Sunday, August 14, 2022

Using the slake test to aggregate stability

In the fifth in our series on soil health tests you can do on-farm, first catch your clump.
Clumps are put through their paces in a slake test.

With our infiltration test we learned how long it took for water to sink into our various soils.  But when rain comes in flooding quantities (which happens more each year now) we also need to know how well it will hold together when there’s too much water around.

Aggregate stability depends on whether there is a good microbiome (fungal/bacterial network of filaments) holding the nodules of organic matter together.  If you haven’t killed it off by repeated tillage or overdosing on chemicals, this is the network that not only holds the soil together, but is also responsible for passing back and forth what both soil and plant roots need at any time.  The mycorrhizal networks (which operate both inside roots and outside in soil) also create sticky stuff called glomalin, which keeps things moving.

The better the networks, the better the aggregate forms (both for air and water), and the better the final yield aboveground.

The slake test is an easy one, which also allows you to compare results from, say, permanent pasture and cropping soils. The equipment consists of two large glass or plastic jars, with “baskets” made of wire or plastic mesh (with about 1cm gaps) to fit into the tops to hold the soil clumps.  The jars are filled to the top with water.

Gently dig or grab a clump of dry-ish soil (or a clump of each type) that will fit into the top of the jar.  Carefully put the clump into the water on top of the mesh. Then watch what happens.

The water will be sucked into the clump of soil, pushing the components apart.  With good stability the clump won’t fall apart, and just a few specks will fall to the bottom of the jar. Soil with less stability will fall apart increasingly quickly, and form a mud layer on the bottom.

Basic soil texture, as well as its soil life, has a big effect, such that sands fall apart regardless, silts holds together pretty well, and clays stick together strongly.

If there are parts of a paddock or crop field that produce markedly differently from each other, it is worth doing a slake test on each to give a clue as to what is happening down there and causing the good, or the bad, results.  It could be that the actual soil type is different, or maybe some areas have had better treatment than others over time.  It can be a waste just bunging more fertiliser on the bad bits, if more organic matter or soil life is what is actually needed to improve the soil itself.

The more you learn about what makes soil work, the quicker and cheaper you will find the right fixit response.


Who am I? Sue Edmonds is a farming and science writer from Waikato.

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