“If you build it, they will come.”
– KRIS NICHOLS
Uproot a plant in healthy soil, and you’ll see tiny pellets clinging to the roots.
To most people, that’s just dirt.
But to farmers in the know, those hard little clumps represent whole towns and cities of soil biota that work together night and day churning out valuable nutrients to sustain crops throughout the duration of the growing season.
Called soil aggregates by scientists, the tiny clumps of organic matter and mineral elements are cemented together with glomalin, a sugary substance secreted by plant roots.
A product of plant photosynthesis, glomalin feeds arbuscular mycorrhizal fungi – the tiny thread-like strands that extend the reach and efficiency of plant roots – which in turn cycle it through a fantastically complicated web of predator-prey and symbiotic relationships involving untold billions of organisms.
Microscopic soil critters go to a lot of trouble to build their helpful little cities, so smart farmers shouldn’t go about wrecking them with tillage or starving them by leaving fields fallow for long periods, said Kris Nichols, a USDA soil microbiologist based in Mandan, North Dakota.
That is, unless they like spending a lot of money on fertilizer, harvesting lower yields, and looking out over failed crops during extended droughts and disease outbreaks.
Mechanical tillage shatters the complex linkages that allow aggregates to form, leading to a temporary spike in nutrient availability as residue decomposers feast on the wreckage. But that practice comes with a high price tag attached in the form of reduced organic matter and tonnes of carbon released into the atmosphere.
“When you have aggregates that are stable formed in your soil, you have a better ability to withstand adverse conditions,” said Nichols in a presentation at the Man-Dak Zero-Till Association annual workshop in Brandon last week.
“You have more efficient nutrient cycling, better water infiltration and holding capacity through increased porosity, and better root penetration. You can do this through management.”
Nichols described an experiment that she used to show the effect of farm management practices on soil health.
She used a series of screens to isolate one particular size of aggregates from soil samples collected from nearby fields under different management styles, then placed them in separate dishes and poured water over them.
Only 14 per cent of the aggregate clumps from a conventional tillage system stayed intact, compared to 47 per cent for a no-till field, and 93 per cent from a moderately grazed pasture.
“The only thing that differed with them was management,” she said.
“If you build it, they will come. If you cultivate your soil organisms by giving them food and appropriate habitat, these are the things that you will get: increased soil organic matter, increased fertility, profitability, and better long-term sustainability.”
In well-aggregated and uncompacted soil, the spaces between the “microbial towns” create air pockets which make room for absorbing and retaining water.
Farmers who favour cultivation often claim that they need to use tillage to aerate their soil to allow the spring snowmelt to penetrate.
In fact, said Nichols, by increasing soil compaction with their cultivator shovels and discs, they are making the problem worse, and end up waiting even longer because the water then has to evaporate before they seed.
She noted that a USDA pamphlet from 1952 explained how low-organic matter leads to compaction, which in turn acts as an obstacle to water infiltration, and that soil with good tilth holds 17 per cent more plant-available water.
She added that a 2004 study in Australia found that for every one per cent increase in soil organic matter, an additional 14.4 litres of water could be stored in the top 30 cm of a square metre of soil.
“We don’t always make the connections. In 1952, we knew that the water doesn’t go in if you have compaction on the surface. How do you not have compaction on the surface? You have soil aggregates. How do you have soil aggregates? You have soil organisms that engineer that soil structure,” she said.
Soil organic matter makes up only five per cent of the soil by weight, but accounts for more than 90 per cent of its activity, she added.
Engineering a superior environment for the proliferation of soil flora to thrive and build organic matter in the form of aggregates is easy: keep them fed by always having something growing in the field.
In a typical wheat-fallow system, the leftover stubble from two months of plant growth cannot provide food for billions of organisms over the entire year.
The result is a gradual loss of organic matter. In the U. S. corn belt, this loss amounted to over half the total available in the decades between the time the virgin sod was first put to the plow and the 1960s, when conservation tillage was first introduced.
Cropping every year, using more diversified rotations, and including cool-and warm-season cover crops, boosts the amount of glomalin – bug food – that soil organisms need to form stable soil structures in which to live.
This practice improves soil health, traps carbon in the soil where it belongs, and rewards the farmer with more efficient plant growth on less inputs. An added benefit comes in the form of reduced disease problems because the increased diversity means that the soil organisms are better able to keep each other in check.
“How can you not grow cover crops?” she asked. “More diversity above ground, more diversity below ground. More diversity helps you to bank more carbon and create more food because you’re feeding more organisms.”
Even better, using “cocktails” of more than one species allows soil biological processes to continue even under adverse conditions such as drought by providing root penetration at different depths and tighter shade cover.
At such times, the reduction in soil surface temperatures by up to 20 can mean the difference between life and death for living organisms and ultimately, profit and loss for the farmer, she said. [email protected]