Soil Quality: More than a Soil Test

Productive soil builds the foundation for any successful cropland. The higher soil quality, the better it performs. Whether producing organic matter, cycling nutrients, filtering water or other critical functions, soil quality is a key ingredient in the Core 4 Conservation recipe for enhancing soil and water resources, improving farm profitability and working toward a brighter future.

“Soil quality is the capacity of a specific soil to function,” said Mike Hubbs, agronomist with NRCS Soil Quality Institute. “Soil quality is not the end, but a means to an end. By improving soil quality, we improve soil function, like soil productivity, and generate higher yields.”

So, assessing, maintaining and improving soil quality are essential considerations when selecting crop types, inputs and management options.

SOIL QUALITY ASSESSMENT

“Determining soil quality requires more than a soil test,” Hubbs said.

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 quality 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 will improve soil function.

SOIL BIOLOGICAL CHARACTERISTICS

Respiration

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 quality.

When oxygen is added to the plow depth, as in conventional tillage methods, biological activity increases temporarily and microorganisms rapidly decompose organic matter. 


Mike Hubbs (left), agronomist with NRCS, collecting core sample for bulk density while waiting on respiration test.

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 quality.

“Crop rotations, cover crops, no-till or conservation tillage can improve soil respiration,” said Hubbs.

Earthworms

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.”
Tennessee NRCS soil scientists and Dr. Don Tyler (second from right), University of Tennessee Experiment Station, collect earthworms at the Milan Experiment Station, Milan, Tenn.

Conventional tillage usually eliminates 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. 

(Not all soils support earthworms. In some cases, earthworms were not introduced to the soil, or environmental conditions do not support earthworm populations.)

SOIL PHYSICAL CHARACTERISTICS

Structural Stability

Aggregate stability quantitatively measures soil vulnerability to destructive forces, such as water or wind. Like respiration, soil stability is correlated with organic matter levels.

Mike Hubbs collects core sample for bulk density while waiting on respiration test.

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 Rate

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.

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.”

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.

pH

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.”

Nitrate

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.

Following nutrient management protocols or using cover crops, such as rye or wheat, can help reduce nitrate loss from soil and protect the environment, Hubbs said.

Understanding soil quality is the first step to making management decisions that maximize soil productivity, Hubbs said. Conservation practices, including conservation tillage, buffers, weed and pest management (IPM) and crop nutrient management, can help increase organic matter and infiltration rates, support earthworm populations and maintain ideal soil chemical conditions.

“Improving the soil quality is a critical step to improving and enhancing soil and water quality, generating greater profits and securing a brighter future on the farm,” said Hubbs.

More information on soil quality is available on the NRCS Soil Quality Institute web page

Photo credits: Cathy Seybold, Soil Quality Institute