Soil Quality Indicators

Soil pH

Soil pH generally refers to the degree of soil acidity or

alkalinity. Chemically, it is defined as the log10 hydrogen

ions (H+) in the soil solution. 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 they are below 7, the solution is acidic. It is important to

recognize that because the 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. Sources of H+ ions in soil solution include

carbonic acid produced when carbon dioxide (CO2) from

decomposing organic matter, root respiration, and the soil

atmosphere is dissolved in the soil water. Other sources of

H+ ions are root release, reaction of aluminum ions (Al+3)

with water, nitrification of ammonium from fertilizers and

organic matter mineralization, reaction of sulfur

compounds, rainwater, and acid rain. Certain soils are

more resistant to a drop or rise in pH (buffering capacity).

Therefore, the lime requirement, which is the quantity of

limestone (CaCO3) required to raise the pH of an acid soil

to a desired pH, must be determined specifically for each

field before amending the soil.

Factors Affecting

Inherent - The natural soil pH reflects the combined

effects of the soil-forming factors. Weathering and

associated chemical reactions release cations (Ca+2, Mg+2,

K+1, Na+1) that remain freely in soil solution, form

complexes with dissolved organic carbon compounds, or

are adsorbed at the surface of clay minerals and humus.

Over time, the leaching of free cations, basic oxides, and

carbonates, as well as the removal of cations by plant

uptake and harvesting, and the continuous formation of

carbonic acid result in soil acidification. When cations are

lost, H+ ions replace them in the soil solution and on the

surface of clay minerals and humus. In very acid soils

(where pH is less than 5), aluminum becomes soluble and

its ions (Al+3) tend to react with water to produce H+ ions,

which further exacerbate acidification. In arid climates,

soil weathering and leaching are less intense, cations

accumulate, and OH- ions offset H+ ions. As a result, the

soil becomes neutral or alkaline. Soils with coarse textures

may acidify easily compared to clay soils, because they

have low organic matter content, a low buffering capacity,

a low cation-exchange capacity (poor cation retention), and

high rates of water percolation and infiltration. Clay and

organic matter in mineral soils act as buffers to resist pH

variations. Soil parent material influences soil properties

including pH as shown by the contrasting pHs of soil

formed in calcareous and granitic materials. The effect of

vegetation on soil pH is due partly to the type of humus

produced as certain types of humus are soil acidifying.

Water erosion removes surface horizons, which can be rich

in organic matter, creating a pH gradient along the slope.

Dynamic - The conversion of uncultivated land into

cultivated soils can result in drastic pH changes after a few

years. These changes are caused by the removal of cations

by crops, the acceleration of leaching, the effect of

fertilizers and amendments, and the variations in organic

matter content and soil buffering capacity. Inorganic

amendments (lime and gypsum) and organic amendments

rich in cations increase soil pH. Ammonium from organic

matter mineralization (nitrification), ammonium-based

fertilizers, and sulfur compounds lower the pH. High rates

of water percolation and infiltration increases leaching of

cations and accelerate soil acidification.

Relationship to Soil Function

Soil pH affects the soil’s physical, chemical, and biological

properties and processes, as well as plant growth. The

nutrition, growth, and yields of most crops decrease where

pH is low and increase as pH rises to an optimum level

USDA Natural Resources Conservation Service

Indicator

C

Test

F

Function

F/N

Helping People Help the Land...

Phosphorus deficiency in corn. Source: R.L. Croissant, Bugwood.org
(see table 1). Many crops grow best if pH is close to

neutral (pH 6 to 7.5) although a few crops prefer acid or

alkaline soils. In acid soils, calcium and magnesium,

nitrate-nitrogen, phosphorus, boron, and molybdenum are

deficient, whereas aluminum and manganese are abundant,

sometimes at levels toxic to some plants. Phosphorus, iron,

copper, zinc, and boron are frequently deficient in very

alkaline soils. Bacterial populations and activity decline at

low pH levels, whereas fungi adapt to a large range of pH

(acidic and alkaline). Most microorganisms have an

optimum pH range for survival and function (see table 2).

At very acid or alkaline pH levels, organic matter

mineralization is slowed down or stopped because of poor

microbial activity linked to bacteria. Nitrification and

nitrogen fixation are also inhibited by low pH. The

mobility and degradation of herbicides and insecticides,

and the solubility of heavy metals are pH dependent. The

effects of soil pH on cation availability influence aggregate

stability since multivalent cations, such as calcium ions,

act as bridges between organic colloids and clays. Some

diseases thrive when the soil is alkaline or acidic. Take-all,

which is caused by the fungus Gaeumannomyces graminis,

is favored by alkaline pH and infects wheat, barley, rye,

and several grasses.

Problems with Poor Soil pH Levels

Deficiencies of many nutrients, decline of microbial

activity and crop yield, and deterioration of environmental

conditions are associated with pH levels as discussed in the

previous section.

Improving Soil pH

Liming, addition of organic residues rich in basic cations,

and crop rotation to interrupt the acidifying effect of

leguminous crops increase soil pH. Applying ammonium

based fertilizers, urea, sulfur/ferrous sulfate, irrigating with

acidifying fertilizers, or using acidifying residues (acid

moss, pine needles, sawdust) decrease soil pH. Increasing

organic matter increases buffering capacity.

Measuring Soil pH

Soil pH is measured in the field using a portable pH pocket

meter as described in the Soil Quality Test Kit Guide,

section I, chapter 6, page 15. See also section II, chapter 5,

pages 63 to 66 for interpretation of results.

Specialized equipment, shortcuts, tips:

Always calibrate the pH meter with the appropriate buffer

solution before using it and report the soil to water ratio

used to take the reading. Wait 10 to 15 minutes after

measuring electrical conductivity to allow soil particles to

settle.

Time needed: 10 minutes

References:

Brady CN and RR Weil. 2002. The Nature and Properties of

Soils, 13th Edition. Prentice-Hall.

Karlen D, Andrews S, Wienhold B, and Zobeck T. 2008. Soil

quality assessment: Past, present and future. Journal of

Integrated Biosciences 6(1):3-14.

Smith JL and JW Doran. 1996. Measurement and use of pH and

electrical conductivity for soil quality analysis. In Methods for

assessing soil quality. Soil Science Society of America Special

Publication 49: 169-182.

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Table 1. Relative yield of selected crops grown in a corn, small

grain, legumes or timothy rotation at different pH levels.

(adapted from Smith and Doran 1996)

Crop

pH

4.7 5 5.7 6.8 7.5

Relative Average Yield

Corn 34 73 83 100 85

Wheat 68 78 89 100 99

Oats 77 93 99 98 100

Barley 0 23 80 95 100

Alfalfa 2 9 42 100 100

Soybean 65 79 80 100 93

Timothy 31 47 66 100 95

Table 2: Maximum, minimum, and optimum pH

values for microbial groups. (adapted from Smith

and Doran 1996)

Microorganisms Range Optimum

Bacteria 5 - 9 7

Actinomycetes 6.5 - 9.5 8

Fungi 2 - 7 5

Blue green bacteria 6 - 9 > 7

Protozoa 5 - 8 > 7