Magnesium in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 40

If magnesium is present in excessive quantities, the clay tends to be very dense
and hard with a prismatic or blocky structure, and is sticky when wet. This results
in low hydraulic conductivity with water soaking into the soil profile only very
slowly. High magnesium also limits the ability of plant roots to penetrate the clay,
the roots often only travelling down the cracks between the clay prisms, with little
if any movement into the clay prism itself (Figure 13 on p. 105). This obviously
limits nutrient availability to plants.
Magnesium is capable of raising soil pH higher than calcium, and in an acid
soil deficient in magnesium, application of magnesium oxide, magnesium
carbonate or dolomite will help raise soil pH as well as correct a magnesium
deficiency. It can also be applied in conjunction with calcium compounds such as
lime (calcium carbonate), hydrated lime (calcium hydroxide) or quicklime
(calcium oxide) to adjust cation ratios as well as raise soil pH. Magnesium can be
leached reasonably easily from coarse textured acid soils and leaching is increased
by nitrification and soils high in sodium.
Magnesium in plants
Magnesium is the only mineral found in chlorophyll and is actively involved in
photosynthesis. A high proportion of the total magnesium found in plants is
involved in regulating the pH level of plant cells and the anion/cation balance. It
plays a key role in phosphorus transport in the plant and is involved in many
enzyme reactions. If a plant is deficient in magnesium there will be impaired
transport of the products of photosynthesis from the leaves to the roots. This
initially shows up as mottling or yellowing of leaves between veins that remain
green, or, in some species, interveinal chlorosis with reddening bordering the
chlorotic zone. This is followed by growth of distorted young leaves and the plant
appearing water stressed. Plant uptake of magnesium can be reduced by too much
potassium, phosphorus or lime, or high applications of nitrogen fertiliser, leading
to a state of induced magnesium deficiency.
Magnesium in animals
Magnesium plays a vital role in the health of animals, including humans. If
animals graze pasture grown on magnesium deficient soils, they will have low
blood serum levels of magnesium, which can lead to grass tetany
(hypomagnesaemia) that if left untreated will be fatal in a short time from the
onset of symptoms.
A chronic shortage of magnesium or boron can also lead to a number of
conditions related to weak bones. Some other conditions directly related to