PHOSPHATE FERTILIZERS 175 



heavy applications of superphosphates at long inter- 

 vals, because the process of fixation may take place to 

 such an extent that crops are unable to utilize the fer- 

 tilizer. Lighter and more frequent applications, as 

 100 to 200 pounds per acre, are preferable. Phos- 

 phates should not be mixed with lime carbonate 

 before spreading ; 22 it is best to apply the fertilizer 

 directly to the land. Phosphates may be used in con- 

 nection with farm manures. Many soils which con- 

 tain liberal amounts of phosphoric acid are im- 

 proved by phosphate dressing of 75 pounds per acre. 

 Such soils, however, should be more thoroughly 

 cultivated, and manured with farm manures, to 

 make the phosphates available. There is frequently 

 an apparent lack of phosphoric acid in a soil 

 when in reality the trouble is due to other causes, 

 as lack of organic matter to combine with the phos- 

 phates or to a deficiency of lime. Before using phos- 

 phate fertilizers, careful field tests should be made to 

 determine if the soil is in actual need of available 

 phosphoric acid. Directions for making these tests 

 are given in Chapter X. 



227. How to Keep the Phosphoric Acid Available. 

 Phosphoric acid associated with organic matter in 

 a moderately alkaline soil, is more available than that 

 in acid soils. Soft phosphate rock may be mixed with 

 manure or materials like cottonseed meal and made 

 slowly available for crops. Soils which contain a 

 good stock of phosphoric acid, when kept well ma- 

 nured, and occasionally limed if necessary, have a lib- 

 eral supply of available phosphoric acid. As an illus- 



