Recently, Mike Hubbs conducted a training session with Adam Daugherty, Greg Brann and eleven other NRCS and Conservation District employees from Middle Tennessee. Adam Daugherty, District Conservationist for Coffee County is conducting a study of soil health in conjunction with a grant from Tennessee Department of Agriculture. The grant will examine soil health indicators with farmers using conservation practices such as no-till, cover crop combinations, crop rotations, nutrient, and pest management in order to quantify the benefits of soil health. He invited Mike Hubbs newly hired by TACD to work with Districts and NRCS to promote soil health.
Mike Hubbs showing NRCS and District employees several tests from the Soil Health Kit to assess soil health, September 16, 2014 in Manchester, Tennessee.
Soil Quality: More than a Soil Test
Productive soil builds the foundation for any successful cropland and grazing land. The higher soil health, the better it performs. Whether producing organic matter, cycling nutrients, filtering water or other critical functions, soil health is a key ingredient in the Conservation recipe for enhancing soil, water, plants, animals, and air resources, improving farm profitability and working toward a brighter future.
“Soil Health: The continued capacity of soil to function as a vital living organism, sustaining and improving plant, animal, and human resources.”
Soil health is not the end, but a means to an end. By improving soil health, we improve soil function, like soil productivity, improve the environment, and generate higher yields.”
So, assessing, maintaining and improving soil health are essential considerations when selecting crop types, inputs and management options.
Soil Health Assessment
Hubbs advises farmers to consider their soil’s physical, chemical and biological characteristics, some of which are affected by management practices. Organic matter content, degree of compaction and infiltration rate are dynamic soil properties and will respond to manipulations near the soil surface. Other inherent soil properties, such as texture, mineralogy and depth to bedrock, are relatively fixed.
The USDA Soil Quality Kit, developed by USDA-ARS, is one on-farm tool farmers can use to measure soil characteristics. The kit, or other commercially available tools, can help determine changes in soil health when different management systems are applied to a given soil type. For an overall assessment of soil quality, one site can be monitored over time to determine long-term impacts of management changes. With either assessment, the goal is to better understand soil properties and which management practices improve soil function.
NRCS worked with ARS in the late 1990s to develop a Soil Quality Kit Guide. Information on the Kit Guide can be found: http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/health/assessment/?cid=nrcs142p2_053873
The Soil Quality Test Kit Guide describes procedures for 12 on-farm tests, an interpretive section for each test, data recording sheets, and a section on how to build your own kit. The NRCS does not build or sell soil quality test kits. Test kits can be purchased from Gempler's or Murray FFA in Murray, Iowa.
The percent of CO2 is read and recorded and put into an equation to calculate the pounds of CO2 per acre per day.
All soil breathes. The rate of respiration, which is determined by measuring carbon dioxide production in the soil, indicates biological activity. High respiration does not always indicate good soil health.
When oxygen is added to the plow depth, as in conventional tillage methods, biological activity increases temporarily and microorganisms rapidly decompose organic matter. Even conservation tillage equipment such as vertical tillage breaks down aggregates and increase decomposition of organic matter.
Soil Biological Characteristics
“Determining soil health requires more than a soil test,” Hubbs said.
This high rate of biological activity in a system of low residue inputs decreases soil organic matter. Less organic matter degrades overall soil quality.
A no-till system with crop rotations and/or cover crops balances decomposition with organic matter inputs from crop roots and residues, providing a more stable system. As a result, organic matter levels are maintained, or even increased, and biological activity is improved. High respiration with high inputs indicates good soil health.
“Crop rotations, cover crops, no-till can improve soil respiration,” said Hubbs. Also excellent grazing systems in perennial vegetation improves soil health.
Earthworm activities in the soil improve water movement, break down residues, distribute residues, improve nutrient availability and enhance soil structure and soil stability. While not essential to high-quality soil, earthworms usually indicate a healthy system with favorable moisture conditions.
“Measuring for earthworms is easy,” said Hubbs. “Dig a one-cubic-foot hole and count.”
Conventional tillage without adding significant organic material severely reduces earthworms in soil. Each initial tillage pass destroys about 25 percent of earthworms, Hubbs said, and other worms leave or die because of higher soil temperatures, destroyed burrows, and reduced food supply. Balancing pH, adding residue, keeping the soil covered, and keeping roots growing as continuous as possible promote soil biology and earthworms.
Soil Physical Characteristics
Aggregate stability quantitatively measures soil vulnerability to destructive forces, such as water or wind. Like respiration, soil stability is correlated with organic matter levels.
Because of its weak structure, overly tilled soil will lose integrity, or fall apart quickly, and crust when exposed to rainfall. Soil with more organic matter and surface residue, however, will remain stable and will not crust.
“The key for improving soil structural stability is to produce plenty of roots and residue and leave it intact,” Hubbs said.
Infiltration is a measure of how fast water enters the soil. Water entering too slowly can lead to increased run-off, erosion, and less available water for plant growth.
Infiltration rate, the rate at which water moves into the soil surface, correlates with organic matter levels, earthworm numbers and soil stability. Good infiltration reduces erosion and helps keep vital topsoil and organic matter in place. In addition, water that infiltrates into soil is less likely to run off fields and carry soil, nutrients and chemicals to nearby water sources.
High-residue no-till systems can have infiltration rates four to eight times greater than conventional tillage systems, Hubbs said. Similar findings have been found with properly managed rotated pastures versus overgrazed continuous grazing. Continuous roots growing and maintain cover are keys to increasing organic matter resulting in good infiltration. Also managing soil compaction is key to good infiltration.
Bulk density is the weight of soil for a given volume. It is used to measure compaction and porosity. In general the greater the density, the less the pore space for water movement, root growth and penetration, and seeding germination.
Staying off fields when wet, reducing traffic and managing traffic will prevent against increased bulk density. Also increasing organic matter will reduce bulk density.
Soil Physical Observations
Digging a hole is sometimes the best assessment of soil health. Measure depth of topsoil can provide evidence of increasing or reducing soil health. After digging a hole, examine structure. Soil structure is how soil particles are grouped together in stable collections or aggregates. Look for roots and probe for compaction. Use metal rod in kit to probe side walls of hole for any compacted zones.
Soil Chemical Characteristics
Electrical Conductivity (EC)
Soil electrical conductivity (EC) assessments measure the soluble salts in soil and can be correlated with soil properties that affect crop productivity. Excess salts, which may occur naturally or as a result of management, inhibit plant growth and disturb the soil-water balance.
“Any process that changes the soil-water balance may affect the movement and accumulation of salts in the soil,” Hubbs said. “And, when excess water on the soil surface evaporates, salts accumulate on the surface.” Although salts are not commonly a problem, it is a useful test especially when using animal manures. Practices to reduce excess salts include irrigation management, crop rotations, manure application, and conservation tillage, all of which improve infiltration and permeability. In addition, deep-rooted crops will remove excess water from seep recharge areas.
Soil pH, the acidity or alkalinity of soil, affects plant nutrient availability, microorganism activity and mineral solubility.
“Managing soil pH according to the needs of the crop will help stimulate plant growth,” Hubbs said. “Factors to consider are nitrogen fertilization, temperature and rainfall.” Balancing a pH to 6.5 will increase earthworm numbers as improving nutrient uptake.
Nitrate is the inorganic form of nitrogen most available to plants. Nitrate levels in soil can be determined by measuring the biological breakdown of organic matter into nutrients (also called mineralization). Soil usually will have higher nitrate levels in the spring or summer. High nitrate levels at the end of the growing season, though, indicate potential nitrate loss through leaching and denitrification. Proper rotational grazing with natural distribution of manure, and rest of pastures will assure better nitrate uptake.
Use of cover crops are important in cycling nutrients. Following nutrient management (Four Rs,: right amount, right method application, right timing, and right source) and using cover crops, such as rye or wheat, can help reduce nitrate loss from soil and protect the environment, Hubbs said.
Understanding soil health is the first step to making management decisions that maximize soil productivity, Hubbs said. Keeping the soil covered, roots growing, reduce soil disturbances, using nutrient and pest management, and adding diversity are all foundational to improving soil health.
“Improving the soil health is a critical step to improving and enhancing soil, air, plant, animal and water quality, generating greater profits and securing a brighter future on the farm,” said Hubbs.