It just might be that the most important living beings on any farm aren’t found in the barn — but in the soil.
Bacteria, mycorrhiza, fungi, nematodes and even earthworms are key to understanding and promoting plant growth, University of Manitoba soil scientist Mario Tenuta told the Manitoba Forage Seed Association’s annual conference in Winnipeg last week.
“We have to think, not just about growing the plants, but about growing soil and micro-organisms and feeding soil,” he said.
And those organisms and micro-organisms aren’t just important, they’re also numerous.
“The numbers are incredible, in just a millilitre of soil, just a little pinch of soil in your palm, there are more organisms in terms of numbers than all of the humans on earth together, that is how incredibly small they must be, and then we have the fungi and other organisms, and those higher critters too,” Tenuta said. “If we were to take a hectare of soil and grab all the bacteria, fungi, nematodes and weigh them, it would come down to the equivalent weight of 20 head of cattle.”
Perhaps the best-known example of the symbiotic relationship between micro-organisms and plants is the relationship between soybeans — a legume — and the rhizobia bacteria that allows it to fix nitrogen.
“When the bacteria are associated with the (soybean) plant they can fix a lot of nitrogen, soybeans fix 150 kgs of nitrogen per hectare easily… but the same bacteria free, living in the soil, and not in a nodule, contribute less than a kilogram, they need to be tacked into a root,” said the researcher, adding that the plant provides the bacteria with what it needs to survive, so that it can provide nitrogen.
Other plants turn to mycorrhiza for assistance, which changes the root structure of a plant. Crops with heavy mycorrhizal activity will have roots that are both thicker and stubbier.
“They kind of in essence give up exploring and let the fungus explore for them, the fungus becomes an extension of their root system,” Tenuta said.
That fungus then obtains phosphorus for the plant, while also contributing to soil aggregation, which is important for drainage and aeration, he said.
The best example of this might be flax, which depends heavily on mycorrhiza.
“With flax a lot of its phosphorus acquisition is actually through mycorrhiza, regardless of the soil’s phosphorus,” said Tenuta. “Whereas other plants, such as rye and buckwheat, they’re dependent on it, but if the soil test phosphorus is high enough, they’ll give up on the fungus.”
Then there are plants that don’t rely on mycorrhiza, in fact they diminish their numbers.
“Some plants like canola, or anything in the mustard family, doesn’t use mycorrhiza, they acidify the soil with their roots, which produce organic acids to solubilize phosphorus,” Tenuta said.
And that means crop rotation needs to factor in how plants and micro-organisms interact.
“So what can you do to grow soil organisms? Rotate wisely,” said Tenuta, adding you never want to follow canola with flax.
“Why? The flax won’t get enough phosphorus, because the mycorrhiza will have not been fed and promoted in terms of their growth by the canola,” he said.
High biomass crops, reduced tillage and perennials can also help to improve soil health, but don’t expect to see results overnight. It takes about a decade for improvements to show up on soil tests, although visual signs may appear sooner.
Improved soil with higher levels of organic matter will drain better, be darker in colour and crumble well, Tenuta said.