184 SOILS AND FERTILIZERS 



vals, because the process of fixation ma}- take place to 

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

 tilizer. Lighter and more frequent applications are 

 preferable. Phosphates should not be mixed with 

 lime carbonate or with loam before spreading. 21 It is 

 best to apply the fertilizer directly to the land. Phos- 

 phates ma}- be used in connection with farm manures. 

 Many soils which contain liberal amounts of total 

 phosphoric acid are improved with a light dressing of 

 superphosphates. Such soils, however, should be more 

 thoroughly cultivated, and manured with farm ma- 

 nures, to make the phosphates available. There is 

 frequently an apparent lack of phosphoric acid in the 

 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 have a good 

 stock of phosphoric acid, when kept well manured, 

 and occasionally limed if necessary, contain a lib- 



