by Deborah Jeanne Sergeant
Want healthy soil? Plan for it is Sarah Hirsh’s advice. As an Extension agent with University of Maryland, she presented “Growing Healthy Soil” as a webinar through MidAtlantic Women in Agriculture.
“Soils tends to be an area that especially people not in agriculture or gardening tend to overlook,” Hirsh said.
She noted that few mainstream news stories cover soil health because it’s not exciting as other news. “No one is wondering about the soil beneath them,” she said.
Outside of ag periodicals, a few environmental-oriented articles are the only exception. But she said USDA and other government organizations have always paid attention to the importance of soil health.
“Soil health is not a new idea,” Hirsh said.
Showing USDA handouts from about 90 years ago about “green crops” (now known as cover crops), she pointed out “a lot of these strategies have been around a long time.”
Hirsh shared the USDA-NRCS’s definition of soil health as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans.”
But soil quality, as defined by Doran and Parkin in 1994, is “the capacity of a soil to function within ecosystem and land use boundaries to sustain productivity, maintain environmental quality and promote plant and animal health.”
“Soil health and soil quality are considered synonymous and can be used interchangeably, with one key distinction,” Hirsh said. “Soil quality includes both inherent and dynamic quality.”
That includes the soil’s natural type influenced by long-term factors and processes of soil formation. It generally cannot be influenced with human management. Dynamic quality includes things that over time humans can change.
Hirsh also said dynamic properties include soil density, porosity, water holding capacity and nutrient availability and retention.
“A lot of times we think about the environmental benefits, but as farmers who are trying to make a living, the economic aspects are driving factors,” Hirsh said.
The economic benefits of maintaining and improving soil health include reduced risk of yield loss during periods of environmental stress, like heavy rain, drought, pest or disease outbreak; better field access during wet periods; reduced fuel costs by requiring less tillage; and reduced input costs by decreasing losses and improving use efficiency of fertilizer, pesticide, herbicide and irrigation applications.
Hirsh said soil physical properties don’t change over time, such as changing clay soil to a more favorable type of soil.
She also broke down the volume composition of a shovelful of surface soil in good condition for most plants: mineral (45%), air and water (20-30% each) and organic matter (5%). The size of the mineral particles determines the soil texture.
“The difference between sand, silt and clay is not what they’re made of, but the size of the particles,” Hirsh said. She added that a soil’s organic matter doesn’t affect soil texture.
USDA uses different textural classes of soil depending upon its composition: clay, silt, sand, loam and various combinations thereof. The water-holding capacity, susceptibility to erosion, leaching potential and nutrient-retention capacity varies depending upon soil type.
“You are stuck with your soil texture,” Hirsh said. But farmers can work with their soil to increase its fertility.
“Roots help form soil aggregates – granular structure,” Hirsh said. That’s at the topsoil level.
“Subsoil layers tend to have more clay content and less organic matter,” Hirsh said. They form blocky chunks.
Soil porosity refers to spaces between solids filled with air, water and biota.
“Air provides oxygen required for cell functioning in aerobic organisms, including plant roots and a lot of biota,” Hirsh said. That’s why soil compaction is such a big issue, since it limits air in soil.
“Soil compaction is the opposite of porosity,” Hirsh said. “It’s detrimental to agriculture.”
She said a soil solution composed of water and dissolved nutrients can provide a source of nutrients to plants.
Cation exchange can help soil hold nutrients in reserve. It’s the process whereby cations can move back and forth between soil solution and negatively charged locations on clay-sized particles and humus.
Soil pH ranges from lower than 2 to greater than 10. Hirsh said on the low end of the spectrum are acid sulfate soils, which can be aluminum-toxic. The scale goes up to forest soils, cropland in humid regions, arid region soils and, at the highest end, sodic, degraded soils, which can be sodium-toxic.
“The pH affects how well your crop can access the nutrients in the soil,” Hirsh said.
She listed adverse conditions due to soil acidity, which include calcium and magnesium in inadequate amounts; aluminum, iron, zinc, manganese and copper in excess or even at toxic levels; phosphorus and molybdenum being tightly bound to clays and not available to plants; and bacteria unable to thrive in acidic soils.
Hirsh broke down soil organic matter as 10% biomass, such as living components, decomposition and biopores; 15% residues and byproducts, like crop residues, dead roots, dead critters and exudates; and 75% humus, the end products of residue decomposition, which increases water- and nutrient-holding capacity.
“Earthworms digest some of the nutritious material and egest the remainder as casts,” Hirsh said. “This helps break down organic matter.” The helpful worms also mix up the matter into the soil and alleviate compaction with their soil pores.
Arthropods like spiders, mites and others also break down organic matter as well as mix the soil. Tiny predatory mites may prey on nematodes, springtails, other mites and the larvae of insects.
Bacteria and fungi play a role in soil health. “They produce digestive enzymes that they release into their surroundings,” she said. “Then they absorb the breakdown products.”
She also said bacteria and fungi also release nutrient ions for plant uptake. They’re important for carbon and nutrient cycling.
“Mycorrhizal fungi can exploit water and nutrients in small spaces in the soil that are inaccessible to roots and feed plants,” Hirsh said. “In return, fungi receive energy in the form of sugars from the plant, used for growth and metabolism.”
The tiny little “hairs” on a plant root are actually fungal extensions that reach out into the soil. These form with most vascular plants. Exceptions include spinach, sugar beets, lupins and brassicas.
To maintain healthy biota, Hirsh recommends a sufficient carbon supply and an initial source of food – an organic material – that drives soil food web. Examples include leaves, roots and exudates. Air and water should also be available and minimal soil disturbance like tillage. This could take the form of no-till, strip-till or ridge-till.
With no-till, “there is minimal disturbance of soil or residue, limited to opening and closing the seedbed,” Hirsh said. With strip-till, “tillage is limited to a width of six to eight inches per row and a depth of six to eight inches.” Ridge-till farming means “crops are grown on permanent ridges that are tilled after harvest and rebuilt during the seasons through cultivation between the ridges.”
“There are a lot of ways you can add health to the soil,” Hirsh said. “When we till, that’s burning up organic matter. It’s exposing it to oxygen, which is allowing for more cation exchange.”