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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.



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