Rock-phosphate
Categories:
COMMERCIAL SOURCES OF PLANT-FOOD
While the greater part of our soils contain relatively
scant stores of phosphoric acid, the deposits of this plant constituent
in combination with lime are immense. The rock now chiefly used in this
country is found in South Carolina, Tennessee, and Florida. It varies
greatly in content of phosphoric acid. When pulverized for direct use
on land, without treatment with sulphuric acid to make the plant-food
available, a
rade running 28 per cent phosphoric acid, or less,
usually is selected, the higher grades being reserved for treatment
with acid or for export. This untreated rock, pulverized exceedingly
fine, often is known as floats.
The value of a pound of phosphoric acid in floats, as compared with
that of a pound in the treated rock, known as acid phosphate, is a
matter upon which scientists differ widely. Only a small percentage of
the plant-food is immediately available, and the question of wise use
hinges upon the degree of availability gained later, and the time
required. The large amount of experimental work that has been done
affords data that causes the following opinion to be stated here:
Rock-phosphate, known as floats, is not a profitable source of
plant-food for soils deficient in organic matter, when compared with
acid phosphate. It is more nearly profitable in an acid soil than in
one that has no lime deficiency. It gives more satisfactory results
when mixed intimately with stable manure than when used upon land that
remains deficient in organic matter. Applications should be in large
amount per acre--500 to 1000 pounds--in order that the amount of
readily available phosphoric acid may meet the immediate need of
plants. Dependence should be placed upon the readily available acid
phosphate in all instances until experiment on the farm shows that the
rock-phosphate is a cheaper source of plant-food than the acid
phosphate.