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Soil Science Spotlight:
The Dr. John Doran/ USDA Soil Quality Test Kit Guide, Part 2
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. I do not plan to restate all the excellent information in the Guide. Instead, I mean to expand on the information you can derive from these simple tests. For years, I knew about Dr. Doran’s Guide, but somehow did not realize its potential until recently; it has become clear to me that the Guide provides farmers who lack access to soil laboratory analysis with the ability to better understand a soil and to determine ways to improve its fertility. Even for those who have access to a lab, the tests in this Guide can provide additional information about your soil that a lab cannot. As our world continues to lose soil fertility at an increasing rate, and with the majority of farmers in the world lacking access to soil testing services, developing the ability to assess a soil and know how to improve it is increasingly important. The Guide, in combination with an approach like Test Your Soil with Plants (my book, which describes how to tell what fertilizers to use to optimize your garden’s health and productivity by observing the plants growing in your garden), can allow an observant and thoughtful farmer to understand their soil and know how to improve its health and fertility even if they cannot afford a soil laboratory test and professional recommendation.

The Guide describes tests for: infiltration, bulk density, soil respiration, electrical conductivity, pH, nitrate, aggregate stability, slaking, earthworms, physical observations, and water quality. I'll be discussing the first two in this segment. These tests can be used in two ways:

  • First, they can be used to compare two different soils. For example, perhaps one soil produces lower yields compared with another soil, and you want to understand why. Or you want to compare differences in a soil when using two different growing methods – say, GROW BIOINTENSIVE and Biodynamic farming. Or to determine what changes occur in a soil when growing more deep-rooted compost crops. These types of comparisons are ideal for the Soil Quality Test Kit Guide.

  • Second, the test results can be used to generate a numeric value, which you can interpret using the Guide’s reference tables, allowing you to see where your soil stands in relationship to other agricultural soils, and how to improve it. For example, using a pH strip with a range close to agricultural soils, you can determine the approximate pH of your soil. Then, using the Guide’s reference chart, you can see if your soil is a little too acidic or alkaline. Another example is bulk density (g/cm3), which is a very useful quantitative soil measurement. Looking up your soil’s bulk density and soil texture (which you can determine yourself) in a chart can help you determine if the soil is compact enough to affect root growth and may need cultivation, or if it is loose enough that you can skip tilling the soil to conserve more organic matter.

The Infiltration test is very simple, but useful. With this test, we are measuring how quickly water can enter the soil. Simple enough! But from this, we can also get an idea of how good the soil’s structure is, how prone it is to water erosion, and indirectly, its level of organic matter, which is critical for so many soil functions.

The Soil Bulk Density test is another measurement you can make yourself to get a sense of how much organic matter is in the soil. The more organic matter a soil contains, the lower the bulk density, because organic matter improves a soil structure and increases the air space in a soil, and therefore decreases the soil’s weight/cm3. By knowing the soil’s texture (a characteristic you can determine yourself, not described in the Guide but easy to find online, for example, using the USDA Soil Texture Calculator materials/soil-texture-calculator), you can determine if the soil’s bulk density will negatively affect root growth due to excessive soil compaction. If you determine that your soil has a dense texture, you know that it makes sense to loosen your soil, and if you find the soil’s bulk density will not prevent root growth, then you can be assured that tillage is not needed, allowing you to conserve your soil’s organic matter this season.

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