Healthy soil for healthy plants
Soil health. It’s an idea that’s getting a lot of attention, but what is it? It’s vague and often misunderstood, yet tech service providers and agribusinesses are investing loads of money and effort into the idea. This is for good reason; healthy soil is key to growing healthy plants.
Southeast Iowa farmer Jeff Olson got hooked on the idea of soil health many years ago. A neighbor was digging out ditches that had filled in with eroded topsoil, and offered to spread that soil onto the ridge tops in one of Olson’s fields. “We had a whale of a corn crop on that ground that year,” Olson says. Since then, he’s been trying to replicate those conditions. “You can’t get it all trucked in, so we have to try to build it in the field.”
It sounds simple enough. But consider that for decades the idea of good soil stewardship has been to minimize loss, with 3 to 5 tons lost per year being “tolerable.” Olson asks, “How can we get back to gains, to building soil, to creating organic matter?” It’s a different can of worms (and fungi, nematodes and bacteria).
Soil biology and soil health
Olson farms near Winfield. His family’s integrated crop and livestock operation produces corn, soybeans, small grains, hay and cattle. He uses cover crops, a diverse rotation and grazing to support soil biology on his farm.
In January, Olson attended a short course on soil health as part of the Practical Farmers of Iowa annual conference. A total of 65 people attended the course, and over 800 attended the PFI conference. Olson says he learned a lot about soil biology, and wants to look more at diversifying his cover crop mixes as a result.
“The soil is alive, teeming with millions of organisms that keep our soils productive, so it’s important that what we do on our farms nurtures and promotes this life,” said Sarah Hargreaves. Hargreaves is a soil microbial ecologist who studied microbes in corn and switchgrass systems during her Ph.D. dissertation work at Iowa State University, and is currently a soil health consultant. She presented at the PFI soils course, along with Ajay Nair, professor of horticulture at ISU, and others.
“In order to understand how soil biology affects our crops, we need to understand a little about the organisms that live there,” said Nair. He explained that soil organisms can be roughly divided into three groups: macro-, meso- and microorganisms — or large, medium and small.
The large ones, macrofauna, are creatures you can easily see, like earthworms, centipedes, and even mice and voles. These organisms do the heavy lifting, mixing residue and shredding it into smaller chunks, so the microbes can more effectively break them down. Olson sees it happen: “We strive to support soil life, and the residues just disappear. I know the biology is working,” he says. Earthworms are creating channels for water infiltration and are also pulling all that residue below the surface, where other soil life can get to it.
Though they are considered medium, the mesofauna are quite small. This group includes nematodes, springtails and mites. Some are pathogens to crops, while others are beneficial, preying on the pathogens and reducing disease pressure. These organisms are important for decomposition and for cycling of nutrients.
“Many nematodes feed on bacteria,” Nair explained to the farmers, “and bacteria are made up of more nitrogen than the nematode needs.” So for every two bacteria the nematodes consume, one serving of nitrogen is excreted in a form readily available to a crop. “It is fertilizer applied right in the root zone,” says Nair. In this way, a healthy soil food web supplies a crop with a consistent nutrient supply. Nitrogen stored in a living bacteria won’t leach away, but there is a steady turnover as they die or are eaten.
The microbes are largely invisible; bacteria and archaea are single-celled. Fungi can be seen; the most recognizable form is a mushroom. In a shovelful of healthy soil, the white root-like structures are often fungal hyphae.
Along with being the food for other organisms, bacteria and fungi are decomposers. Hargreaves says soil microbes function similarly to the microbes in our stomachs. She explains: “Instead of digesting the material inside of their bodies, microbes release enzymes that break down organic material into smaller compounds, such as sugars, nitrogen and phosphorus.” Microbes digest residues into forms available for uptake by plants, meaning microbe activity is extremely important for plant nutrition.
From crop residue to soil
There are two outcomes of decomposition: the release of nutrients and the creation of soil organic matter, or humus.
Soil organic matter is different from organic material. Organic materials are bits of plants, whether on the soil surface or below, which are in the process of decomposing. Humus is material that has decomposed into a stable state, a form that can’t decompose anymore.
Think about humus as the building block of healthy soil. It is the backbone of soil structure; little pathways and tunnels necessary for oxygen, water, root and earthworm movement are held open because the soil particles are held together by organic matter. “It’s somewhere in between ‘If you build it, they will come,’ and the chicken and the egg, ” Hargreaves hedges. “A soil high in organic matter has habitat for a diverse number of microbes, but you need a diversity of organisms to create soil organic matter in the first place.”
This article published in the March, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.
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