50 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
crops. The reaction of the sulphuric acid with the calcium phos- 
phate produces in addition to acid phosphate, calcium sulphate 
(gypsum), to which may be attributed some of the benefit secured by 
the use of the acid phosphate. 
On account of its ready availability, acid phosphate may be used 
in moderate amounts so as to supply only the phosphorus needed by 
the crops of one or two years. Since it usually has 6 or 7 per cent of 
phosphorus, crops requiring from 12 to 14 pounds of that element 
would need 200 to 300 pounds of the acid phosphate to furnish 
sufficient phosphorus for a single year. Where the crops grown are 
such as require large supplies of this element, as in the case of clover, 
alfalfa, cabbage, turnips, and certain other crops, a larger applica- 
tion would be better. 
Slag -phosphate . — When pig iron from ores rich in phosphorus is con- 
verted into steel by the basic process in which an excess of lime is 
used, a by-product, or basic slag, results. When produced by proper 
methods, the basic slag contains about 8 per cent of phosphorus, 
together with a considerable quantity of lime, from which the slag 
may derive a part of its benefit to the soil. Slag phosphate is pro- 
duced in large quantities in Europe, and to some extent in the United 
States. 
Potassium in the soil (Ref. No. 4, pp. 214, 215). — Potassium exists 
in large quantities in most soils, having been left as a residue from 
the incomplete decomposition of minerals rich hi that element such as 
feldspar and mica. The total amount in sand, silt, and clay soils 
varies from 0. 5 to 2.5 per cent. A large part of this is still combined 
with silica in an extremely insoluble form, and it becomes available 
only through the further decomposition of these silicates. The 
availability of these great natural stores of this element depends 
largely upon the presence of an abundant supply of organic matter 
in the soil. Peat and muck soils, which have been chiefly formed 
from vegetation which has grown in water or in very wet marshes, 
have usually had a considerable portion of the potassium leached out 
after the death of the plants, so that the resulting peat or muck 
contains relatively small quantities of this element. The average 
content of potassium in muck and peat soils is only from one-twentieth 
to one-fiftieth of that contained in upland earthy soils. It is true 
that the rapid decomposition of the organic matter of such soils which 
takes place when they are drained and broken generally leads to a 
fair supply of this element for a few years, but in practically all cases 
heavy applications of potassium are required sooner or later, and 
of phosphorus also in most cases. 
Potassium taken by crops (Ref. No. 4, p. 213). — Cereal crops require 
relatively small amounts of this element, ranging from 20 to 40 pounds 
per acre annually, of which from one-third to one-fifth only is con- 
