The Soil Carbon Project

Understanding your soil structure is very important as soil structure affects root penetration, water availability and soil aeration.

There is a standard method to understand the structure of your soil which was developed in Scotland called the Visual Evaluation of Soil Structure, or VESS for short.

This proxy test is very easy to do and the results can inform future management of the field.

Method:

• Sample at three locations across the field to gain an overall understanding
• Extract a soil block one spade’s depth and width.
• Examine soil structure by manipulating the soil block to see how easy it is to break up.
• Use the VESS scorecard to record a score for topsoil and subsoil.

The VESS Scorecard can be found here. The scale ranges from 1 (friable soil) to 5 (very compact).

Aggregate Stability refers to the degree to which a lump of soil maintains its structure when submerged in water.

Healthy soil will have a low aggregate stability score, organic matter and carbon acts as “glue”, binding soil particles together to give a good soil structure. If an aggregate has a high score, the soil is more susceptible to erosion and compaction as there is nothing holding particles together.

Method:

• While you have the soil block extracted for the VESS test, take a fistful of soil home with you
• Place the soil on a tray to air dry for 3-5 days
• Once dry, extract three lumps 3-5mm in diameter and place them in a dish of water so they are fully submerged
• Score the samples using the aggregate stability score (below) after 5 minutes and 2 hours

Aggregate Stability Scores:

• 0. if the lump remains intact,
• 1. if the lump collapses around the edges but remains largely intact,
• 2. if the lump collapses into angular pieces,
• 3. if the lump collapses into small rounded pieces, forming a cone,
• 4. if the lump completely collapses into single grains.

The rate at which water infiltrates into the soil is a good indicator of soil structure, particularly compaction.

Healthy soil has a short infiltration time, because pore spaces allow water to infiltrate down into the soil. These pore spaces encourage root development, soil aeration and the retention of water for use by plants.

Compacted soil has fewer pore spaces and so infiltration times will be longer.

Method:

• Choose three sites across the field that represent ‘normal’ soil conditions (not in gateways, tramlines or tracks), clear away any vegetation or residue from the surface.
• Insert a cylinder (e.g. tin can with both ends removed) a few inches into the soil
• Pour 107ml of rainwater into the cylinder and start the stopwatch
• Time how long it takes for water to infiltrate, stop timing when the water is gone and the soil is glistening

A detailed guide to carrying out the infiltration test can be seen below and here

Burying cotton pants is a good way of looking at the microbial activity in the soil. Healthy soil supports communities of microbes that are important for decomposition and the cycling of carbon and nutrients.

Method:

• When you have dug a soil pit for the VESS test, place a pair of white, 100% cotton pants in the pit
• Make sure the pants are spread wide in the pit, not scrunched up at the bottom
• Fill in the pit and mark the location
• Return in three months to dig up the pants and look at the degradation

Healthy soil will have lots of microbes that will break down the pants and create holes in them. Unhealthy soil won’t have the important microbial communities and the pants will remain intact.

Check out the hashtag #soilmyundies on Twitter to see how the test works in the field.