90 AGRICULTURAL BACTERIOLOGY 



needed by the growing plant, and is often found in such 

 small amounts in an available form as to limit the yield of 

 the crop. It occurs in the soil in the form of calcium 

 phosphate, iron, and aluminum phosphate, and in organic 

 matter. All these forms are insoluble in water. 



In the Southern States there are great deposits of cal- 

 cium phosphate, or rock phosphate as it is often called. 

 This rock is ground to .a. very fine powder which is sold 

 under the name floats. This source of phosphatic fertil- 

 izer is the most important one. Superphosphate, which 

 represents a more soluble form of phosphate, is often used. 

 It is produced by treating the ground-rock phosphate with 

 sulphuric acid, forming the acid phosphate. Phosphoric 

 acid contains three atoms of hydrogen that can be replaced 

 by such a base as calcium. The acid phosphates that are 

 obtained when but one or two of the hydrogen atoms are 

 replaced by calcium are soluble in water, while the normal 

 phosphate is not. Superphosphate is added when a quick 

 acting phosphatic fertilizer is desired. 



If the insoluble phosphates in the soil are to be made 

 available to the green plant, they must be rendered soluble 

 by processes similar to those used by the manufacturer of 

 superphosphate. The acids to accomplish the change must 

 be those formed in the decomposition of the organic matter 

 added to the soil. It is generally recommended that 

 ground-rock phosphate be mixed with barnyard manure, 

 or that it be applied directly to the land when the green 

 manuring process is to be tried. It would be useless to 

 add rock phosphate to a light sandy soil that is quite devoid 

 of organic matter. The phosphorus in organic matter be- 

 comes available to the green plant on the decomposition of 

 the material. The various chemical changes through 

 which it passes are not known. 



Potassium. Potassium is found in the soil largely in 



