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Soil pH Affects Nutrient Availability



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
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
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
pH Available
Boron (B)
Zinc (Zn)
Manganese (Mn)
Iron (Fe)
Copper (Cu)
Molybdenum (Mo)
Chlorine (Cl)
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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
Table 2. Extractable manganese concentration
(lb/acre) necessary to grow soybeans in sandy soil
(Camberato, 2000)
Soil pH
Mn (lb/acre)
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
Understanding Soil Type and Crop Needs is
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.
For more information on this and other topics visit the University of Maryland Extension website at
Adams, F. 1984. “Crop response to lime in the
southeastern United States,” Soil Acidity and
Liming. Dinauer, R.C. (Ed). ASA-CSSA-SSSA. Madison,
Brady, N.C., R.R. Weil. 1999. Elements of the Nature
and Properties of Soils. Abrid 12 ed. Prentice-Hall.
Upper Saddle River, NJ.
Camberato, J.J. 2000. Manganese Deficiency and
Fertilization of Soybeans. Clemson Extension.
Foy, C.D. 1984. “Physiological effects of hydrogen,
aluminum, and manganese toxicities in acid soil,”
Soil Acidity and Liming. Dinauer, R.C. (Ed). ASA-
CSSA-SSSA. Madison, WI.
Gascho, G.J. and M.B. Parker. 2001. “Long-term
liming effects on coastal plain soils and crops,
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Havlin, J, J.D. Beaton, S.L Tisdale, W.L. Nelson. 1999.
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Saddle River, NJ.
Jones, J.B. 2012. Plant Nutrition and Soil Fertility
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Lanyon, L.E., B. Naghshineh-Pour, and E.O. Mclean.
1977. “Effects on pH level on yields and
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Lathwell, D.J. and S.W. Reid. 1984. “Crop response
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Jarrod O. Miller
This publication, Soil pH Affects Nutrient Availability FS-1054, is a series of publications of the University of Maryland Extension. The
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Thoroughly updated and now in full color, the 15th edition of this market leading text brings the exciting field of soils to life. Explore this new edition to find: A comprehensive approach to soils with a focus on six major ecological roles of soil including growth of plants, climate change, recycling function, biodiversity, water, and soil properties and behavior. New full-color illustrations and the use of color throughout the text highlights the new and refined figures and illustrations to help make the study of soils more efficient, engaging, and relevant. Updated with the latest advances, concepts, and applications including hundreds of key references. New coverage of cutting edge soil science. Examples include coverage of the pedosphere concept, new insights into humus and soil carbon accumulation, subaqueous soils, soil effects on human health, principles and practice of organic farming, urban and human engineered soils, new understandings of the nitrogen cycle, water-saving irrigation techniques, hydraulic redistribution, soil food-web ecology, disease suppressive soils, soil microbial genomics, soil interactions with global climate change, digital soil maps, and many others Applications boxes and case study vignettes bring important soils topics to life. Examples include “Subaqueous Soils—Underwater Pedogenesis,” “Practical Applications of Unsaturated Water Flow in Contrasting Layers,” “Soil Microbiology in the Molecular Age,” and "Where have All the Humics Gone?” Calculations and practical numerical problems boxes help students explore and understand detailed calculations and practical numerical problems. Examples include “Calculating Lime Needs Based on pH Buffering,” “Leaching Requirement for Saline Soils,” "Toward a Global Soil Information System,” “Calculation of Nitrogen Mineralization,” and “Calculation of Percent Pore Space in Soils.”
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It is known that soil acidity can limit crop yield, but additional research is needed to identify more precisely optimum soil pH for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] and the within-field variation in yield response to liming. The objective of this study was to identify optimum soil pH for these crops by studying the variation of soil pH and grain yield response to liming within several Iowa fields. Fourteen 4-yr strip-trials were established in acidic Molisols from 2007 to 2009. The methodology used global positioning systems (GPS), dense soil sampling (0.12-0.18-ha cells), yield monitors, and geographical information systems (GIS). One-time treatments replicated two to five times were an unlimed control and limestone at 6.72 Mg ha-1 effective calcium carbonate equivalent (ECCE), incorporated into the soil in fields managed with tillage. Soil samples (15-cm depth) were collected before liming and annually after crop harvest. The lowest initial soil pH at each site ranged from 4.75 to 5.70. Maximum pH increase was reached 1 to 3 yr after liming. Grain yield response to lime varied greatly. Corn yield responded more frequently than soybean yield but the magnitude of the response did not differ consistently. Liming seldom increased yield with pH > 6.0 in soils having a high subsoil pH (≥ 7.4) and CaCO3 within a 1-m depth but often increased yield up to pH 6.5 with lower pH subsoil. The results provided improved criteria for site-specific soil pH and lime management. © 2015 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved.
Understanding the effect of soil pH on the distribution of micronutrients among soil forms is a necessary precursor to designing micronutrient extractants that extract the form of the metal available to plants. An experiment was undertaken to determine effects of soil pH changes from different liming sources on the distribution of Mn, Cu, Fe, and Zn among soil fractions and to relate the amounts of micronutrients in the fractions to that extracted by the Mehlich I and DTPA extractants. A sandy soil was amended with rates of 0, 0.99, and 1.98 g kg ⁻¹ of three liming materials: calcitic limestone, dolomitic limestone, and an industrial by‐product containing 203 g kg ⁻¹ Mg. Following a greenhouse experiment, the soils were extracted with DTPA and Mehlich I and fractionated into exchangeable, organic, Mn oxide, amorphous Fe oxide, crystalline Fe oxide and residual fractions. The extracts and fractions were analyzed for Mn, Cu, Fe, and Zn, and there were very few differences among lime sources. Increasing lime rates decreased exchangeable Zn and increased organic fraction Zn and Mn. This increase in Zn and Mn in the organic fraction as pH is increased shows that pH does not influence metals in some fractions in the same manner as it does plant availability. Iron decreased in the exchangeable and organic fractions as pH increased. Correlations between DTPA‐ or Mehlich I‐micronutrients and those in the fractions were significant ( P = 0.05) only for the exchangeable and organic fractions. DTPA‐extractable Fe and Zn values were well‐correlated with plant uptake (0.842** and 0.377*, respectively). The results indicate that a micronutrient soil extractant should remove the exchangeable fraction but none of the oxide fractions and for Mn and Zn, none of the organic fraction.
Ground limestone rates of 0, 2.24, and 8.96 metric tons/ha as calcite, dolomite or a mixture of the two were applied to a Kalmia s1 (Typic Hapludult) in 1970 and 1973. Soil pH was 5.7 at the low rate and 6.4 at the high rate in 1977. Corn ( Zea mays L.) yield was significantly lower at pH 6.4 compared to pH 5.7. At pH 6.4 dolomite resulted in lower corn yields than did calcite or a mixture of the two. Yield of corn was negatively related to soil pH and higher soil Mg and positively with plant Mn. Extration of soil samples with the double acid extractant from 5 min to 48 hours showed that there was considerable non‐exchangeable Ca and Mg at the high lime rate (pH 6.4), but little or none at the low rate (pH 5.7). The nonexchangeable Ca and Mg appear to be reaction products from dolomite. The ratio of nonexchangeable Mg/Ca is 4.48 on a meq basis which is unfavorable for plant growth and indicates why continued use of dolomite on sandy Coastal Plain soil with a low CEC results in a yield decrease.
Bulk samples of soils representative of widely differing degrees of weathering (Mollisols, Alfisol, Ultisols, and Oxisols) were collected. Aliquots of each were adjusted to different pH levels, fertilized, and cropped successively to one cutting of pearl millet (Pennisetum glaucum L. Willd.) and three cuttings of alfalfa (Medicago sative L.). As postulated, the trends in crop response to pH differed with crop and with degree of soil weathering. Pearl millet responded very little to increase in pH, except that with very highly weathered soils yields increased significantly up to pH 5.6 to 6.2 followed by a significant decrease at higher pH. Significant increases in alfalfa yields with increased pH were obtained on all soils. However, yields from some of the soils increased prograssively to the highest pH; others leveled off at an intermediate pH, and still others decreased at the highest pH. The latter generally occurred in the most highly weathered soils. Soil and plant analyses as well as visual symptoms suggested that toxic levels of Mn (all soils) and insufficient Ca and high Al (one soil only) were major causes of low alfalfa yields at low pH. Depressed yields of alfalfa in the highly weathered soils at higher pH levels may be related to low levels of available P. The yield response to soil pH vs. degree of weathering relationship was not as clear cut as postulated, but this may not be so much a shortcoming of the hypothesis as it is a consequence of the difficulty in selecting individual soils representative of the highest levels of soil classification, and of the effect of specific parameters important to crop response-soil pH relationships which transcend such soil groupings