouration and softness of stems (Figure 3). The control was planted slightly earlier than the treatment areas.
Figure 3: Comparison of leek and pea tendrils from the different treatments prior to harvest, April 2025.
In addition, analysis of the soil pits in the compost treated areas showed high carbon content (lignin) with strong leek root growth at 0-10cm depth (Figure 4).
Figure 4: Lignin from compost with leek root growth, April 2025.
Stay tuned for further updates, including more detailed results and a case study over the coming months.
