July 2018 Issue of Wines & Vines
 
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Growing a Revolution

An excerpt from a book that examines the roots of the underground economy and how to improve soil health

 
by David R. Montgomery
 
 

For much of human history, the role of organic matter in soil fertility was no secret. Farmers and philosophers alike believed that humus - soil organic matter - nourished plants. Until, that is, two key discoveries undercut this long-held belief. The first was the discovery of photosynthesis - that plants obtained their carbon, and thus most of their mass, from the air and not the soil. The second was the observation that most humus was insoluble and could not be sucked up by plant roots. So soil organic matter did not feed plants.
What replaced the humus theory was the idea of the soil as a chemical reservoir from which plants drew sustenance. In the first half of the 19th century, German chemist Justus von Liebig demonstrated that a lack of availability of key nutrients could limit plant growth. He also established that adding elements in relatively short supply dramatically boosted plant growth. Farmers working degraded fields found that adding calcium, phosphorus, or potassium could bring crop yields back up to levels not seen since their grandfather's day.
So did the addition of nitrate- and phosphate-rich guano, bird droppings that were aggressively mined from South Pacific islands. As the supply began to run low in the late 19th century, European and North American crop yields were threatened due to centuries of soil loss and degradation. Getting enough nitrogen to crops became a top priority.

Nitrogen gas (N2) bathes our world - it makes up almost 80% of Earth's atmosphere. So you might think that plants could grab all the nitrogen they need from the air. That's what they do for carbon through photosynthesis. But the triple bond between the two atoms in a molecule of nitrogen gas is incredibly stable. While nitrogen is essential for making amino acids, proteins, and DNA, not much of it is biologically available. This means that nitrogen is often the limiting element for plant growth - especially in soils with little organic matter.

But there was another strategic motivation for securing a steady supply of nitrogen: it was essential for making high explosives. In 1909, a pair of German chemists - Fritz Haber and Carl Bosch - figured out how to synthesize ammonia (NH3). Using hydrogen gas as a feedstock, they developed a high-pressure, catalyst-based process that worked at high temperatures. The ability to manufacture nitrogen both prolonged the nightmare of the First World War and produced the miracle of cheap fertilizers that could boost crop yields on degraded land, of which there was no shortage.

After the war the Allies demanded the secret to the Haber-Bosch process so they could modernize their own munitions factories. Decades later, after the Second World War, the same Allied countries converted idled munitions factories to fertilizer production, a change that could have been quickly reversed had the Cold War heated up. The widespread availability of cheap fertilizers, coupled with the new fertilizer-loving wheat and rice varieties of the Green Revolution, doubled global crop production.
Although fertilizers can quickly boost crop yields on degraded land, the increase in returns on rich, fertile soil is marginal at best. And as it is, only about half the nitrogen applied as fertilizer gets taken up by crops. The part that isn't does not stick around, causing problems off the farm. Most chemical fertilizers readily leach into groundwater because they are soluble by design. Add a lot in the fall and by spring much of it can end up in a river, reservoir, or water well.
Law of return
Soon after Haber and Bosch uncorked the nitrogen genie, an observant English agronomist began to question the new agricultural gospel. Based on years of work developing large-scale composting methods for commercial plantations in India, Sir Albert Howard proposed his Law of Return in the 1930s to explain why returning organic matter to the fields was essential to soil health, healthy crops, and bountiful harvests. At a time when there was little knowledge of how nutrients reached plants, Howard thought that mycorrhizal fungi played a big role.

In his experience, well-made compost boosted growth of mycorrhizal fungi. And fields with abundant mycorrhizae consistently produced abundant healthy crops. This led Howard to see fungi as nature's recyclers. He suspected that mycorrhizal fungi fed on decaying organic matter and served as root extensions that provided essential nutrients to plants. In Howard's view, chemical fertilizers could not replace soil organic matter, because adding a few elements could never provide all the mineral nutrients and substances in soil that fungi rounded up and delivered to plants.

While Howard grasped the general pattern, he could not really explain why fungi helped nourish plants. To agronomists, Howard's talk of altruistic fungal magic seemed just that. Still, he was sure that the agrochemical bandwagon was speeding down a dead-end street.

"There is a growing conviction that the increase in plant and animal diseases is somehow connected with the use of artificials. In the old days of mixed farming, the spraying machine was unknown, the toll taken by troubles like foot-and-mouth disease was insignificant compared with what it is now. The clue to all these differences-the mycorrihizal association-has been there all the time. It was not realized because the experiment stations have?.?.?.?[thought] only of soil nutrients and have forgotten to look at the way the plant and the soil come into gear." (An Agricultural Testament, A. Howard, 1940, London, Oxford University, p. 158.)

Howard's lack of an explanation for exactly how fungi and other microbes helped plants undermined the scientific community's interest in his challenge to conventional wisdom. Besides, the clear evidence of the near-miraculous effects of fertilizers in reviving flagging crop yields on degraded fields spoke for itself. In short order, Howard's ideas were eclipsed by the Green Revolution's fertilizer-intensive approach to boosting crop yields.

Life of the soil
Another influential agricultural myth is an innocent half-truth I learned in college - that chemistry and physics govern soil fertility. In particular, I was taught that a soil's fertility lay in its cation exchange capacity - its capacity to hold positively charged ions, essential nutrients like potassium (K+) and calcium (Ca2+), loosely enough for soil water to take them up. This is not wrong, there's just more to the story.
When farmers send samples off to a commercial lab to find out what is in their soil, it is with an eye toward what they need to add to boost plant growth. But the standard soil chemistry tests only measure the soluble fraction of what is in the soil, the stuff that water percolating through the soil can readily pick up and hand off to plants.

Nutrients tightly held in soil organic matter do not show up in conventional soil tests. Neither do all the nutrients locked up in slow-to-dissolve minerals. At any one time, just a fraction of the elements in a soil is in an exchangeable, soluble form plants can take up. Standard soil chemistry lab reports are missing something big: the potential for soil life to convert nutrients from mineral soil and organic matter into forms plants can use. Since the 1980s, advances in soil ecology and microbiology have radically changed our understanding of how microbial life and organic matter interact to govern nutrient cycling and influence soil fertility.
This wouldn't have surprised Sir Albert Howard, or the philosopher - farmers who founded this country. It should not surprise farmers today. Good farmers may not know all the details behind what makes for fertile soil, but they know it when they touch and see it. I have seen how they pick up soil and rub it between their fingers, asking themselves: Is it crumbly or dusty, slick or firm? Does it aggregate and hold together, or disintegrate into dust at the touch? Above all, how much organic matter does it hold?

In a way, it is easy to see whether soil is healthy or degraded. The darker the soil the more organic matter - and carbon - it contains. Several generations ago, the amount of organic matter in the soil set the price of agricultural land. Every farmer knew that soil rich in organic matter was more fertile, and so did their bankers.

You might think of healthy soil as a particular mix of soil organisms, organic matter, and minerals that forms a thin skin on our planet, like a grand version of lichen coating an alpine boulder. Part alive and part dead, the average thickness of topsoil ranges from about one to three feet. Soil accounts for a thin sliver of Earth's 4,000-mile radius, but its proportions belie its importance. This delicate blanket of rotten rock is what makes our terrestrial world habitable. As the dynamic frontier between the living world of biology and Earth's rocky bones, soil is the realm in which microbial life recycles the remains of higher life into the raw materials for new life.

The history of life on land is a collaborative tale of plants harvesting solar energy, and microbial life mining and recycling nutrients. The first land plants evolved some 450 million years ago. They had partners right from the start - mycorrhizal fungi that hooked up with their roots.

Like today's plants, the earliest ones periodically shed dead roots and leaves, and eventually died. All that organic matter became food for soil organisms that then mined more nutrients from the mineral soil and recycled the dead stuff back into nutrients for the plants to consume. More plants led to more organic matter, which led to richer, more fertile soil. Soon, and for ages ever since, vegetation covered all but the rockiest, driest, or ice-covered landscapes.

Why was this partnership crucial? Consider where plants get their elemental building blocks. They use solar energy to combine carbon dioxide from air with hydrogen from water to make carbohydrates (sugars). Plants also get their nitrogen either indirectly from the air, with microbial assistance from nitrogen-fixing bacteria living in specialized root nodules, or from nitrates they absorb through their roots. Other elements plants need to make their bodies come from rocks and decaying organic matter. Mycorrhizal fungi and soil-dwelling microbes extract mineral nutrients from soil particles and rock fragments and help break organic matter back down to soluble nutrients that plants can suck up through their roots.

Yet roots are not simply straws. They are two-way streets through which carefully negotiated and orchestrated exchanges occur. Plants release into the soil a variety of carbon-rich molecules they make, and which can account for more than a third of their photosynthetic output. For the most part, these exudates consist of proteins and carbohydrates (sugars) that provide an attractive food source for soil microbes. In this manner, plant roots feed the fungi and bacteria that pull nutrients from the soil - from the crystalline structure of rock fragments and organic matter.

When enough microorganisms are present, root exudates do not last long. Microbes chow down on and assimilate most within hours, modifying and re-releasing them in other forms. In addition, with the help of soil-dwelling bacteria certain mycorrihizal fungi use their thread-thin, root-like hyphae to seek out and scavenge particular biologically valuable elements, like phosphorus, from rocks or decaying organic matter. Then they trade the scavenged elements, now in plant-available form, for root exudates. This sets up an exchange through which both sides benefit from the commerce of the original underground economy.

Likewise, the dead cells that roots slough off last for only a few days before microbes consume and reprocess them. The resulting microbial metabolites include plant-growth-promoting hormones and compounds that bolster plant health or aid in plant defense. Some form stable carbon-rich deposits that, in turn, help structure communities of beneficial bacteria in the rhizosphere, a biologically rich zone around plant roots.

Curiously, rhizosphere-dwelling bacteria are more effective at promoting plant growth once a critical microbial density is reached, triggering a process known as quorum sensing. When enough individual bacteria of the right kind are present, they coordinate the release of compounds that aid in promoting plant growth. But, if the population of soil microbes drops too low, they turn off the tap. In other words, they only work if there are enough of them to make a difference to the plant, which in turn produces a healthy exudate return for the microbes. By pushing enough exudates out into the soil, the plant can cultivate microbial populations that produce compounds useful to the plant. The complexity and adaptation belowground, mirrors that aboveground, as plants recruit and feed specialized communities of bacteria and fungi in relationships every bit as specialized as those between flowers and pollinators.

Where do you think you would find the most bacteria in soils? Where the food is, of course - around plant roots. And where are the most bacteria-eating protists and nematodes? Also around the roots, where the bacteria are. This is another link in the soil food chain - after saprophytic fungi and bacteria consume organic matter, they become enriched with nutrients. Predatory arthropods, nematodes, and protozoa feast on them, then release those nutrients back into the soil in plant-available forms. Because the excrement of these microscopic predators is rich in nitrogen, phosphorus, and micronutrients, it makes excellent micro-manure.

In these ways, soil life makes soil fertile. Major elements like calcium, magnesium, phosphorus, potassium, sodium, and sulfur that plants need to make their bodies, and we need to make ours, ultimately come from rocks via the soil. So do essential trace elements like copper, iodine, manganese, molybdenum and zinc. At every step along the way microbes are intimately involved in making most mineral-derived elements available to plants. And the more of these microbes that are on the job, the more nutrients that are available to plants.

Most (though not all) soils have enough of most elements to grow healthy plants - as long as those elements are unbound from mineral grains and organic matter and in forms that plants can take up. This is the microbes' job. Microbes facilitate getting a suite of essential micronutrients into plants - things like copper and zinc that we don't tend to think of as nutrients but that healthy plants and people alike need in small quantities. Soil-dwelling microbes work like little chemists to convert nutrients to plant-available forms. But in a soil sparsely populated with life, crucial nutrients remain parked outside of a plant's root zone, like goods on a ship stranded at sea far from port.

Biological bazaar
From bacteria to beetles, soil life forms an underground community that breaks down organic matter, yielding organic by-products and metabolites rich in nitrogen and mineral-derived elements. Soil life also influences the ability of plants to defend themselves - when insects or herbivores graze on foliage, some plants exude compounds that rhizosphere-dwelling microbes metabolize. Plants then use the microbial metabolites to drive away the herbivores. In other words, plants outsource the production of pest repellent to microbes that get paid with root exudates. When the rhizosphere is full of beneficial microbes, pests and pathogens have a harder time finding a seat at the crowded table.

The slow pace of rock weathering and limited availability of biologically-critical elements on Earth's surface means that recycling these elements is essential to growing and sustaining abundant life. Over geologic time, microbially-mediated processes refined and built up the stock of ingredients circulating through terrestrial ecosystems. Soil life not only turns the wheel of life, it procures and stores nutrients essential for new life and keeps them from leaching out of the soil.

Heavy fertilizer applications can alter soil microbial communities, make the soil acidic and harm beneficial microbial life. Although crops have access to nitrogen through the fertilizer, elements that microbes previously converted into usable forms may remain inaccessible when the right soil organisms are not around to do their job. When plants get all the macronutrients they need for free from fertilizers, they shut off their expensive exudate faucet, denying the microbes that are left in the rhizosphere a much-needed food source. This turns crops into botanical couch potatoes and helps make degraded farmland dependent on nitrogen fertilizers. It also means that while plants may get certain major elements they need to grow, they lose out on the microbial allies that help procure the mineral micronutrients they need to be healthy and mount a robust defense against pests and pathogens.

More than one-half century after Sir Albert Howard first proposed his Law of Return, we finally understand how it works. There is a biological basis for the central role soil organic matter plays in growing healthy crops and sustaining bountiful harvests. Fertility isn't only about chemistry and physics. Soil ecology and nutrient cycling driven by microbial life also matter. Even when standard soil chemistry tests say you need to add fertilizers, the right soil life - if present and abundant - may be able to supply what plants need.
Growing evidence shows that synthetic fertilizers work like agricultural steroids, propping up short-term crop yields at the expense of long-term fertility and soil health. Consider fertilizers and agrochemicals as like antibiotics - a godsend if you really need them, but foolish to rely on for regular use. And this, of course, is exactly what we've been doing for decades.

In hindsight, we know that our dependence on the plow and fertilizers to pump-up crop yields depleted soil organic matter and disrupted the beneficial fungi that extract crucial micronutrients from rocks and deliver them to crops. When we take out mycorrhizal fungi - eliminating or limiting their role in nutrient acquisition - and compromise microbial roles in pest and pathogen control, we have to replace them with fertilizers and pesticides.

But we can reverse this by cultivating beneficial microbial life. The key to doing this seems to be practices that build soil organic matter - feed them and they will come. Farming practices that maintain high levels of soil organic matter support the diversity of beneficial soil life that in turn supports plant health. Organic-matter rich soils promote beneficial soil nematodes over plant-parasitic nematodes as well as bacterial communities that suppress pathogens. And it is well established that they are more fertile.

Speaking at farming conferences for the past few years, I met farmers discovering how to rebuild fertile soil. They are showing how highly productive agriculture can cultivate soil fertility, using modern technology to update traditional methods and restore productivity to degraded farmland, while sustaining high yields with decreasing energy and input use. Their experiences challenge the wisdom of currently conventional agronomic practices and prove that farming practices that build soil health can reverse trends millennia in the making.

The key to maintaining soil health lies in the world of soil life, in the microbial cycling and recycling of nutrients from mineral and organic matter. Herein lies the good news. For the short lifespan of microbial life means that restoring life and fertility to the soil-and increasing the productivity of marginal farms - is not only possible, but can happen faster than we ever imagined.


David R. Montgomery is a professor of Geomorphology in Department of Earth and Space Sciences, University of Washington, and author of Dirt: The Erosion of Civilizations and Growing a Revolution: Bringing Our Soil Back to Life, and co-author of The Hidden Half of Nature: The Microbial Roots of Life and Health. Connect with him at Dig2Grow.com or on Twitter (@Dig2Grow).

 
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