Considering Soil Health

About 250 different soils have been identified in Mississippi. Soils differ across landscapes because each soil is a site-specific product of five fundamental factors – parent material, local biology, topography, climate, and time. Biological, chemical, and physical processes are continuous. Management such as tillage, land-forming, clearing, or drainage influence these changes.

These soils are the foundation for agricultural production. They provide plants with mineral nutrition, water storage, and physical support for growth. Soils sustain productivity, maintain quality of air and water, and provide habitat for many organisms. The capacity of soil to perform these functions is called ‘soil quality’. Soil degradation is an issue as we face feeding and clothing the coming generations.

‘Soil health’ has supplanted ‘soil quality’ in public discussions in recent years. This has led to some confusion. If health is a snapshot of where humans are on the path to a quality of life, soil health assesses where we may be on the road to soil quality. However, the terms are often used interchangeably.

Because soils are diverse, the capacity of a particular soil to function (i.e. quality) varies. Soils respond to management but the degree will vary because of the same site-specific factors which formed the soil – parent material, topography, climate, biology, and time, and the chemical, biological, and physical processes that follow.

Many soil properties associated with soil health are centered around organic carbon in the soil. Increasing it impacts the soil in many ways (adapted from Lal, 2011):

• increased soil aggregation and aggregate stability,
• improved total and macro-porosity,
• decreased losses of soil water through increased water infiltration rate and reduced evaporation,
• improved plant available water capacity,
• reduced susceptibility to crusting, compaction and erosion by water and wind, and decreased non-point source pollution of rivers and lakes,
• increased soil cation and anion exchange capacity,
• increased plant nutrient reserves, both total and available,
• increased microbial biomass C, along with activity and species diversity of soil biota (such as earthworms),
• increased methane oxidation capacity, and moderated rates of nitrification and denitrification,
• reduced leaching losses of soluble plant-nutrients,
• increased soil buffering capacity, and moderation of elemental balance, and
• more efficient use of energy-based inputs.

Managing land and crops to increase soil carbon, and thus soil health is not a simple process, especially in warm, humid climates such as Mississippi. Use of cover crops, rotations, and reduced tillage should increase soil organic matter and thus soil health. However, issues such as herbicide resistance, economics, cover crop termination, insect pressure, and economics require consideration. More information is coming on these aspects.

Economics always entered into the winter crop discussion, in 1938 it was estimated 442,000 acres of winter legumes were planted to provide nitrogen for cotton in the Mississippi Delta. The rapid commercialization of cheap nitrogen fertilizers after World War II led to the discontinuation of planting winter legumes during the 1950’s.

While measuring soil organic matter, and thus soil carbon is straightforward, measuring soil health is not a simple process. Soil health is the intersection in the soil environment of biology, chemistry, and physics. Relevant changes take time. Author experience is that tillage system changes require three to five years to take measurable effect.

Soil chemistry is measured routinely through traditional soil testing, and that needs to be maintained because you need both the starting point and mileage markers along the way. Soil testing should

• accurately determine available nutrient status of soils,
• indicate any deficiency or excess,
• show any fertility issues, and
• permit an economic evaluation of any recommendations.

Nutrient management recommendations from competent laboratories are based on calibration and correlation. Recommendations vary because nutrient replacement philosophies used by laboratories vary. Additionally, different extraction chemistries are used among laboratories, so be careful what is compared.

Physical measurements in soils such as soil compaction or aggregate characterization and stability are be more involved, and may require some specialized training and equipment.

Biological testing is an arena of intense current research activity. Some soil biological activity tests are offered to the public, however consensus is lacking within the soil science community on their utility, consistency, or costs. One very useful, widely available test is the simple soil organic matter test mentioned above. It is provided on all row crop soil samples processed by Mississippi State, and by request on other samples.

Soil health improvement is needed but it will require long-term goal oriented management tailored to the soils on-site.