Will the coming "brown" revolution be as big as the green revolution? The challenge is that it's out of sight, out of mind. And that’s an expensive attitude. The invisible world of soil microbes beneath your feet convert sunlight, water, CO2 and crop residue into crop income, courtesy of the “most incredible zoo,” says Diana Wall, soil ecologist at Colorado State University.
They are out of our sight, but too vital to ignore.
When you apply phosphorus (P) fertilizer, for example, the P may become chemically unavailable before the plant can absorb it, and soil microbes make it available through the growing season by unlocking chemical bonds.
During floods and droughts, soil microbes are what have built soil highways for efficient transport and storage of soil moisture, gases and nutrients.
“Soil microorganisms decay organic matter and cycle nutrients back into forms that plants can use,” says Jill Clapperton,rhizosphere (root zone) ecologist and president of Rhizoterra, Lolo, Mont.“Tiny soil animals like protozoa, amoebae, nematodes and mites feed on organic matter, fungi, bacteria and each other. Together these activities stabilize soil aggregates, build a better soil habitat and improve soil structure, tilth and productivity.”
Untilled soil has $2,600 worth of free crop nutrients in the top 6 in., says soil microbiologist Kris Nichols with the USDA-ARS Northern Great Plains Research Laboratory, Mandan, N.D.
Soil carbon is the invisible plant and microorganism fuel produced by photosynthesis that feeds the magic microbes that work for us underground. The bottom line, Nichols says, is that working with soil microorganisms improves their fuel efficiency to produce your crop. Just as you control weeds and fertilize soil to optimize field environment aboveground, you can work with soil microorganisms underground to speed their conversion of residue into plant-available forms of N, P and other key nutrients. Your crop changes sunlight to fuel (soil carbon), which builds the machine (plant material and roots) that creates crops.
The valuable link between soil carbon and your crop aresoil microorganisms. They decay organic matter and cycle nutrients back into forms that plants can use, says Clapperton.“Tiny soil animals like protozoa, amoebae, nematodes and mites feed on organic matter, fungi, bacteria and each other. Together they stabilize soil aggregates, building a better soil habitat, improving soil structure, tilth and productivity.”
Soil microbes, fed by carbon, release crop nutrients that are strongly locked to soil calcium, iron or aluminum, depending upon soil pH, Nichols says. Without microbes, vital soil nutrients like P remain present but unavailable to your crop.
Feeding this cycle by extending the growing season (with cover crops and diverse crop rotations) increases the underground fuel (carbon) for your soil microbes, which feed your crop. “By mixing plant species with different rooting depths and architecture we can fill the soil profile with roots, build soil structure (secure the foundation), and feed soil organisms a diverse and varied diet,” Clapperton says.
There are 10,000-50,000 species in just one gram of soil, whose nutrient-cycling services amount to $1.5-20 trillion in benefit annually, making it the world’s most valuable ecosystem, according to Dance and Tillman et al. (Soil organisms alone worth $1.5 trillion.)
These “soil livestock” are more diverse and numerous than tropical rain forest species, according to Eric Triplett, microbiologist at University of Florida, Gainesville.
How else can you improve your soil’s miles per gallon? Keep their roads open.
Root metabolism is handicapped when their underground highways (soil pore space) are blocked and their root networks shattered by tillage. These root networks are created mainly by beneficial mycorrhizal fungi (see photos). The bodies of these fungi are strands of fungal hyphae or thin threads, which increase the surface area contact with soil to gather more nutrients from the soil.
“Tillage is very costly microbially, because it breaks down these aggregates, clogs soil highways and ruptures fungal networks that deliver nutrients to your crops,” Nichols says. “Tillage also exposes valuable organic matter to the air, releasing more soil carbon (microbes’ fuel) to the atmosphere.
“Compare a vase full of marbles to a vase full of sand,” Nichols says. Air and water move easily through the broad spaces between the marbles, but not through what little space exists between fine grains of sand (see photo). “In the same way, a healthy soil made of aggregates (marbles) has more pore space than a compacted soil (the sand).”
Preserving soil structure, or aggregates, preserves broad soil pathways for efficient two-way exchange of vital nutrients, gases and moisture. Tillage and compaction clog underground avenues by disintegrating soil aggregates that choke traffic.”
We know how vital moisture is, and the recent drought drove home soil aggregates’ key role in storing soil moisture more efficiently.
Underground highways, or pore spaces, are also vital storehouses of moisture. Organic matter (OM) evens out soil’s water distribution for future access and reduces ponding. Research by USDA-SCS Soil Scientist Beman Hudson correlates soil’s available water-holding capacity more closely with OM content than any other soil property. As soil OM increases from 1% to 3%, its available water-holding capacity doubles, regardless of soil texture.
Soil OM content has more of a “pronounced effect on water-holding capacity” than previously realized. For example, the available water-holding capacity of a silt loam with 4% OM by weight (about 15% by volume) was more than twice that of a silt loam containing 1%OM by weight in his research. The surface layers of most cultivated soils in the U.S. contain from 1to 6% OM by weight. For every 1% increase in soil OM, water-holding capacity increased by 3.2 times that much, Hudson concludes. “Organic matter’s pronounced effect on soil available water-holding capacity suggests that soil OM’s a key factor in agricultural productivity.”
At today’s fertilizer prices, each 1% of soil OM provides $750/acre of soil nutrients…free, according to Jim Hoorman, Ohio Extension assistant professor, cover crops and water quality.
Your “soil factory” needs as much continuous soil cover as possible, because those living roots provide food for soil microbes. That food is carbon. This complex system banks a lot of carbon in your soil, the key to soil fertility.
Soil pore spaces are also vital pathways for soil carbon, the major feedstock for soil microbes. During the growing season, 10-35% of the total soil carbon is provided by crops fromroot material, microorganism wastes and other soluble products (Juma, 1993).
Each plant species provides different nutrients to different soil organisms because each plant species has a different carbon-to-nitrogen ratio. This is the rationale behind cover-crop cocktails, which diversify the aboveground species to expand underground.
“This interplay of the biological, physical and chemical soil components continues unless altered by tillage,” Nichols says. Tillage reduces your bank of soil carbon and destroys soil highways.
Another benefit of soil cover is reduced soil temperature on hot summer days. In the scorching summer of 2006, uncovered soil near Mandan, N.D. registered 107.4 degrees F, versus 87.6 degrees, Nichols says. “High soil temperatures kill soil microbes and reduce your nutrient supply.”
Gold beneath your feet
Push the zoom button on your soil and you will see a bag full of sticky strings, soil particles, plant leftovers and organic matter, says Kris Nichols, soil microbiologist with USDA-ARS Northern Great Plains Research Laboratory, Mandan, N.D. The strings are crop roots and fungal hyphae, or bodies, coated with glue snagging soil particles into aggregates, clods or pellets. They are the bag.
The finer the roots, the more efficient they are at grabbing nutrients, while requiring less valuable carbon as fuel. They are lean, high-mpg soil workhorses. Piggybacking on them are friendly fungi, arbuscular mycorrhizae, which unlock chemical bonds to release phosphorus (P), sulfur, nitrogen (N) and micronutrients into forms that crops can use. These fungi can take up soil P up to six times faster than the root hairs.
Nutrients made available through biological processes are absorbed and metabolized more easily and efficiently than synthetic fertilizers, says Jill Clapperton, rhizosphere (root zone) ecologist and president of Rhizoterra, Lolo, Mont. “Soil biological processes are responsible for about 75% of the available N and 65% of the available P in the soil. Practices like crop rotation and tillage affect the number, diversity and functioning of the micro- and larger organisms in the soil community, which in turn affect the nutrient content of the crops we grow,” she says.
Magic soil glue
Glomalin is a belowground sticky gum holding onto your most valuable soil resource: carbon. “The more glomalin in a soil, the more efficient and healthier that soil probably is,” says Kris Nichols, soil microbiologist, USDA-ARS Northern Great Plains Research Lab, Mandan, N.D.
“Glomalin’s stickiness creates soil aggregates (or pellets or clods), so vital to soil nutrient exchange and water movement. The pore space between aggregates is key to exchanging, storing and transporting carbon, water and gases. They reduce ponding and run-off after heavy rains, and exchange gases between the soil and the surface to support plant roots’ growth and respiration. Without these underground highways created by soil aggregates, crops require more commercial fertilizer to produce the same yields,” Nichols says.
“Glomalin stabilizes soil clods by coating them with a waxy substance to keep water from flowing rapidly into the soil clod and washing away everything,” Nichols says. “This slows down the rate of water entering an aggregate, keeping it more stable.
“Think of soil aggregates also as a soil carbon vault. The vault slows carbon decomposition, similar to time-release fertilizer pellets. This creates a steady level of N and P release through the growing season where the roots can use them, rather than a more rapid release, where they can be quickly lost to wet spring conditions. Glomalin stores carbon in aggregates where only slow-acting microbes live, storing carbon like a slow-release fertilizer to fuel plants and roots.”
Glomalin’s presence in soil reflects soil health. Nichols correlates glomalin levels with less tillage, more diverse crops and longer-season crops. Diversity above ground correlates with diverse carbon forms below ground, available to feed the millions of microorganisms improving your crops’ efficiency, Nichols says.
This wonder substance, glomalin, is produced in and around plant roots by beneficial fungi, fueled by carbon. These arbuscular mycorrhizal fungi have tiny hair-like threads called hyphae that extend roots’ reach, delivering more soil nutrients to plants in return for carbon that came from decomposed surface residue and photosynthesis.
These finely branched, tiny root threads are the workhorse of nutrient procurement and delivery to your crops. Till your soil and you damage this root network, reducing their productivity. As someone raised on a corn-soybean farm in southwestern Minnesota, Nichols knows this concept can be a hard sell. “Tillage also ruptures soil aggregates, clogging soil avenues for nutrient, water and gas exchange,” she says.
Besides producing glomalin, these same arbuscular mycorrhizal fungi work with beneficial soil bacteria to release chemical bonds that prevent soil P from being available to your crops. By breaking these chemical bonds, the AMF literally free more P and other nutrients.
Estimating Fertilizer Value of Soil Organic Matter
- 2,000,000 lbs. soil in top 6 in.
- 1% organic matter = 20,000 lbs.
- Nitrogen: 1,000 lbs. x 60¢/lb. N = $600
- Phosphorus: 100 lbs. x 50¢/lb. P = $50
- Potassium: 100lbs. x 50¢/lb. K = $50
- Sulfur: 100 lbs. x 50¢/lb. S = $50
- Carbon: 10,000 lbs. or 5 ton x $0/ton = $ 0
Value of 1% SOM Nutrients/Acre = $750