Searching for Goldilocks in the Soil?


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Andrew McGuireFAFDL CONTRIBUTOR: Andrew McGuire | Agronomist | Washington State University Center for Sustaining Agriculture and Natural Resources | @agronomistag

Andrew McGuire is an agronomist working in the Columbia Basin’s irrigated cropping systems. His current focus is on helping farmers build soils, save money, and maintain yields through high residue farming systems and cover cropping.

This piece previously appeared on CSANR blog in two separate posts. It appears here by permission of the author.

TOO COLD: Are you to killing your soil?

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 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.

Soils are very hard to kill. Soil scientists wanting to sterilize the soil 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. It is hard to imagine how the equivalent to 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.

An autoclave, how researchers kill soils. Wikimedia Commons

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.

So, if soils are hard to kill and resilient, why the talk about dead soils? I think that 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. 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 occur 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.

USDA photo | Flickr

TOO HOT: Are You Micromanaging Your Soil?

As a means to improve soil management, I commend the high interest in soil biology among farmers and gardeners. However, I have noticed the tendency for this interest to be combined with the thought that we should be able to fine-tune our soil biology for the good of our crops, health, sustainability, democracy, justice, and peace.. OK, mostly just crops and soil health, but exaggeration does seem to be rampant when it comes to expectations. The problem is that the idea that we can accurately fine-tune our soils is wrong. Unfortunately, this has not stopped a swarm of salesmen from swooping upon budding soil micromanagers hawking their bio-products. And this is not just at organic farming conferences where biological products are commonplace. I collected this list of products at last year’s Pacific Northwest Vegetable Association conference in Kennewick: Bio Secure, Bio Safe, Bio Innovator, Bio Flora, Bio Generator, Byo Soil, Byo Gon, Bio Forge, Bio Works, Bio Terra Plus, and BioBurst.

Rhizosphere microbes, can you manage them? Courtesy of Pacific Northwest National Laboratory.

I did not evaluate these products and I am not saying that none of them work. My targets are those practices, potions, and promises that claim to “fine-tune your soil biology for better ____.” Fill in the blank with balance, crop yield, crop quality, nutrient concentrations, pest control, and my favorite, which comes from a recent phone call from a salesman, “stripe-rust control.” (Stripe rust is a foliar fungal disease of wheat with spores that blow from state to state. How a soil product would cure this, I haven’t a clue).

The problem here is not the desire to manage soils; it is the degree of control that they are claiming. Peddlers of these products claim to be able to give you “a perfectly balanced soil,” “equilibrium”, “fine-tuning your soil’s biology” (all taken from actual product advertising). They make it sound like managing the soil is like blending a fine wine, a little of this, a little of that, and we can solve any problem. However, fine-tuning implies understanding, and we do not understand “soil biology” at anywhere near the detail to allow us to micromanage it by adding this enzyme or that bacterial species. To think otherwise is folly.

Soil scientists estimate, don’t ask me how, that we know only about 10% of the soils species. Most species do not grow in the lab and so we cannot study them in traditional ways. With new genetic methods, researchers can better determine what is in the soil, but even this data has to be compared with DNA libraries of known species. In addition to the unknown species, the sheer diversity is staggering. I have seen estimates of 10,000 bacterial species in a gram of soil. Who can say they understand this underground jungle enough to fiddle with the details?

Getting It More or Less Just Right

What to do in the face of such complexity? First, don’t try to tweak the details of your “soil biology.” Rather, take the big view and focus on the principles, which are, as I see them:

  1. Feed the soil – If you want to change a bulk physical property of the soil, you have to use bulk materials. The hitch with many non-fertilizer “bio-products” is the small amounts actually added to the soil. If you do the calculations, many are adding about a drop of product per square foot, sometimes much less. Although some pesticides are effective at such levels, I don’t think it is reasonable to expect one drop to change your soil biology. Bulk organic materials, on the other hand, provide food for your soil biology. These can be grown on-farm, like cover crops and green manures, or imported materials like manure, compost or organic waste. Once fed, the soil biology does what it does, building structure, cycling nutrients, etc.
  2. Stay out of the way – Conserve what the soil does naturally. This includes minimizing disturbance (tillage) and keeping the soil covered as much as possible.

Sounds too simple? The principles ARE simple, but the details of doing this can be challenging. That’s why it is tempting to look for “organic-matter-in-a-jug” solutions. Resist the urge to micromanage and save your money for proven soil building practices.

If you do want to test some of these non-bulk products, I recommend looking for those that are focused on doing one specific thing at one specific location in the soil, such as inoculating of plant growth-promoting bacteria in the seed furrow. Then work with your local Extension educator to set up a reliable on-farm test. Be very skeptical of everything else.

Further reading:
• Crop Probiotics: How More Science and Less Hype Can Help Farmers
• The 4 Essentials of Sustainable Agriculture

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