50 BULLETIN 355, U. S. DEPAKTMEIS'T OF AGEICULTTJRE. 



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. Smce 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 

 sufiicient 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 m 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 sUica 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 up.on 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 resultmg 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 contamed in upland earthy soils. It is true 

 that the rapid decomposition of the organic matter of such soils which 

 takes place when they axe 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 talcen hy 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- 



