Calcium can help stabilize aggregate structure of
soils. The most commonly used calcium sources include gypsum, agricultural
lime, and a few other calcium salts. In some soils, existing calcium minerals
can be dissolved, releasing the calcium they contain.

Negatively charged soil clay particles can be bound together into clumps
or aggregates by positively charged molecules (cations). The formation of
stable soil aggregates, a process also called flocculation, encourages water
infiltration and drainage and prevents surface soil crusting. Flocculation is
promoted by high levels of salinity (which may not be conducive to plant
growth) and by the presence of cations that are strong flocculators. The
dominant soil cations in medium to high pH soils are the monovalent cations
(one positive charge per molecule) sodium (Na+) and potassium (K+),
and the divalent cations (two charges per molecule) magnesium (Mg2+)
and calcium (Ca2+). In highly acidic soils the trivalent aluminum
cation (Al+3) may be present.

The ability of the dominant soil cations to flocculate
soil clays, a function of their charge and size, is shown in Table 1. In this
table the flocculating power of Na+ is assigned a value of 1, and
the other cations valued relative to Na+. We can see that K+
is a stronger flocculator than Na+, but that Mg2+ and Ca2+
are much more powerful flocculators than either of the monovalent cations.
Calcium is clearly the cation of choice for flocculating soil clays.

Ion

Chemical
Symbol

Relative Flocculating Power

Sodium

Na+

1.0

Potassium

K+

1.7

Magnesium

Mg2+

27.0

Calcium

Ca2+

43.0

Relative flocculating power of major soil
cations.

In soils without adequate soluble Ca2+, increasing the Ca2+ in solution
will help to flocculate clay particles. There are two methods that can be used
to accomplish this. One is to solubilize calcium already present in the soil;
the other is to add a supplemental calcium source.

Let’s look at the first option, solubilizing existing
soil calcium. This strategy works only if there is an excess of calcium
carbonate minerals in the soil. Soils with excess or solid-phase calcium
carbonate (CaCO3) are referred to as calcareous soils. They can be identified through a soil analysis.
Look for free lime on the soil test.
It will usually be reported in general categories such as ‘high’, ‘medium’ or
‘low’. You can test for the presence of carbonates yourself by putting a drop
of dilute acid on them and observing whether or not they effervesce (fizz) as
the CaCO3 reacts with the acid (sulfuric acid in the equation below)
to produce carbon dioxide (CO2) gas:

In calcareous soils, acid can be applied to dissolve
soil calcium carbonate. The products of the reaction of calcium carbonate and
sulfuric acid are CO2, water (H2O), sulfate (SO42-),
and Ca2+. The Ca2+ released from the soil CaCO3
can now act as a flocculant.

Any acid can dissolve soil calcium carbonate and
release the bound calcium. Sulfuric acid is most common because it is
relatively inexpensive and adds less salt to the soil than hydrochloric acid
(HCl). Sulfurous acid (H2SO3) can be produced by
combustion of elemental sulfur in a ‘sulfur burner’, which is a popular
alternative to sulfuric acid. Additionally, acid-forming materials such as
elemental sulfur can be used. Elemental sulfur is converted to sulfuric acid by
sulfur oxidizing bacteria, producing the same effect as sulfuric acid. Sulfur conversion is a biological process,
however, and requires several weeks to several months to take place (depending
on soil conditions), unlike acids which react instantly.

Acids and
acid-forming materials will only be effective in calcareous soils!
The soil should effervesce when acid is applied, or have ‘medium’ to ‘high’ or
‘very high’ free lime soil test levels.

Now let’s look at calcium additives. There are several
calcium bearing salts that can be used to add Ca2+ to soil but in
order to be effective they must be soluble. A salt is a compound made up or
a cation and an anion (a negatively charged molecule). Calcium salts, of
course, contain Ca2+ as their cation. The anion could be sulfate (SO42-)
for calcium sulfates, carbonate (CO32-) calcium
carbonate, chloride (Cl-) for calcium chloride, nitrate (NO3-)
for calcium nitrate, etc.

The most widely used calcium soil additive is gypsum.
Gypsum is one of the family of calcium sulfates. The chemical formula for
gypsum is CaSO4.2H2O. This means that each
gypsum molecule contains one calcium cation, one sulfate anion, and two waters.
There are other calcium sulfates, such as calcium sulfate anhydrite (CaSO4).
Chemically, these two salts are closely related, the difference being that
calcium sulfate anhydrite does not contain water. Consequently, calcium sulfate
anhydrite contains more calcium on a weight basis than gypsum. Calcium sulfate
anhydrite contains 29.4% calcium, whereas gypsum contains 23.2% calcium.

Both of these calcium salts are mined, and then ground
into a powder for use as soil additives. However, by-product gypsum materials,
waste products of phosphate fertilizer production (phosphogypsum) or from power
plant stack scrubbers (flue gas desulfurization gypsum), are also used.

Gypsum is a good choice for calcium addition because
it is inexpensive, non-toxic, and safe to handle, and it is relatively soluble.
We are interested both in solubility (how much of the salt will dissolve in the
soil water) and the rate of dissolution (how fast the salt dissolves in water).
Mined gypsum is well-crystallized, having formed over millions of years. Waste
gypsum, on the other hand, is formed rapidly during industrial processes, and
is less crystallized. Although they generally have the same chemical formula,
the waste gypsum materials dissolve more rapidly than mined gypsum. Sometimes
powdered gypsum is prilled in order to reduce dust and to improve handling
properties, and this slows its rate of dissolution. A study that compared
dissolution rates of gypsum sources found that flue gas gypsum dissolved 3.6
times faster than mined gypsum, whereas phosphogypsum dissolved 2.2 times
faster than mined gypsum. The rate of dissolution is
particularly important for treatment of soil crusting, which is caused by
dispersion of clay particles at the soil surface. In this situation, rapid
dissolution is critical to maintain a high level of dissolved Ca2+
in the surface soil as raindrops or irrigation water leach cations from the
uppermost layer of soil. However, for general treatment of soil structure, the
rate of dissolution is less important than the overall solubility.

Calcium carbonate or limestone is another mined
calcium salt. It’s often referred to as lime or agricultural lime. The main use
of calcium carbonate is to raise soil pH (to reduce acidity). In the same
manner that CaCO3 neutralizes sulfuric acid in the equation above,
it also neutralizes acidity in low pH soils. Unlike gypsum and calcium sulfate
anhydrite, calcium carbonate solubility is dependent on soil pH. Its solubility
increases in acid soils and decreases as soil pH increases. When soil pH is
above approximately 8.2, calcium carbonate becomes quite insoluble. This is why
most soils with a pH above this threshold are also calcareous, meaning that
they contain solid mineral calcium carbonate. In acidic soils, supplemental
calcium carbonate will dissolve, but in alkaline soils it will not; adding
calcium carbonate to calcareous soils accomplishes nothing.

Calcareous soils may benefit from addition of soluble Ca2+, even though
they contain excess calcium in the form of insoluble
calcium carbonate. Below approximately pH 7.0, calcium carbonate is more soluble than gypsum; above this pH
calcium carbonate is less soluble
than gypsum. So in neutral to alkaline soils, gypsum is the calcium additive of
choice. Gypsum is also the favored additive for improving aggregation in acidic
soils also, for reasons that cannot be adequately described in this short
article.

Geologic deposits of gypsum often contain some calcium
carbonate. It is therefore important to look at the composition of agricultural
gypsum. It is not sufficient to know the calcium content of gypsum for this
does not distinguish between calcium combined in sulfate or carbonate salts.
Pure gypsum contains 23.2% Ca2+, and 18.6% S or 55.7% SO42-
by weight, or a weight ratio of Ca2+:SO42- of
about 2.4 to 1 (a comparable ratio for Ca2+:S is 1.25 to 1). If the ratio
is much lower than this, the presence of calcium carbonate is indicated. In
acidic soils, the calcium carbonate ‘impurity’ will slow down the dissolution
of gypsum, whereas in alkaline soils it will primarily reduce the pound for
pound effectiveness of gypsum.

Calcium salts that contain Ca2+ and a
monovalent anion such as Cl- or NO3- are very
highly soluble. They are not usually used as calcium amendments because of
their expense and their high salt content. Applying enough of these salts to
promote soil aggregation would generally increase soil salinity to unacceptable
levels.

Acid or acid-forming amendments are acceptable
additives for increasing soluble Ca2+ in calcareous soils only. In
all soils, regardless of pH, gypsum is a good Ca2+ additive. Calcium
sulfate anhydrite will also supply Ca2+, but it will dissolve more
slowly than gypsum. With either gypsum or calcium sulfate anhydrite it is
important to know the composition of the material.