More Soil Biology!

 A SoilFoodweb youtube video.

You may have heard the concept that the soil system is founded on 3 legs – chemical, physical, and biological. And from a management perspective, what has been our “conventional” approach to these legs? – 1) We do soil tests for soluble or “available” nutrients then correct a perceived deficiency with an inorganic salt (chemical), 2) we describe structure and apply that to water movement models  though we really don’t do much to manage it excepting no-till/minimum till practices (physical). (Tillage destroys structure and creates hard pans.), and 3) we apply herbicide and pesticide to wipe out the bad guys… and good guys (biology).

So where has this gotten us?

Despite our best efforts and new technological approaches, essentially we have still been slowly destroying  our soil system foundation, all 3 legs. We still have far too much agriculture induced erosion, high crop susceptibility to  pathogens and drought, nutrient leaching (leaky systems), and overall land degradation or even desertification. Many would also say that we have nutrient poor soils, but that’s not exactly true. The nutrients are there, they are just not available to the plant. Why? What are we doing wrong?

The biology is missing!  The soil biology is the “deep rooted leg” that anchors everything else and specifically the other 2 legs, chemical and physical. It’s the biology that completes the nutrient cycle in the soil making mineral and organic nutrients available for plant uptake. In a natural setting the soil biology is the “initiator” of almost every other property in the soil (Please see my previous post regarding Soil Organic Matter) and it’s the leg that we have been ignoring the most, and even destroying it.

I spent the first week of February delving deeper into the intricacies of the Soil Food Web in a class taught by Dr. Elaine Ingham. It was a great 5 days loaded with information and learning starting with the food web basics, then exploring the concepts and issues in making quality compost and compost teas (essentially soil inoculants). We finished off with some microscope training and real time critter ID (good guys and bad guys). I would highly recommend this class to anyone who has interest in building soil and plant health – it’s all about the biology.

Some key concepts summarized:

1) Plants select for the symbiotic biology they require through root and leaf exudates, whether they need nutrients via the microbiology or defense against pathogens. Nutrients are always there in the soil but you have to have the biology present to make them available for plant uptake. Applying herbicides, pesticides, and salt fertilizers selects against beneficial soil biology (kills it) and selects for pathogens and weeds by putting the soil system back to square one in the natural successional process. Obviously there are times when “you gotta do what you gotta do” to deal with an imbalance (like a bumper crop of weeds) – just understand what you’re doing to the soil biology so you can try to remediate.

2) The bacteria/fungal ratio is different for specific plant biomes. In a natural setting the ratio typically is bacteria dominant during primary succession and becomes increasingly fungal as the succession moves towards a climax of shrubs and trees. So, depending on what “biome” you are managing for (corn, grass pasture, vegetable garden, apple orchard, etc.) you can select or make your compost & compost tea inoculants to match it. The following diagram should clarify this point a bit.


3) Compost and compost teas can be poorly made and result in something much less useful (low nutrient mulch) or worse, a source of pathogens. Pay close attention to the conditions if you are making your own. If you purchase a product make sure you know how it was made or examine a sample yourself (Microscope knowledge is quite handy, if not essential to making good decsisions here).

4) Our focus on killing the “bad guys” is 180 degrees skewed. We need to focus on helping the “good guys”. Then they can deal with the bad guys in their own (and better) way through resource competition and more direct methods of micro-life “warfare”. It’s amazing how similar it is to the human immune system through gut biology. We essentially need to feed the soil some pro-biotics.

5) The predator functional groups Protozoa and Nematodes are critical to completing the organic matter mineralization cycle in the soil. They are the critters that eat the fungi and bacteria and convert the organic nitrogen into plant available forms (NO3 and NH4). Unfortunately many, if not most, farm soils are devoid of predator protozoa.

Moving along with the biological focus, I would like to direct you to a couple of articles that specifically address soil biology and soil health forwarded to me via some colleagues (Thanks Dave and Suzanne). The first is from “The Cattleman” magazine featuring grazing practices influencing soil health. The right hand side bar directly addresses the role of soil biology. I’m really glad to see these articles coming out in magazines like this. The second, in “The Scientist” magazine, one of the science contributors neatly summarizes some recent research into how we can “Fight Microbes with Microbes“, (that good guy/bad guy thing).

So here is the “nutshell” version of the point of this post: We need to build and bolster our soil biology because of it’s positive influence on every other aspect of a healthy soil system. Here’s how we can do that:
1) Inoculate biologically deficient soils with good biology through composts, compost teas, focused to the needs of the crop you are growing. 2) Use herbicides, pesticides, and salt fertilizers sparingly, if at all. They kill the good guys too. If you do use them understand what it is that you are doing and refer back to point 1.  3) Feed your underground herd (through cover/companion crops). Diversity is key to a healthy system. A diversity of plants (and their roots) will equate to a diversity of soil biology to aid in nutrient cycling and disease reduction. 4) Till as little as possible. Soil biology is what builds soil structure and every time we till it’s left to what ever biology is left to re-build. Tilling also knocks out the predator (and fungal) populations and breaks the nutrient/mineralization cycle.

I know this entry is a bit choppy and obviously there is a LOT more to the details. But hopefully this provides a decent summary or intro to the basics of soil and plant health via soil biology. So let’s focus on helping the good guys and reinforcing that important biological leg.


About Chuck

Soil Scientist USDA/NRCS, Owner -
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