April 12, 2018
By Robert Fears
Soil health has become a frequent topic of conversation and for good reason—it’s the basic element of the cattle industry. Healthy soil grows abundant forage which keeps cattle producing in good body condition. Unhealthy soils can cause ranchers to file for bankruptcy.
“Soil health is the capacity of a soil to function as a vital, living ecosystem that sustains plants, animals and humans,” says Steven Shafer, chief scientific officer with the Soil Health Institute. “Key words in this definition are vital and living. Soil health is vital to our livelihood and soil is alive with physical, chemical and biological components.”
“Soil has structure, which is the arrangement of primary particles into secondary units called aggregates. Soil aggregates are clumps of soil particles held together by moist clay, organic matter, polysaccharide gums produced by bacteria and fungi and fungal hyphae (strands),” says Dennis Chessman, Southeastern regional soil health team leader, NRCS Soil Health Division.
“Pores between aggregates contain water and air and allow roots to grow. Structure affects water infiltration, water holding capacity, water and air movement, nutrient availability and root growth,” he explains.
An example of poor soil structure is plating, which is horizontal layers of soil particles created by compaction or lack of root growth. Plating prevents downward movement of water, nutrients and roots and reduces soil productivity.
“Soil texture is the percent of sand, silt and clay particles and determines water holding capacity,” Shafer says. “Water is lost to deep percolation below root zones in sandy soils, whereas clay soils hold water too tightly for it to be available to plants. Available water capacity occurs in medium textured soils between levels of field capacity and wilting point.”
Organic matter is an important part of soil chemistry and is derived from the remains of organisms such as plants, animals and their waste products. Benefits of soil organic matter include its major role in aggregate forming and its improvement of water infiltration, water holding capacity and available water at field capacity.
Organic matter mineralizes nitrogen, making it available to plants and accounts for 30% to 90% of the cation exchange capacity (CEC) of soil. CEC is a measure of soil fertility and nutrient retention capacity. In addition, soil organic matter is a major source of plant-available phosphorus and sulfur.
“Metals such as iron, manganese, zinc and copper are chelated by organic matter, keeping them available to plants. Organic matter improves plant root environment and contributes to favorable habitat for soil biology,” Chessman says.
Creatures living in the soil are critical to soil health and include bacteria, fungi, protozoa, nematodes, arthropods and earthworms. They affect soil structure, soil erosion and water availability. Soil biology is important for decomposition, nutrient cycling and plant growth. Soil is home to most of the biodiversity in the world.
“Soil bacteria fall into four functional groups,” says Elaine Ingham, chief scientist at the Rodale Institute and a soil ecology consultant in Corvallis, Ore. “Most are decomposers that convert energy in soil organic matter into forms useful to other organisms. Decomposers are especially important for retaining nutrients, such as nitrogen, in their cells, preventing loss from the rooting zone.”
A second group of bacteria, the mutualists, form partnerships with plants. Most well-known of this group are the nitrogen-fixing bacteria.
The third group is the pathogens that form galls on plants. Some species of the fourth group, called lithotrophs or chemoautotrophs, are important in nitrogen cycling and degradation of pollutants. Bacteria from all four groups perform important services related to water dynamics, nutrient cycling and plant disease suppression.
Then there are protozoa, which are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter and sometimes fungi.
“They play an important role in mineralizing nutrients, making them available for use by plants and other soil organisms,” Ingham says. “Ratio of carbon to nitrogen for protozoa is 10:1 or much more and 3:1 to 10:1 for bacteria. As a result, bacteria eaten by protozoa contain too much nitrogen for the amount they need.”
The protozoa release the excess nitrogen in the form of ammonium. This usually occurs near plant root systems. Bacteria and other organisms rapidly take up most of the ammonium, but some is used by plants.
Another role of protozoa is regulating bacteria populations. When they graze on bacteria, protozoa stimulate growth of the bacterial population and in turn, decomposition rates and soil aggregation. Protozoa are also an important food source for other soil organisms and help depress plant disease by competing with or feeding on pathogens.
Moving up the soil food chain, “Nematodes are non-segmented worms and like protozoa, mineralize or release nutrients in plant-available forms. They help distribute bacteria and fungi through soil and along roots by carrying live and dormant microbes on their surfaces and in their digestive systems,” Ingham adds.
“Many bugs, known as arthropods, make their homes in the soil,” says Andrew Moldenke, a plant pathologist at Oregon State University. “Arthropods get their name from their jointed (artoros) legs (podos).”
Large arthropods frequently seen on the soil surface are shredders that chew up dead plant matter. The most abundant are millipedes, sowbugs, termites, certain mites and roaches. Arthropods that gaze on fungi, and to some extent bacteria, include most springtails, some mites and silverfish. They consume bacteria and fungi off root surfaces releasing plant-available nutrients.”
“Earthworms dramatically alter soil structure, water movement, nutrient dynamics and plant growth,” says Clive Edwards, The Ohio State University. “They stimulate microbial activity, mix and aggregate soil, increase infiltration, improve water-holding capacity, provide channels for root growth and bury and shred plant residue.”
“Grazing lands are managed to improve soil health by minimizing disturbance with appropriate stocking densities and adequate forage rest through pasture deferment. Keep the soil covered,” Chessman advises. “In pastures, soil should never be visible when looking down into plant canopy. Increase plant biodiversity and maintain good root structures in the soil.”
Fears is a freelance writer based in Georgetown, Texas.
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