How to kill your soil

posted in: Commentary | 5

I recently saw an infographic that stated, “There are no life forms in the soil, which is sterilized…” What was it talking about? Soils on the moon? A toxic chemical spill? Soils around Chernobyl? Nope, this was the description of soils under so-called industrial agriculture. I have heard it before, the epidemic of “dead soils” caused by “chemicals.” This may make good copy for organic food advertisements, but it is not good science.

Dead soils on Mars
Dead soils on Mars

Soils are very hard to kill. Soil scientists wanting to sterilize  soil have to expose it to high-pressure steam for 30 minutes or more in an autoclave. Often, because soils are notoriously hard to sterilize, they repeat the process more than once. It is hard to imagine how the killing power of an autoclave could be occurring in fields, regardless of management. Even fumigation, the most drastic of attempts to kill off soilborne pests, does not kill everything. The huge diversity of bacteria and fungi in soils, and the variety of microhabitats available to them, means that much life survives.

Soils are also very resilient. After fumigation, farmers know that they have a limited time before the pests and beneficial organisms rebound. Fumigating every year, rather than working better, tends to select for those organisms that use the applied chemicals as an energy source (they eat it). These organisms proliferate and diminish the effects of future applications. This should not be a surprise, after all, most pesticides are organic chemicals, and bacteria and fungi are experts at organic chemistry.

If soils are so resilient and hard to kill, why the talk about dead soils? I think much of it is just marketing hyperbole – “dead soils” stir emotions. However, there are soils that do not function well, that can seem dead. The cause of this problem is generally not “chemicals,” but rather a lack of organic matter, especially fresh organic matter.

The rich, deep color of this soil indicates exactly what healthy soil looks like. Image by NRCS Soil Health via Flickr.
The rich, deep color of this soil indicates exactly what healthy soil looks like. Image by NRCS Soil Health via Flickr.

Organic matter can be categorized by its age. Humus is very old (50-10,000 years) organic matter that is stable because it is highly resistant to further decomposition by soil organisms (the process of organisms extracting energy from, or “eating” organic materials). Material that is 5-30 years old is on its way to becoming humus, but can still be decomposed further by a select group of organisms. These two pools are large, but do not provide much food for the majority of soil microorganisms.

It is fresh organic matter, “the recently deceased,” that feeds the soil. When soils do not receive a regular supply of this fresh organic matter, biological activity decreases. The same can happen when soils are tilled often, which stimulates rapid decomposition. With no fresh organic materials to eat, the soil biology slows down and eventually many microorganisms go into a resting state. These soils are not dead, but they are starving. This view of organic matter and soil biology is simplified, but is useful for thinking about soil management and what soils really need.

A good place to learn more about organic matter and soil health is the book Building Soils for Better Crops, from the USDA Sustainable Agriculture Research and Education program, available in print or for download.

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Andrew McGuire is an Irrigated Cropping Systems Agronomist for Washington State University Extension. He works with farmers in the Columbia Basin of Central Washington in improving soils through cover crops and high residue farming systems.