Sally Brown

June 7, 2018 | General

Connections: Soil Health Indicators

Sally Brown

Sally Brown
BioCycle June 2018

It is pretty easy to tell if you are healthy. You go to the doctor and they test your blood pressure, weight and every couple of years you get a blood panel. There are even tools to make sure you stay healthy. I recently read that if you eat well and not too much, stay active, don’t smoke and only drink in moderation you get 12 to 14 extra years. That suggests that if I continue (pretty much) following all five, I have a chance of lasting past 85.
There are no such guidelines for soils. Soils are old to start with. Eighty-five years in soil development is the equivalent of a couple of hours in human time. It is also very difficult to do a blood panel on a soil. However, we are starting to realize that just like people, soils can be considered as healthy or unhealthy. The Soil Health Institute was established a few years ago to promote healthy soils. Recently, the Institute unveiled a North American project to come up with the soil equivalent of a blood panel. The next step after the blood panel is to come up with the tools to keep soil healthy.
While neither myself nor many of the other soils people I associate with were invited to weigh in on possible tools, our answer is straight forward and pretty simple: Add organic matter. This can come in the form of cover crops, crop residue, manures, composts or biosolids. All make for a healthy, clean living soil. Add these and you reduce the need for fertilizers and pesticides. Just read Growing a Revolution by David Montgomery (see “Closing The Loop,” Sept. 2017) and you will see the light.

Soil Health “Blood” Panel

Let’s take a look at the Soil Health Institute’s version of the blood panel and see how the indicators of soil health are related to soil organic matter (SOM). Its first tier indicators are:
• Soil pH: How acidic or alkaline? Varies based on amount of rainfall and fertilizer use (rain and nitrogen fertilizer makes soil acidic, arid areas usually have basic or high pH soils).
• Soil electrical conductivity (EC): How salty? In dry climates soils build up salts and this is measured by testing the soil’s ability to conduct an electrical charge.
• Cation exchange capacity (CEC): Soil’s ability to hold onto nutrients. The CEC is the charge on the surface of soil particles. This is an inherent property of soils.
• Percent (%) base saturation: How much of the charge in the CEC is covered by bases, typically nutrients like calcium, magnesium and potassium, that are good for plants.
• Extractable potassium, calcium, magnesium, sulfur, iron, zinc, manganese, copper and sodium: Measure of plant available (different from total or water soluble) nutrients. The exception here is sodium — bad for people and bad for plants.
• Soil organic carbon: Finally! Carbon in soils can be present as both organic matter and as a component of limestone. This measure is only the portion of carbon associated with organic matter.
• Soil texture: Relative proportions of sand, silt and clay in a soil. Nothing you can do to change this. It’s akin to genetics determining how tall you are going to be.
• Aggregate stability: How well those sand, silt and clay particles are held together. This is something you can change. High aggregate stability is a sign of a good soil.
• Available water holding capacity: Measures how well a soil holds onto water. Makes a difference for irrigation requirements and potential susceptibility to drought.
• Bulk density: Same deal as weight in people. An unhealthy soil weighs a lot (more than about 1.3 g cm3); a healthy soil weighs less.
• Soil penetration resistance: Another way to measure how well structured a soil is, e.g., how hard it is to stick a probe into the soil.
• Water infiltration rate: How fast water soaks into (rather than runs off the top of) a soil.
• Crop yield
• Short-term carbon mineralization: Measures how quickly the microbes in the soil will eat soil carbon. A soil with a fast mineralization rate means a healthy microbial community. So fast is good.
• Nitrogen mineralization rate: Same as above but specific to nitrogen. Here the transformations are from organic matter nitrogen (as in manures, composts, and biosolids) to ammonia and then from ammonia to nitrate. Plants typically like to use nitrate.

Soil Properties

It goes without saying that I think increasing SOM will make the universe a better place to live. But to add some outside credibility to my arguments, I am going to look at The Nature and Property of Soils, the classic soil science textbook by Brady and Weil (Ray Weil was my professor for introductory soil science). The tests can be divided into three basic categories: soil chemistry, physical properties of soil, and soil biology.
Soil Chemistry
Soil chemistry covers pH, EC, CEC, and the tests for fertility — topics that don’t traditionally include a consideration of organic matter. However, looking at the textbook, you can see that organic matter leads to higher availability of nutrients. How? First, the organic matter typically contains the nutrients. Dead plants contain the essential nutrients that let plants grow in the first place; same deal with animal and human wastes. The second part is that the organic matter itself has charge, just like the soil particles. Not only does SOM contain nutrients, it contains the ability to hold onto them once they are released into soil solution. The textbook also notes that organic matter helps to stabilize soil pH and reduces the toxicity of aluminum in cases of acid soils. I’ve also read that SOM makes salts less toxic at high pH, but that isn’t in the text. So SOM is related indirectly to just about all soil chemical parameters listed.
Physical Properties
Soil physical property indicators include soil texture, bulk density, and aggregate stability. Measurements for water infiltration, water holding capacity and penetration will all be impacted by those three physical property indicators. Soil consists of the solid material and pore space. How well the solid material is held together determines how much pore space there will be. That in turn determines how easy it is for water to soak into soil, how much water is held in the soil and how hard it is to push a probe into the soil.
It is generally recognized that pushing a probe through air is much easier than pushing a probe through rocks — even if those rocks are already shattered into tiny pieces. Once again, we turn to the source, Brady and Weil. Excepting texture (the one equivalent to the genetic code of soils), all soil health indicators are improved with increased organic matter. Organic matter helps hold soil particles together, creating stable aggregates. That in turn creates more pore space, which reduces bulk density. That in turn lets more water soak in and stay there. Organic matter does for soil physical properties what exercise does for personal fitness.
Soil Biology
Biological properties include carbon and nitrogen mineralization. This is a no brainer. Even according to Brady and Weil. Soil microbes are responsible for both of these. If you have more food for the microbes to eat (read SOM) you get more microbes and higher rates of carbon and nitrogen mineralization. To quote the text, “As food source, [SOM] increases soil faunal and microbial diversity and activity.” All helping lead to increases in that final variable: crop yield.
An initial read of the first tier testing recommendations from the Soil Health Institute may not scream soil carbon at you but once you start looking in depth and seeing the connections, soil organic matter makes everything better. Making soils live longer and be healthier (i.e., high in organic matter) is much easier than doing the same for people. Just give them a balanced diet and you are good to go.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.

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