Soil pH Affects Nutrient Availability

Abstract

Fertilizers, whether commercial or from manure sources, will not be effective if soil pH isn’t managed. Besides nutrient availability, soil biology and pesticide efficacy can also be pH dependent.
Unfortunately, due to varying soil types and crop needs, there isn’t a single pH that is ideal. While most crops are assigned a suitable pH range (6.0 to 6.5), producers can maximize yields by better understanding soil properties and crop response.

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Fertilizers, whether commercial or from manure
sources, will not be effective if soil pH isn’t
managed. Besides nutrient availability, soil biology
and pesticide efficacy can also be pH dependent.
Unfortunately, due to varying soil types and crop
needs, there isn’t a single pH that is ideal. While
most crops are assigned a suitable pH range (6.0 to
6.5), producers can maximize yields by better
understanding soil properties and crop response.
Acidic Soils have a pH Less than 7
The pH scale (0-14) measures hydrogen (H)
concentration, which causes acidity. A pH of 7 is
considered neutral while anything lower is acidic.
Any pH greater than 7 (alkaline) will have more base
(OH) than H. Pure water is a combination of H and
OH (H2O).
It may seem best to maintain a neutral pH 7, but H
doesn’t reach plant toxicity levels until below pH
4.5. It is elements like aluminum (Al), iron (Fe) and
manganese (Mn) that can cause plant toxicity in
acid soils.
While Acidity is Caused by H, There can be Many
Different Sources
Minerals in the soil (Al and Fe) cause acidity by
splitting water and releasing H (Figure 1). These
minerals are called acidic cations. Calcium (Ca),
magnesium (Mg) and potassium (K) are base cations
because they do not split water, and therefore
don’t create acidity.
Figure 1. Dissolved aluminum can react with water
to create acidity
Ammonium (NH4+) fertilizers (e.g. urea), manure or
compost also add acids to soil. Some products, such
as elemental sulfur (S), are applied to purposefully
lower the soil pH.
Soils can Become More Acidic as Base Cations are
Leached out by Rainfall
Although pure water has a pH 7, natural rainfall is
slightly acidic due to carbon dioxide (CO2) in the
atmosphere. When CO2 dissolves in rainwater, the
pH is approximately 5.6. This should not be
confused with “acid rain,” where sulfur from fossil
fuels creates sulfuric acid, depressing rainfall pH
below 5.
Soil pH in Maryland will range from 5 to 7, similar to
the slightly acidic rainfall the state receives (Figure
2). The opposite can be seen in dry, arid
environments, where less rainfall can lead to
alkaline soils (pH > 7).
Soil pH Affects Nutrient Availability
Fact Sheet FS-1054
July 2016
Al+3 + 3H2O Al(OH)3 + 3H+

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Figure 2. Rainfall in Maryland (orange soil) can
leach out basic cations and leave behind acids (Al
and H). Desert soils (brown) may actually have
more evaporation leading to salt accumulation and
an alkaline pH at the surface
Soil Type and Weathering can Predict the Amount
of Base Cations (Ca, Mg, K, and Na)
Younger soils typically have less acids and more
bases. As these soils are leached and weathered
over time, they will become more acidic (Figure 2).
Most soils in Maryland can be considered highly
weathered, particularly those with greater clay
content. It is a good rule of thumb that soils high in
clay are also high in Al.
Parent materials that contribute bases back to the
soil will maintain a moderate pH, such as those on
the Piedmont (central Maryland). In the Ridge and
Valley province of the Appalachians, soils may form
from carbonate limestone bedrock and maintain a
neutral to alkaline pH. Soils lacking a ready source
of dissolvable nutrients (e.g. Eastern Shore sands)
are likely to be more acidic. In addition, quartz
sands have low cation exchange capacity (CEC). The
CEC is a measure of the capacity of a soil to hold
nutrients, so that quartz sands with a low CEC will
not retain as many bases.
Soil CEC Holds Acids and Bases, but not all Soils
have the Same CEC
Clay soils with higher CEC hold more bases (Ca, Mg,
K, and Na) as well as acids (Al, H). Producers must
manage their soil type properly, understanding that
soil with greater CEC will require more lime to raise
the pH. Soil with greater CEC will also acidify slower.
The Al and H held on the CEC is referred to as
reserve acidity. The amount of acids held on the soil
must also be measured by a testing lab to calculate
the correct lime requirement to reach the target
pH.
Crop Being Grown Determines the Target Soil pH
Most field crops prefer a pH range of 6 to 7, while
some plants thrive in more acidic conditions
(azaleas or blueberries). Crops like potatoes may be
more susceptible to diseases at alkaline soil pH.
With different varieties or hybrids, it is increasingly
difficult to predict variability within a crop. Studies
of soybeans and corn indicate that some varieties
may be more susceptible to Mn and Al toxicity. The
susceptibility of your variety of choice may not be
well known, which requires field observations to
discern any differences.
While crop ranges are good guidelines, they do not
take other important soil characteristics into
account. This includes the toxicity of elements such
Al and Fe, as well as the availability of macro and
micronutrients.
Soil pH Affects the Availability of N and P
Nitrogen (N), from urea fertilizers or mineralized
from organic matter, is in the form of ammonium
(NH4+). In alkaline soils, NH4+ becomes ammonia
(NH3), and can be volatilized (lost as a gas). In acid
soils, the additional H helps maintain NH4+
concentrations, which can adsorb to the CEC.
Uptake of nitrate (NO3-) by plants is best at a lower
pH, while NH4+ is absorbed more efficiently at a
neutral pH. For legumes, a pH < 6 restricts
nodulation on alfalfa, but not as much on red

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clover. The dissolved Al observed in acid soils also
can be toxic to rhizobia and plant roots, limiting
legume production of N.
Denitrification, which transforms NO3- into gaseous
N2 in waterlogged soils, occurs at lower rates in acid
soils (pH < 5).
Optimum P availability is at pH 6.5. Below 6.5, P
becomes insoluble Al/Fe minerals or absorbs to
oxides and clay. Above 6.5, P bonds with Ca to form
solid minerals similar to Ca-phosphate fertilizers.
Relatedly, Ca-phosphate fertilizers added to acid
soils will readily dissolve and release P, but will have
limited solubility in alkaline soils.
Potassium (K), Calcium (Ca) and Magnesium (Mg)
are Indirectly Affected by pH
Potassium, Ca and Mg are less available in acid soils
because they have been leached out, not
necessarily due to solubility issues. Al can also
dominate the CEC, limiting the soils ability to absorb
and hold K. Compared to K, Ca and Mg are more
competitive with Al for CEC sites. In addition, toxic
levels of Mn and Al may damage plants roots,
preventing uptake of Ca, Mg and K.
While alkaline soils are associated with greater
concentrations of Ca, this can be in the form of
precipitated CaCO3 (lime).
Sulfur is Available in Soils as the ion SO4- over a
Wide Range of pH
The ion SO4- form of sulfur is negatively charged and
is retained better by acidic soils. It is important to
remember that when elemental sulfur (S) is added
to soil, it creates sulfuric acid (lowering pH).
However, compounds containing SO4- (gypsum) do
not have the same ability to lower pH.
Table 1. Micronutrients and their availability
related to soil pH
As pH Rises, Micronutrients Bond to the Soil or
Become Insoluble Minerals and Cannot be Taken
up by Plants
All the known micronutrients (Table 1) decrease in
availability as pH rises, except for molybdenum
(Mo). Zinc (Zn), Cu and Mn decrease 100 fold in
concentration with every one unit increase in pH.
These nutrients are not lost, but rather
preferentially sorb to soil surfaces, where they are
not plant available. When concentrations are high
(e.g Fe), they will precipitate as solid minerals.
When severe, deficiencies will cause obvious
symptoms in the field (Figure 3). If a micronutrient
deficiency is observed in an acidic soil, it is probably
related to lower concentrations and the leached
nature of the soil.
Figure 3. A soybean field with a manganese
deficiency due to higher pH
Micronutrient
pH Available
Boron (B)
Acidic
Zinc (Zn)
Acidic
Manganese (Mn)
Acidic
Iron (Fe)
Acidic
Copper (Cu)
Acidic
Molybdenum (Mo)
Alkaline
Chlorine (Cl)
N/A

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Concentration Controls Whether a Micronutrient is
Deficient or Toxic
Sandy soils are typically lower in micronutrient
concentrations, so the pH has to be carefully
managed. For soybean growth, sandy soils with a
pH 5.5 required 4 lbs Mn/acre, but at least 17
lbs/acre was needed when the pH was 7 (Table 2).
This indicates that a greater amount of
micronutrient is necessary in a sandy soil to ensure
availability.
Table 2. Extractable manganese concentration
(lb/acre) necessary to grow soybeans in sandy soil
(Camberato, 2000)
Soil pH
Mn (lb/acre)
5.5
4
6.0
9
6.5
13
7.0
>17
Finer textured clay soils often have greater
micronutrient concentrations and can tolerate
higher pH than sandier soils. As a result, pH
recommendations from the University of Delaware
for sandy soils may be 6.0, while those with finer
textures can be up to 6.5.
Deficiency will be more common for the weathered
soils in Maryland, but toxicity should still be
considered in some cases. Elements like Fe and Mn
are more available in acid soils, and if their
concentration is too high, they can be toxic to
crops. If a producer has over-applied a
micronutrient to acid soil, the excessive
concentration could reduce yields.
Al Toxicity Should be a Concern
When soil pH drops below 5.5, Al becomes soluble
and is toxic to plant roots. All soils have some free
Al, although it is more likely to be higher in
weathered, clay soils (Figure 4).
Therefore, sandy soils low in Al could possibly
tolerate pH lower than 5.5. In addition, soils very
high in organic matter can remove Al from the soil
solution, and can also tolerate lower soil pH for crop
growth. In Maryland, weathered soils with high clay
content are the most likely to experience Al toxicity
when the pH is below 5.5.
Figure 4. The red color
of this soil indicates
oxide coatings and
greater weathering.
Soils like this will have
more Al, but can also
show greater tolerance
to higher pH and
micronutrient
availability.
Understanding Soil Type and Crop Needs is
Essential
There are some important things to keep in mind
regarding the pH of a soil:
• A neutral pH of 7.0 is not needed to
maximize production.
• Reduce Al toxicity by maintaining a pH
greater than 5.5.
• Micronutrients may have a narrow range of
availability versus toxicity. Watch for
deficiencies when the pH is raised.
• Sandy, low CEC soils will leach nutrients
faster than fields with clay soils. Due to
lower concentrations, a high pH will also
cause micronutrients to bond tightly to soil
surfaces. A pH 6.0 or less is best.

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Jarrod O. Miller
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