It is estimated that soil
in over 30% of the world’s cropland is acidic and would benefit from liming and
soil improvement. Most soils have a natural tendency to become acidic over time
through natural and managed factors. Farmers too often fail to monitor soil
acidity, despite its widespread nature.
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| Global variation in soil pH. Red = acidic soil. Yellow = neutral soil. Blue = alkaline soil. Black = no data. |
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| Aluminum toxicity stunts bean roots |
Several natural factors
contribute to the development of soil acidity. The geologic material that weathers
into soil has a large influence on soil pH. Acid soils occur more frequently in
high rainfall areas where leaching removes cations such as calcium and
magnesium from the root zone. Poor plant nutrition is frequently a significant
problem in acid soils due to the lack of adequate calcium. Phosphorus
availability also becomes limited as the soil pH drops. Soil acidity also
limits nitrogen fixation in many legume crops. However, aluminum toxicity is
usually the largest constraint to plant growth in acid soils.
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| Calcium uptake near root tips is hindered in acid soils by high aluminu | m |
Nitrogen fertilizer can
also be a contributor to the development of soil acidity. When urea or ammonium-based
fertilizers are converted to nitrate by soil bacteria, hydrogen ions (acidity)
are naturally released. Any nitrogen source containing ammonium (including
manures, composts, or cover crops) will contribute to the gradual process of
acidification.
There are many examples to
show where decades of repeated nitrogen fertilizer use has led to a gradual
decline in soil pH. This gradual soil acidification can occur even in regions
where acidity problems are not common. For example, this natural process is
often noted in areas where nitrogen fertilizer is repeatedly applied to the
same place in the soil for many years, such as surrounding a drip irrigation
emitter in a permanent crop. Fortunately, measuring soil pH is one of the
easiest analyses to perform in the laboratory.
The addition of ground
limestone to agricultural soils neutralizes acidity and reduces the presence of
soluble aluminum, which is toxic to plant roots. Adding limestone to acidic
soil will also enhance the solubility of phosphate, which becomes more
available for plant uptake as the pH approaches neutral. Finally, limestone
will provide a valuable source of calcium, which is frequently lacking in
acidic soils.
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| Adding lime is an old ag practice |
Limestone requires acidity
to rapidly dissolve in soil. In regions where the soil pH is greater than 6.5,
limestone dissolves very slowly or not at all. Areas with naturally occurring
limestone are classified as having calcareous soils. If there is a need to
supply large amounts of supplemental calcium in non-acidic soils, gypsum
(calcium sulfate) is commonly used. Although gypsum does not rapidly dissolve
in soil, it supplies more soluble calcium than limestone in neutral and
alkaline soils.
IPNI recently released a
publication entitled: Soil Acidity Evaluation & Management, which provides an overview of
issues related to acidity. More information can be found at the IPNI website: http://info.ipni.net/IPNI-3353
