Under the heading of manures that are used purely for their phosphatic
benefit to the soil are superphosphate and basic slag.
_Superphosphate._--This is the typical phosphatic manure, and is the
base of the numerous artificial manures used on the farm.
Superphosphate is made by dissolving raw phosphatic minerals in
sulphuric acid (oil of vitriol), the tribasic phosphate of lime which
these contain being converted into the so-called "soluble phosphate,"
sulphate of lime being formed at the same time. The first impetus to
the manufacture of superphosphate was given by Liebig, when he
suggested, in 1840, the treatment of bones with oil of vitriol in
order to make them act more quickly in the soil. Lawes subsequently,
in 1843, applied this to mineral phosphates, using phosphorite, first
of all, and the great manufacture of mineral superphosphate then
began. Coprolites, as found in Cambridgeshire, Suffolk, Bedfordshire
and elsewhere were the raw materials at first employed in the United
Kingdom. But gradually the demand for the new manure became so great
that distant parts of the world were searched to bring in the raw
material for conversion into superphosphate. Many new sources of
supply have been worked, and many worked out or abandoned in favour of
better and richer phosphates. Among these were the crystalline
apatites of Canada and Norway, French, Spanish and German (Lahn)
phosphates, and, at a later period, Carolina (land and river),
Florida, Tennessee, Somme, Belgian, Algerian and Tunisian phosphates.
In addition to these came other materials which, in their origin, were
really of the nature of guano, being bird deposits the ammoniacal
matters of which were gradually washed out. The mineral matters
remained and altered the composition of the original rock on which the
guano was deposited, thus forming rich deposits of phosphate of lime.
Such were the phosphates obtained from many of the islands of the West
Indies and South Pacific, and known under such various names as
Sombrero, Curaçao, Aruba, Maiden Island, Megillones, Baker Island,
Fanning Islands, Lacepedes Islands, &c. guanos. Few of these are now
worked, but their place has been largely taken by the rich deposits of
Ocean Island and Christmas Island, which are of similar origin. The
principal supplies of phosphatic minerals at the present time come
from Florida, Algeria, Tunis, Ocean Island and Christmas Island. Other
phosphates imported are Redonda and Alta Vela phosphates, but these
consist mainly of phosphate of alumina, and are not used for
superphosphate manufacture but for phosphorus production.
Coprolites, as formerly used, contained from 50 to 60% of phosphate of
lime, but they are not worked now, the richer sources, which are also
better adapted for superphosphate manufacture, having taken their
place. The amount of oxide of iron and alumina in raw phosphates is of
great importance, as phosphates containing these bodies are liable to
cause superphosphate to "go back" or form what is called "reverted"
phosphate, the percentage of "soluble phosphate" being reduced
thereby. For this reason many of the older supplies have been replaced
by newer and better ones. Florida rock phosphate of high grade
contains 75 to 78% of phosphate of lime, and Florida land pebble
phosphate about 70%. Algerian and Tunisian phosphates have from 55 to
65% of phosphate of lime, and are very free from iron and alumina,
this fitting them especially for superphosphate making. Tennessee
phosphate has about 70% of phosphate, Somme and Belgian phosphates 40
to 50%, while Ocean Island and Christmas Island phosphates are of very
high grade and yield over 80 and up to 86% of phosphate of lime.
Superphosphate is made by finely grinding the raw phosphate and mixing
it with oil of vitriol (chamber acid); what actual product is formed
is a matter of some uncertainty, but it is a phosphate soluble in
water, and believed to be mono-calcic phosphate. This is the true
"soluble phosphate," but in commercial transactions it is universal to
express the amount in terms of the original tribasic phosphate which
has been rendered soluble. Ordinary grades of mineral superphosphate
give from 25 to 27% of soluble phosphate and higher grades 30 to 35%.
On reaching the soil, the soluble phosphate becomes precipitated by
the calcium and iron compounds in the soil. But it is precipitated in
a very fine form of division, in which it is readily attacked by the
plant roots. Superphosphate is used practically for all crops,
including cereals, clover and other leguminous crops. Its use tends to
early maturity in a crop. Its value for giving a start to root crops
is particularly recognized, and root crops generally are dependent on
it, as they have little power of utilizing the phosphoric acid in the
soil itself. On land poor in lime superphosphate must be used with
caution owing to its acid nature, and in such cases an undissolved
phosphate is preferable. The quantity in which it is applied ranges
from 2 and 3 cwt. per acre to 5 cwt. It suffers but little loss
through drainage, and will exercise an influence on crops beyond the
year of application.
_Basic Slag._--This other principal phosphatic manure is of more
recent origin, and is an undissolved phosphate. It is the waste
product of steel-making where the Thomas-Gilchrist or "basic" process
of manufacture has been employed. This process is used with ores
containing much phosphorus, the removal of which is necessary in
steel-manufacture. The "converters" which hold the molten iron are
lined with lime and magnesia, and the impurities of the iron form a
"slag" with these materials. For a long time the slag was regarded as
a waste product, but ultimately it was found that, by grinding it very
finely, it had distinct agricultural value, and now its use is
universal. Basic slag is of various grades, containing 12 to 20% of
phosphoric acid, which is believed to exist in the form of a
tetracalcic phosphate. This phosphate is found to be readily attacked
by a weak solution of citric acid, and this probably accounts for the
comparative ease with which plants can utilize the phosphate. With it
is also a good deal of lime, and the presence of this undoubtedly, in
many cases, accounts partly for the benefits that follow the use of
basic slag. It should be very finely ground; a common standard is that
80 to 90% should pass through a sieve having 10,000 meshes to the
square inch.
The principal use of basic slag is on grass-land, especially where the
soil is heavy or clayey. Its effect on such land in causing white
clover to appear is in many cases most remarkable, and without doubt,
much poor, cold grass-land has been immensely benefited by its use. It
is also employed for root crops; but its effect on these, as on
cereals, is not so marked as on grass-land. On light land its benefit
is not nearly so great or universal as on heavier land.