Organic Rob Tilth Indicator

All soils can be looked at as the result of three linked but separate processes:

Physical – Chemical – Biological

Industrial agri-frood is only concerned with the Chemical part of soils through available N-P-K and chemical fertilizer.
Organic agri-food is only concerned with the Biological part of soils through earthworms and animal manures and composts.
For the farmers, the only part of the soil they experience is the Physical, how well the soil works up for small seeds or how easy it is to harvest root crops.

Is there a way to look at all the parts of the soil together?

Organic inspectors also have to be concerned about the physical condition of the fields during the farm visit. The recent revisions to the Canadian Organic Standards (COS, 2015; updated 2018) still requires that tillage practices be used which maintain and/or improve soil structure (tilth). The inspector has to assess the impact of farming practices on soil structure even though chemical soil tests are little help in making this assessment.

What else can be used that would be more effective than chemical soil tests?

ORGANIC ROB TILTH INDICATOR is based on 5 field indicators that help assess the biological part of the soil by a detailed assessment of the physical part of the soil. These indicators are Soil Texture; Available Nitrogen; Depth to Compaction; Electrical Conductivity; Aggregation. This is my effort to answer that key question based on section 5.4.3 of COS 2015 (updated 2018): Are farming practices building up or breaking down soil structure?

Aggregation or soil structure is so important because all beneficial soil organisms are oxygen breathers, like us. Air and water penetrate through the soil profile in the large and small pore spaces. This sponge like structure is held together with a biological glue that is produced by actively growing soil microbes. When the soil structure breaks down, the beneficial soil life suffers and so do profits.

Measuring the physical aggregation helps give us a window into the unseen biological part of the soil.

Let us examine one indicator in detail to see how it can be helpful in assessing whether farming practices are helping or hurting the soil. Depth to compaction was suggested as a field indicator by the noted soil scientist Dr. Weil, during a workshop on soils at RODALE ORGANIC FARM. He suggested straightening out a thick metal coat hanger and walking over a field to see how far down you could stick the metal rod…

Soil compaction means the soil aggregates are breaking down and filling the pore spaces with sand-silt-clay. This reduces how deep crop roots can grow and negatively affects the nutrients and water that is available to plants.

John Jeavons says 12-24″ are needed for bio-intensive yields. Organic research in the Okanagan showed soils had compaction at 6-8″. This is similar to results I found during farm inspection visits. These on-farm field indicator readings are less than ideal. Our company uses 12″ as the operational goal for this field indicator in ORTI.

drawing from 1999 Okanagan Similkameen Soil Management Handbook

The above drawing shows the link between the depth to a compaction zone and the amount of aggregation in a soil. The greater the depth to a compact zone, the greater the aggregation. That is how things are starting to appear to me.

When the % aggregation by soil volume increases, there is an increase in available nitrogen and water holding capacity (more small pore spaces), plus greater resistance to soil erosion.

Increasing aggregation can have a positive impact on net farm income, as more and more research is showing. Organic Rob Tilth Indicator (ORTI) is still a work in progress as farmers help us develop these field indicators into a useful business tool for organic bio-intensive farm managers.

Soil structure or tilth (aggregation) is one of the most important critical organic control points. We need to monitor this more efficiently for the benefit of the soil and farm families and the surrounding communities.

Robert Dixon; Okanagan Organic Services (OK.O.S)
Building Organic Food Security