Minnesota Crop News > 2001-2008 Archives
Value of Gypsum
George Rehm, Extension Soil Scientist
Judging by the number of telephone calls
in recent weeks, use of gypsum for crop production has been
a hot topic of conversation. Because of the variety of questions,
it's appropriate to take time to look at the value of this
material for crop producers.
Gypsum is calcium sulfate. Historical accounts suggest that
it was applied to land in colonial times. When applied to soil,
it dissolves somewhat slowly and separates into the Ca++ and
SO4-- ions. Using chemistry terminology,
this separation is called dissociation.
The chemistry equation is written as: CaSO4 <==> Ca++ +
The calcium (Ca), being positively charges, becomes attracted to clay particles
and is not leached from soils. The sulfate (SO4) is not attracted
to clay particles and remains in the soil water films. It can leach through
soils just like NO3 although not as rapidly.
A Source of Calcium for Plant Growth?
Yes, the calcium from gypsum can be taken up (absorbed) by
plants. Amount of calcium needed for crops, however, is relatively
small. Minnesota soils have high levels of calcium. Therefore,
calcium from an outside source will not be needed to achieve
optimum crop production in Minnesota. Thus, we don't need to
think of gypsum as a fertilizer source of calcium.
A Liming Material?
There has been some suggestion that the application of gypsum
will raise the pH of acid soils. Although gypsum, like lime,
contains calcium, the calcium is not responsible for an increase
in pH where soils are limed. It is true that the use of gypsum
increases pH when applied to acid soils in the southwestern
United States. That increase is due to complex reactions with
soluble aluminum. Soluble aluminum is certainly not a concern
in Minnesota soils. Therefore, we can't justify the application
of gypsum as a liming material.
Will Gypsum Use Reduce Soil pH?
Soybean growers who have problems with iron deficiency chlorosis
in western Minnesota would like to discover a soil amendment
that would lower pH. Some have suggested that the application
of gypsum will do this. Gypsum, however, is a neutral salt.
When it dissolves (dissociates) in soil, there is no change
in soil pH.
Gypsum is used to reclaim saline or saline/alkali soils in
the western United States. With these soils, there is an excessive
amount of sodium (Na) that must be removed. This reclamation
- Application of gypsum at very high rates (hundreds of pounds
- Large amounts of irrigation water
- Drainage to remove this water from the soil system.
The reclamation process can be described as follows:
In this process, the calcium from gypsum replaces the sodium
associated with the soil clay particles. To be successful,
the sodium must be removed from the soil system. This is accomplished
by flushing with high rates (in excess of 12 inches) of irrigation
water and drainage.
In Minnesota, a small number of acres are classified as saline
or saline/alkali. In addition, large amounts of irrigation
water and good drainage are not found where these soils are
present. So, use of gypsum on these soils would have no benefit
The pH of calcareous soils can be reduced temporarily by the
application of high rates of elemental sulfur. When elemental
sulfur is added to soils, it is converted (oxidized) to sulfate
sulfur. This is a biological reaction that produces hydrogen
Elemental Sulfur --->--->sulfate sulfur + H+
To achieve any reduction in pH, high rates of elemental sulfur
are needed and the pH reduction is temporary. Results of research
in Chippewa County show this (see following table).
Effect of application of elemental sulfur on soil pH.
Elemental S applied 5/1969; Chippewa County
||Sulfur Applied (lb./acre)
Results of this trial show that there is no economical way
to reduce soil pH for any length of time.
Gypsum does contain sulfur (approximately 17.5%). But the
sulfur is in the sulfate form and there is no reduction in
soil pH when this material is added to calcareous soils. So,
we don't need gypsum to reduce soil pH.
A Sulfur Fertilizer?
Gypsum has been recognized as a source of sulfur for several
years. Use of gypsum will have a positive effect on crop yield
where sulfur is deficient in soils. In Minnesota, sulfur fertilizers
are needed where crops are grown on soils with a sandy texture
(loams, sandy loams, loamy sands). Use of gypsum and other
sulfur fertilizers will not have a positive effect on yield
when crops are grown on soils that are not sandy.