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Soil Science Spotlight:
The Dr. John Doran/ USDA Soil Quality Test Kit Guide, Part 3
by John Beeby, Ecology Action Soil Fertility Advisor

One of the primary functions of the GROW BIOINTENSIVE method is to allow small-scale farmers everywhere to build and maintain soil fertility levels that will allow the farmers to grow a large amount of food and compost materials in a very small area, with greatly reduced resource use, for an indefinite period of time, sustainably.

Soil testing and the application of the correct type and quantity of organic soil amendments at the correct time is a fundamental part of building and maintaining sustainable soil fertility levels. To introduce the topic of soil testing and the reasoning and methodology involved in soil test analysis and making soil amendment recommendations to a wider audience, John Beeby and Ecology Action are creating a series of topics on the subject called “Soil Science Spotlight”, which is posted to in the “Protocol” section with new posts added often.

Soil Science Spotlight - Grow Your Soil - If we understand a soil we can improve it

In Part 1 of this series, I introduced the USDA Soil Quality Test Kit Guide, developed by Dr. John Doran. The Guide describes tests for: infiltration, bulk density, physical observations, aggregate stability, earthworms, soil respiration, slaking, water quality, electrical conductivity, pH, and nitrate levels.

In addition to infiltration and soil bulk density tests which I discussed in the previous issue, you can follow the instructions in the Guide on how to perform Physical Observations of the color and depth of your topsoil, as well as your soil’s rooting depth. This also points to the Soil Organic Matter (SOM) level, structure, and can help determine if the topsoil depth might be improved with additional organic matter as well as some tillage to allow roots to travel deeper into the soil and deposit root organic matter to create a deeper topsoil over time.

The Aggregate Stability Test is another great way to understand your soil’s structure, its ability to withstand heavy rainfalls, and indirectly, to assess the soil’s organic matter level. You can also run the very simple Earthworm Count Test to understand a soil’s structure, organic matter level, and overall biological health. All of these tests are going to give you slightly different perspectives on similar soil properties, allowing you to make sound decisions on whether your soil’s organic matter level needs to be improved, if the soil needs to be tilled, etc.

Now let’s talk about Soil Respiration. Does a soil breathe? Yes! Soil microbes living in an aerobic soil (a soil with plenty of air) are like us in that they take in oxygen and release carbon dioxide: a process called “respiration”. This carbon dioxide eventually leaves the soil and can either be taken up by plants (especially effectively when plants are grown close together as in GROW BIOINTENSIVE), or if it’s not taken up by plants, it escapes into the atmosphere and adds to the ongoing climate change crisis. However, this is not to say that soil respiration is bad! Soil respiration is a good sign of healthy active soil macro- and microbiological populations, which is good for soil health so long as we replenish the organic matter that was consumed and converted into carbon dioxide.

A healthy soil macro- and microbial population is essential for good soil structure and tilth. So, by determining if a soil is respiring well, you can understand something about the health of the soil’s microbial ecosystem, as well as whether the soil has sufficient organic matter to support that ecosystem. Unfortunately, it can be challenging to acquire the equipment to measure soil respiration and to afford the Draeger tubes that can only be used once. The Solvita system is another way to measure soil respiration, but it also can only be used once and can be challenging to acquire in some areas of the world. So, instead of measuring soil respiration directly, other tests in the Guide (such as the infiltration rate, aggregate stability, bulk density, earthworm count, and physical observations of the topsoil color and depth and the rooting depth) can allow for a similar understanding of the soil’s organic matter and macro- and microbial population health in a way that approximates a soil respiration test.

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