Download The role of calcium and magnesium in agriculture

Soil
The nature of soil
Soil generally has a loose texture and consists of solid mineral
and organic matter, water and air spaces. The mineral portion of
the soil is formed from parent rocks by the weathering action of
physical, chemical and biological processes.
The organic portion of soil consists of the remains of plant
biomass in various stages of decay. Typical soils support high
populations of bacteria, fungi and animals such as earthworms.
Soils exhibit distinctive layers with increasing depth. The layers
are called horizons. The top layer, typically several cm’s in
thickness, is the A horizon or topsoil. It is the layer of
maximum biological activity and contains most of the soil
organic matter. Metal ions and clay particles in the A-horizon
are subject to considerable leaching.
The B-horizon, or subsoil, receives materials such as organic
matter, salts, or clay particles leached from the topsoil.
The C-horizon is composed of weathered parent rocks from
which the soil originated.
The soil minerals, reflecting the composition of the crustal
rocks, are largely silicates.
The role of calcium and magnesium in
agriculture
For healthy plants, an adequate supply of calcium compounds in
the soil is required. This is because calcium is an essential
constituent of plants. Not only are they a principle factor in
controlling the pH of the soil but also they affect the plants
ability to absorb nutrients through the roots.
Magnesium is essential for plant growth. All green plants
contain chlorophyll, which is essential for photosynthesis, and
chlorophyll contains magnesium.
The greater part of the magnesium in plants is present as Mg 2+
ions, associated with both organic and inorganic anions.
Amongst other functions, these ions are necessary for the
functioning of several plant enzymes.
The pH of the soil influences the concentration of plant nutrients
in the soil solution and hence their availability.
Example: A soil with a pH of about 5 has a higher
concentration of plant nutrients, such as aluminium and
manganese, compared to a soil of pH 7. Tea is a well-known
example of a crop, which thrives, in very acid soils and it
contains far more aluminium than most plants.
Calcium in the soil is mostly in the exchangeable form, with
some reserve as calcium carbonate.
Ca-soil + 2NH4+ (aq) → (NH4)2-soil + Ca2+ (aq)
The soil is made up both of inorganic and organic materials. An
important part consists of clay minerals, which carry a negative
charge; the humus derived from the organic matter is similarly
charged.
These charges are balanced by cations in the soil, mainly the H+,
Ca2+, Mg2+, K+ and Na+ ions.
Rainwater, which contains carbonic acid, H2CO3, leads, as it
percolates through the soil, to the replacement of cations such as
Ca2+ by H+. Fertilizers such as ammonium sulphate (NH4)2SO4,
are also involved in the cation exchange.
The NH4+ ions held in the soil are converted first to NO2- and
then to NO3- as the result of bacterial action.
Hydrogen ions are produced simultaneously to balance these
anions and these replace ammonium ions in the exchange
complex, making the soil more acidic (the more exchangeable
H+ the more acid the soil)
Exchangeable calcium and other ions are removed from the soil
by growing crops.
Except in chalk soils the weathering of mineral fragments in the
soil is generally not enough to replace the calcium lost in these
various ways, and the deficiency has to be made good by the
addition of either calcium hydroxide (slaked lime) or calcium
carbonate (ground limestone)
Like calcium, magnesium is removed from the soil by both rain
and crops. In most medium and heavy soils the weathering of
soil minerals restores the loss; in many sandy soils it may be
necessary to add magnesium carbonate.
As grazing animals obtain their food from grass it is necessary
for the grass to provide all the nutrients that they require. Cattle
and sheep feeding on lush spring pastures are liable to suffer
from hypomagnesaemia (commonly called grass staggers) if the
grass has an inadequate level of magnesium – animal feed is
sometimes supplemented with magnesium carbonate powder.