- Soil pH This is a measure of the soil acidity or alkalinity and is sometimes called
the soil “water” pH. This is because it is a measure of the pH of the soil solution,
which is considered the active pH that affects plant growth. Soil pH is the
foundation of essentially all soil chemistry and nutrient reaction and should be the
first consideration when evaluating a soil test. The total range of the pH scale is
from 0 to 14. Values below the mid-point (pH 7.0) are acidic and those above pH
7.0 are alkaline. A soil pH of 7.0 is considered to be neutral. Most plants perform
best in a soil that is slightly acid to neutral (pH 6.0 to 7.0). Some plants like
blueberries require the soil to be more acid (pH 4.5 to 5.5), and others, like alfalfa
will tolerate a slightly alkaline soil (pH 7.0-7.5).
- The soil pH scale is logarithmic, meaning that each whole number is a factor of
10 larger or smaller than the ones next to it. For example if a soil has a pH of 6.5
and this pH is lowered to pH 5.5, the acid content of that soil is increased 10-fold.
If the pH is lowered further to pH 4.5, the acid content becomes 100 times
greater than at pH 6.5. The logarithmic nature of the pH scale means that small
changes in a soil pH can have large effects on nutrient availability and plant
growth.
- Buffer pH (BpH) This is a value that is generated in the laboratory, it is not an
existing feature of the soil. Laboratories perform this test in order to develop lime
recommendations, and it actually has no other practical value.
In basic terms, the BpH is the resulting sample pH after the laboratory has added
a liming material. In this test, the laboratory adds a chemical mixture called a
buffering solution. This solution functions like extremely fast-acting lime. Each
soil sample receives the same amount of buffering solution; therefore the
resulting pH is different for each sample. To determine a lime recommendation, the laboratory looks at the difference between the original soil pH and the ending pH after the buffering solution has reacted with the soil. If the difference between the two pH measurements is large, it means that the soil pH is easily changed, and a low rate of lime will suffice. If the soil pH changes only a little after the buffering solution has reacted, it means that the soil pH is difficult to change and a larger lime addition is needed to reach the desired pH for the crop.
- The reasons that a soil may require differing amounts of lime to change the soil
pH relates to the soil CEC and the “reserve” acidity that is contained by the soil.
Soil acidity is controlled by the amount of hydrogen (H+
) and aluminum (Al+++) that is either contained in, or generated by the soil and soil components. Soils with a high CEC have a greater capacity to contain or generate these sources of acidity. Therefore, at a given soil pH, a soil with a higher CEC (thus a lower
buffer pH) will normally require more lime to reach a given target pH than a soil
with a lower CEC.
Soil Colloids
- During physical and chemical weathering processes in which rocks, minerals,
and organic matter decompose to form soil, some extremely small particles are
formed. Colloidal-sized particles are so minuscule that they do not settle out
when in suspension. These particles generally possess a negative charge, which
allows them to attract positively charged ions known as cations. Much like a
magnet, in which opposite poles attract one another, soil colloids attract and
retain many plant nutrients in an exchangeable form. This ability, known as
cation exchange capacity, enables a soil to attract and retain positively charged
nutrients (cations) such as potassium (K+), ammonium (NH4+), hydrogen (H+),
calcium (Ca++), and magnesium (Mg++). Also, because similar charges repel
one another, some of the soluble negatively charged ions (anions), such as
nitrate (NO3-) and sulfate (SO4=), are not bonded to soil colloids and are more
easily leached than their positively charged counterparts. - Organic colloids contribute a relatively large number of negative charges per unit
weight compared with the various types of clay colloids. The magnitude of the
soil’s electrical charge contributed by colloids is an important component of a
soil’s ability to retain cationic nutrients in a form available to plants.
Cation Exchange Capacity
The ability of a soil to retain cations (positively charged ions) in a form that is
available to plants is known as cation exchange capacity (CEC). A soil’s CEC
depends on the amount and kind(s) of colloid(s) present. Although type of clay is
important, in general, the more clay or organic matter present, the higher the
CEC.
- The CEC of a soil might be compared to the size of a fuel tank on a gasoline
engine. The larger the fuel tank, the longer the engine can operate and the more
work it can do before a refill is necessary. For soils, the larger the CEC, the more
nutrients the soil can supply. Although CEC is only one component of soil fertility,
all other factors being equal, the higher the CEC, the higher the potential yield of
that soil before nutrients must be replenished with fertilizers or manures.
When a soil is tested for CEC, the results are expressed in milliequivalents per
100 grams (meq/100 g) of air-dried soil. For practical purposes, the relative
numerical size of the CEC is more important than trying to understand the
technical meaning of the units. In general, soils in the southern United States,
where physical and chemical weathering have been more intense, have lower
CEC’s (1-3 meq/100 g) than soils in the northern United States, where higher
CEC’s are common (15-25 meq/100 g) because weathering has not been as
intense. Soils in warmer climates also tend to have lower organic matter levels,
and thus lower CEC’s than their northern counterparts.
- Soils high in clay content, and especially those high in organic matter, tend to
have higher CEC’s than those low in clay and organic matter. The CEC of soils in
Maryland generally ranges from 1-2 meq/100 g for coarse-textured Coastal Plain
soils to as high as 12-15 meq/100 g for certain Piedmont and Mountain soils. The
CEC of most medium-textured soils of the Piedmont region ranges about 8-12
meq/100 g.
- There are many practical differences between soils having widely different CEC’s. It has already been mentioned that the inherent fertility (exchangeable
nutrient content) of soils varies in direct relationship to the magnitude of the CEC.
Another important CEC-related property is soil buffering capacity, that is, the
resistance of a soil to changes in pH. The higher the CEC, the more resistance
soil has to changes in pH. The CEC and buffering capacity are directly related to
the amount of liming material required to produce a desired change in pH. Higher
CEC soils require more lime than those with low CEC’s to achieve the same pH
change.
- If CEC is analogous to the fuel tank on an engine, soil pH is analogous to the fuel
gauge. The gauges on both a large and a small tank might read three fourths full;
but, obviously, the larger tank will contain more fuel than the smaller tank. If a soil
test indicates that two soils, one with a low CEC and the other with a high CEC,
have the same low pH, indicating that they both need lime, the one with the
higher CEC will require more liming material to bring about the desired pH
change than will the one with the lower CEC. The reason for this difference is
that there will be more exchangeable acidity (hydrogen and aluminum) to
neutralize in the high CEC soil than in the lower CEC soil. Thus, a soil high in
clay or organic matter will require more liming material to reduce soil acidity (and
raise the pH) than a low organic matter sandy soil will.
