Practicas agricolas y calidad de suelos

Agricultural Management Practices And Soil Quality:
Measuring, assessing, and comparing laboratory and field test kit indicators of soil quality attributes.

Publication 452-400 2000
VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY VIRGINIA STATE UNIVERSITY

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Agricultural Management Practices And Soil Quality:
Measuring, assessing, and comparing laboratory and field test kitindicators of soil quality attributes.
Greg Evanylo and Robert McGuinn; Extension Specialist and Former Research Associate, respectively, Crop and Soil Environmental Sciences, Virginia Tech

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Introduction
What makes a healthy soil? Is soil merely a solid medium that holds nutrients for plant growth? Increasing concern for the sustainability of our natural resources has led to the developmentof a more complex concept of soil health. Karlen et al. (1997) proposed the following as vital soil functions: (1) sustaining biological activity, diversity, and productivity; (2) regulating and partitioning water and solute flow; (3) filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including agricultural, industrial and municipal by-products andatmospheric deposition; (4) storing and cycling nutrients and other elements within the Earth’s biosphere; and (5) providing support of socioeconomic structures and protection for archeological treasures associated with human habitation. The term “soil quality” has been coined to describe the combination of chemical, physical, and biological characteristics that enables soils to perform a wide rangeof functions. We have described in this publication: (1) some indicators of soil that can be measured with a simple test kit developed by the United States Department of Agriculture, Natural Resource Conservation Service (USDA-NRCS); (2) directions for interpreting these measurements; (3) the effects of soil amendments on soil quality attributes; and (4) comparisons of field kit and laboratoryresults.

hyphal growth. Improved aggregation, in turn, increases water infiltration and the ease of plant root penetration. Soil biological activity is considered an integral attribute of a healthy soil.

Soil respiration rate
Soil respiration rate [as assessed by carbon dioxide (CO2) evolution] is an indicator of soil biological activity. Soil CO2 evolution results from the decomposition oforganic matter; thus, soil respiration rate is an indicator of the amount of decomposition that is occurring at a given time. Soil respiration can be limited by moisture, temperature, oxygen, soil reaction (i.e., pH), and the availability of decomposable organic substrates. Optimum respiration usually occurs around 60% of water filled pore space. Soil respiration will decrease under saturated or dryconditions. Biological activity doubles for every 18°F rise in temperature until the optimum temperature is reached (varies for different organisms). Activity declines as temperature rises above optimum. The most efficient soil organic matter decomposers are aerobic; thus, soil respiration rates decline as soil oxygen concentration decreases. Oxygen is most limiting in soils that are saturatedwith water. Greater oxygen flow occurs in wellaggregated soils that have many macropores. Addition of organic materials will generally increase soil respiration. Organic matter provides the food or substrate on which heterotrophic soil microbes feed. Organic materials with low carbon to nitrogen (C:N) ratios (e.g. manure, leguminous cover-crops) are easily decomposed; thus, the addition of thesematerials to soil will increase soil respiration. Materials with high C:N ratios (e.g., compost, sawdust) decompose more slowly but provide a more stable, long term supply of organic material than legumes, biosolids, and manures. Soil microbes will compete with plants for nitrogen when soil is amended with products having C:N ratios higher than 25:1. Agricultural chemicals that directly kill or...