Soil Indicators and Function
Soil quality indicators help the Bureau of Land Management (BLM) meet its mission of land sustainability and its land management objectives. Soil management practices often are associated with manipulating vegetation (after fire, disturbance, chemical/ physical treatments, seeding, planting, etc.) and can influence soil quality. Soil quality is integrated with all relevant BLM programs and activities.
Measures of Soil Functional State
Scientists use soil quality indicators to evaluate how well soil functions since soil function often cannot be directly measured. A soil quality indicator is a chemical, physical or biological property of soil that is sensitive to disturbance and represents performance of ecosystem function in the soil of interest. Indicators are dynamic soil properties, meaning that they change depending on how the soil is managed. Measuring soil quality is an exercise in identifying soil properties that are responsive to management, affect or correlate with environmental outcomes, and are capable of being precisely measured within certain technical and economic constraints. Soil quality indicators may be qualitative (e.g. drainage is fast) or quantitative (infiltration = 2.5 in/hr).
Soil Function and Soil Quality
Soil quality is the capacity of a soil to function. Healthy soils support plant/animal diversity and productivity, regulate and partition water and solute flow, filter and buffer pollutants, store and cycle nutrients, and support structures and protect archaeological treasures. Soil quality is a result of soil management and is integrated with the Bureau of Land Management’s (BLM’s) programs and activities. BLM uses field guides and health score cards to help assess soil quality.
For soil indicators to be useful in providing information on soil quality and function, they should:
- Correlate well with ecosystem processes
- Integrate soil physical, chemical, and biological properties and processes
- Be accessible to many users
- Be sensitive to management and climate
- Be components of existing databases
- Be interpretable
Examples of Soil Quality Indicators
Types of Indicators
Soil quality is evaluated by measuring dynamic soil properties that serve as indicators of soil function. Soil function is difficult to measure directly and observations may be subjective. Indicators of soil function can be classified into the following interdependent groups:
Biological soil indicators increase awareness of soil as a dynamic and living system by providing information about the organisms that form the soil food web that are responsible for decomposition of organic matter and nutrient cycling. The soil food web is a complex system consisting of earthworms, fungi, bacteria, mites, ants, beetles, and numerous other organisms. Information about the numbers of organisms, both individuals and species, that perform similar jobs or niches, can indicate a soil’s ability to function or bounce back after disturbance (resistance and resilience). Measurements of organic matter, earthworm populations, soil respiration, decomposition rates, plant growth, and also the smell of the soil can be used as biological indicators.
Biological Indicator Examples
- Decomposition (measured by cotton strips)
- Earthy smell
- Organic matter (darker soils)
- Soil fauna
- Soil respiration (CO2)
Definition: A measure of biological activity of soil organisms and decomposition of organic matter into inorganic forms available for plant growth.
Importance: As soil organisms consume organic matter and each other, nutrients and energy are exchanged through the food web and are made available to plants.
Measuring/Identifying: Soil respiration is measured by the amount of carbon dioxide given off. Cotton strips measure decomposition. Darker soil indicates more organic matter and better nutrient cycling.
Affects: Soil respiration or the release of carbon dioxide from the soil comes from many sources: aerobic microbial decomposition of soil organic matter to obtain energy for their growth and functioning (microbial respiration), plant root and faunal respiration, and the dissolution of carbonates in soil solution. Reduced soil respiration indicates reduced organic matter or aerobic microbial activity in the soil. Soil properties such as soil temperature, moisture, aeration, and available nitrogen can limit biological activity and decomposition, and adversely affects root respiration, plants and soil organisms.
Management: Increase organic matter by managing grazing. Prevent erosion. Limit soil compaction by grazing and vehicles. Manage pesticide application.
The chemical component of the soil has an effect on the available plant nutrients, soil and plant relationships, soil’s buffering capacity, water quality, and mobility of soil nutrients and contaminants. Chemical indicators provide information about the equilibrium between soil solution (soil water and nutrients) and exchange sites (clay particles, organic matter); plant health; the nutritional requirements of plant and soil animal communities; and levels of soil contaminants and their availability for uptake by animals and plants. Cation-exchange capacity, soil pH, electrical conductivity, and nitrate-nitrogen are among the chemical indicators that help determine the capacity of the soil to store and release nutrients, and can also be indicators of potential soil, plant, and groundwater toxicity.
Chemical Indicator Examples
- Depth to calcium carbonate
- Nitrate-nitrogen levels (measured using test strips)
- pH (measured by pH meter)
- Salt content (measured by electrical conductivity meter)
Electrical Conductivity (EC)
Definition: Soil electrical conductivity, known as EC, is the ability of soil to conduct electrical current. EC is expressed in milliSiemens per meter (mS/m). Traditionally, soil scientists used EC to measure soil salinity. However, EC measurements also have the potential for estimating variation in some of the soil physical properties in a field where soil salinity is not a problem.
Importance: Soil salinity can influence biological, chemical, and physical properties and processes in soils.
Measuring/Identifying: A pocket electrical conductivity meter measures salts in the soil solution.
Affects: Excessive soil salinity can deter plant growth or contaminate water supplies.
Management: Improper water management is a main cause of salinization; however, some soils can be naturally saline because of factors such as gypsum and sulfate concentrations.
Definition: Soil nitrates are good measures of plant-available nitrogen, but they can be readily lost from the soil by leaching and volatilization.
Importance: Microorganisms and plant roots use nitrate as a food source.
Measuring/Identifying: Nitrate can be measured using nitrate test strips; however, the soil lab test is more accurate.
Affects: Nitrate in the soil that is not used by plants may potentially be leached from the root zone and become a pollutant in the water. Insufficient nitrate levels can impede plant growth.
Management: Nutrient management – managing the amount, source, placement, form, and timing of the application of plant nutrients and soil amendments. A high nitrate concentration on BLM lands is often a result of fertilization of surrounding farms and croplands.
Definition: Soil pH is the degree of soil acidity or alkalinity, defined as the log10 hydrogen ions (H+) in the soil solution.
Importance: Soil pH influences the solubility of nutrients. It also affects the activity of micro-organisms responsible for breaking down organic matter and most chemical transformations in the soil. Thus, soil pH affects the availability of several plant nutrients. A pH range of 6 to 7 is generally most favorable for plant growth because most plant nutrients are readily available in this range. However, some plants have soil pH requirements above or below this range.
Measuring/Identifying: A variety of kits and devices are available to determine the pH in the field, including dyes, paper strips, and glass electrodes. Temperature changes the chemical activity, so most measurements of pH include a temperature correction to a standard temperature of 25°C (77°F). The soil pH generally is recorded as a range in values for the soil depth selected.
Affects: Soil pH affects many micro-organisms. The type and population densities change with pH. A pH of 6.6 to 7.3 is favorable for microbial activities that contribute to the availability of nitrogen, sulfur, and phosphorus in soils, which are needed for plant growth. Many heavy metals become more water soluble under acid conditions and can move downward with water through the soil, and in some cases move to aquifers, surface streams, or lakes. Additionally, high metal concentrations are toxic to some plants.
Management: Adding finely ground limestone to the soil is often used to increase the pH (i.e., decrease acidity). Depending on the cause of high pH, acids (for soils with high calcium carbonate concentrations), gypsum or calcium (for soils with high sodium content), anhydrous ammonia, or sulfur may be added to reduce pH (i.e., increase acidity).
- The pH scale ranges from 0 to 14.
- A pH of 7 is considered neutral.
- If pH values are greater than 7, the solution is considered basic or alkaline.
- If pH values are below 7, the solution is acidic.
- pH scale is in logarithmic units, a change of just a few pH units can induce significant changes in the chemical environment and sensitive biological processes.
- For example, a soil with pH 5 is 10 or 100 times more acidic than a soil with pH 6 or 7, respectively.
Physical characteristics of the soil refer to the arrangement of solid particles and pores. Physical indicators provide information about soil hydrologic characteristics, such as water entry and retention that influences availability to plants. Some indicators are related to nutrient availability by their influence on rooting volume and aeration status. Other measures tell us about erosional status. Physical indicators such as texture, infiltration rate, bulk density, soil compaction, aggregate stability, and soil crusting can help determine how well water and roots are able to move through the soil and how stable the soil resource is to the effects of climate. Top soil thickness, soil color, subsoil exposure, sediment fans, and soil structure are visual examples of physical indicators.
Physical Indicator Examples
- Aggregate stability
- Bulk Density
- Infiltration Rate
- Soil Compaction
- Water (available capacity)
Definition: Ability of aggregates (groups of soil particles that are bound to each other more strongly than to adjacent particles) to resist degradation.
Importance: Soils with stable aggregates at the surface are more resistant to water erosion than other soils because soil particles are less likely to be detached and the rate of water infiltration tends to be higher on well aggregated soils. Soil aggregates protect organic matter within their structure from microbial attack. Formation and preservation of aggregates allows organic matter to be preserved in the soil.
Measuring/Identifying: Measure using the soil stability kit. Soil samples are removed from the top 1/4 to 1/2 inch of the soil (for Rangelands), placed in water, and sieved. Granular structure indicates a stronger aggregate stability than massive structure. Slaking (breakdown of large, air-dry soil aggregates (> 2–5 mm) into smaller sized microaggregates (< 0.25 mm) when they are suddenly immersed in water) indicates the stability of soil aggregates.
Affects: Expansion and contraction of clay particles as they become moist and then dry can shift and crack the soil mass and create or break apart aggregates. Calcium in the soil generally promotes aggregation, whereas sodium promotes dispersion.
Management: Maintain the optimum amount of live vegetation and litter to maintain the content of organic matter and soil structure and control erosion. Decrease number and size of bare areas and minimize soil surface disturbances. Management activities that disturb soil and leave it bare can result in a rapid decline in soil organic matter, biological activity, and aggregate stability.
For more information on aggregate stability see U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Rangeland Information Sheet Rangeland Soil Quality — Aggregate Stability.
Soil samples before being placed in water
Soil samples in water — Healthy soil holds together in water (soil at left), while poor soil falls apart (soil at right)
Definition: Indicator of soil compaction - moist or wet soil aggregates are pressed together and the pore space between them is reduced.
Importance: Increased soil density and the decreased pore space limit water infiltration, percolation, and storage; plant growth; and nutrient cycling.
Measuring/Identifying: Bulk density is calculated as the dry weight of soil divided by its volume. Higher bulk density indicates higher compaction, which reduces infiltration.
Affects: Bulk density is linked to respiration, aeration, infiltration, and root growth. Bulk density reflects the soil’s ability to function for structural support, water and solute movement, and soil aeration. Dry soils are much more resistant to compaction than moist or wet soils. Sandy loams, loams, and sandy clay loams are more easily compacted than other soils. Gravelly soils are less susceptible to compaction than non-gravelly soils. Increased vegetation and soil organic matter decrease compaction potential.
Management: Minimize grazing, recreational use, and vehicular traffic when the soils are wet. Use only designated trails or road and/or reduce the number of trips. Maintain or increase soil organic matter by improving plant cover and plant production.
|Soil Texture||Ideal bulk densities for |
plant growth (g/cm3)
|Bulk densities that restrict |
root growth (g/cm3)
|Sandy||< 1.60||> 1.80|
|Silty||< 1.40||> 1.65|
|Clayey||< 1.10||> 1.47|
Definition: Process of water soaking into the soil.
Importance: Water in soil is replenished by infiltration. Rate of infiltration is greater in dry soils and declines as water temperature approaches freezing.
Measuring/Identifying: Increased soil density indicates decreased filtration. Water infiltration can be easily measured by a coffee can infiltrometer.
Affects: Decreased infiltration increases runoff.
Management: Anything that decreases runoff will likely increase infiltration. Increase plant cover and organic soil matter. Avoid intensive grazing and use of machinery on wet soil. Management that reduces soil cover, disrupts continuous poor space, compacts soil, or reduces soil organic matter negatively impacts infiltration. Solutions for maintaining or improving infiltration include practices that increase soil organic matter and aggregation, and reduce soil disturbance and compaction.
For more information on infiltration see U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Rangeland Information Sheet Rangeland Soil Quality — Infiltration.