A Te Ahikawariki / VICE Project
SLAKES: a cost effective measure of soil structural stability is one of two projects I have been running as a part of my Te Ahikawariki/VICE internship. A key part of the project was engaging with growers to demonstrate and support valid paddock sampling and correct use of the test methods. Our aim was for growers in each regional community to know how to use and understand results from a standardised assessment methodology consistent with SHI guidelines. As part of the Slakes project, we ran individual farm workshops with growers in Hawke’s Bay, Pukekohe, Palmerston North/Levin, and Gisborne.
Soil aggregate stability
Soil aggregate stability is a measurement of aggregates’ ability to resist degradation and maintain structure when being affected by external forces such as rainfall, vehicle traffic, and tillage. It is correlated with soil erosion, water retention, aeration, root growth, soil microorganisms, and soil carbon.
Soil aggregate stability is one of the three key soil health indicator tests recommended by the Soil Health Institute (SHI), alongside soil organic carbon concentration and carbon mineralization potential. The SHI recommends the Slakes method for testing aggregate stability as it is practical and cost-effective.
The Slakes method uses a free app that was developed by the SHI and the University of Sydney, to test aggregate stability. The app is used to take images of sets of three aggregates, initially dry and again after they have been submerged in water for 10 minutes. The Slakes app takes photos, times submersion, and calculates an Aggregate Stability Index, which is the ratio of initial to final soil area.
The Slakes Project outline was to:
- Compare the Slakes aggregate stability index and Manaaki Whenua – Landcare Research wet sieving aggregate stability results
- Correlate the results to a visual soil assessment (VSA) score, bulk density, and total carbon
- Engage with growers while demonstrating valid paddock sampling and how to use the Slakes app
Most samples collected throughout this project were taken from an uncropped and a cropped area within the same paddock. When comparing the Slakes aggregate stability index and the Manaaki Whenua Landcare Research wet sieving aggregate stability results, there was a clear trend in both methods that the uncropped areas had higher aggregate stability.
When comparing the VSA and aggregate stability results, most of the cropped paddocks had good VSA structure scores but weak aggregate stability. This suggests that the tilth created by cultivation looks good, but it lacks the strength of true aggregates that are the basis for good soil structure.
There was a moderately strong relationship between the Slakes aggregate stability index and the total carbon percentage in the soil (R2 = 0.69), with the uncropped areas (blue dots) having higher total carbon and aggregate stability than cropped areas (brown dots).
Throughout the Slakes project, we received positive feedback from growers about the Slakes app, as it provides a quick, simple, and affordable way to gauge the soil’s aggregate stability without needing to send samples away to a lab. However, some growers asked how they can use it to aid their management. Most are keen to know if there is an aggregate stability index threshold that indicates when poor stability will impact productivity. They recognise the test is low cost, but some would like the option to send it to a laboratory to save time.
Many thanks to Te Ahikawariki, the Vegetable Industry Centre of Excellence, for supporting me in my internship, and for funding LandWISE to undertake the Slakes research project. And thanks also to the farmers, growers and consultants who spent time trying out the Slakes method with us and giving us their considered feedback.
Olivia