Helping People Help the Land

USDA Natural Resources Conservation Service

Soil Quality Indicators

Physical, Chemical, and Biological Indicators

for Soil Quality Assessment and Management

Indicator

?

Test

?

Function

?

A series of information sheets for physical, chemical, and biological indicators is available to help conservationists 

and soil scientists with soil quality assessment. Use this guide to learn more about selecting appropriate soil quality 

indicators to assess specific soil functions. Visit http://go.usa.gov/zUAH for more information and to download copies of 

the information sheets.

What is soil quality?

Concise definitions for soil quality include “fitness for use” 

and “the capacity of a soil to function.” Combining these, 

soil quality is the ability of a soil to perform the functions 

necessary for its intended use.

Soil functions include:

● sustaining biological Diversity, activity, and 

productivity

● regulating Water and solute flow

● Filtering, buffering, degrading organic and inorganic 

materials

● storing and cycling Nutrients and carbon

● providing physical Stability and support

TIP: The Function icon at the top right corner of 

each information sheet uses D, W, F, N, or S to 

show the function(s) that is most affected by the 

subject indicator.

How is soil quality measured?

The quality of a soil, or its capacity to function, is 

evaluated using inherent and dynamic soil properties. 

These properties serve as indicators of soil function 

because it is difficult to measure function directly and 

observations may be subjective.

Inherent, or use-invariant, soil properties change 

very little or not at all with management. Inherent soil 

properties form over thousands of years and result 

primarily from the soil forming factors: climate, 

topography, parent material, biota, and time. Examples of 

inherent properties are soil texture, type of clay, depth to 

bedrock, and drainage class.

Dynamic, or management dependent, soil properties are 

affected by human management and natural disturbances 

over the human time scale, i.e., decades to centuries. 

Significant changes in dynamic soil properties can occur in 

a single year or growing season. There are many dynamic 

soil properties, several of which are the subjects of this 

information sheet series.

Soil indicators are often divided into Physical, Chemical 

and Biological categories depending on how they affect 

soil function. However, these categories are not always 

clearly defined since a soil property or indicator can affect 

multiple soil functions or categories.

TIP: The Indicator icon at the top right corner of 

each information sheet uses P, C, or B to show 

the category in which the indicator best fits.

Depending on the indicator and the method used 

to evaluate it, properties are assessed in the Field, 

Laboratory, or even an Office when no special equipment 

is required.

TIP: The Test icon at the top right corner of 

each information sheet uses F, L, or O to show 

where indicator assessment takes place for the 

method highlighted on the information sheet.
USDA is an equal opportunity provider and employer. January 2015

Selecting soil quality indicators

A soil function – indicator matrix (fig. 1) can be used to 

select appropriate indicators for assessing a particular soil 

function. Additionally, if an indicator is already being 

measured, the matrix reveals the indicator’s relationship to 

other soil functions, thus maximizing the usefulness of the 

collected data.

Each indicator listed in the matrix below is linked to its 

accompanying information sheet. The information sheets:

● define and describe the indicator

● relate the indicator to soil function

● discuss inherent and dynamic factors influencing it

● suggest management practices to improve soil function

● provide a reference for an assessment method

Figure 1. Soil function – indicator matrix: when a direct relationship exists between the function and indicator, 

increasing reliability and ease of use of the associated assessment method is shown with increasing stars.

Soil Quality Indicator

Soil Function

Sustain 

biological 

diversity, 

activity, and 

productivity

“D”

Regulate and 

partition 

water and 

solute flow

“W”

Filter, buffer, 

degrade, 

detoxify 

organic and 

inorganic 

materials

“F”

Store 

and cycle 

nutrients 

and carbon

“N”

Physical 

stability and 

support for 

plants and 

structures 

associated 

with human 

habitation

“S”

Aggregate Stability a,c,f ——

Available Water Capacity a,g —— ——

Bulk Density a,h ——

Earthworms b,d ——

Infiltration b,e,i —— —— ——

Particulate Organic Matter a,c

Potentially Mineralizable Nitrogen a,c —— —— ——

Reactive Carbon a

Slaking b,e,i,j —— —— ——

Soil Crusts b,d —— —— —— ——

Soil Electrical Conductivity b —— —— —— ——

Soil Enzymes a —— —— ——

Soil Nitrate b —— —— ——

Soil pH b,d ——

Soil Respiration a,b,c ——

Soil Structure and Macropores b,d

Total Organic Carbon a

a

 laboratory/office method

b

 field method

c

 time consuming

d

 simple visual observation

e

 variability requires large sample number

f perhaps the most informative physical

indicator

g

 important for drought prone areas

h important for weight to volume conversions, small sampling

errors result in significant interpretation problems

i

 effective educational method

j

 qualitative