ARTIFICIAL AND CONCENTRATED MANURES. 95 



form phosphates ; these forms are believed to be less 

 readily taken up by the plant than the dicalcic form. 



The time required for this fixation, as well as the form 

 it takes, depends upon the character of the soil ; though on 

 soils in a good state of fertility the fixation is quite rapid. 

 On very sandy soils the fixation is sometimes incomplete, 

 because of the absence of lime and iron. 



Use of Superphosphates. Because of the tendency 

 of soluble phosphoric acid to form in time relatively in- 

 soluble compounds in the soil, it is often recommended 

 to use a mixture of superphosphate and of animal bone, 

 instead of either alone; the soluble for immediate use, 

 and the less soluble for use at later stages of growth, or 

 for the improvement of fertility. 



Superphosphates are never better or more available 

 than when applied ; phosphates are probably never less 

 available than at the time applied. 



Recent experiments and studies show that fine ground 

 phosphates are very desirable under certain conditions, 

 and their use is gradually growing in favor. This point 

 has reference, however, to the economy of use, which 

 will be discussed in detail in its proper place. 



Potash Manures. Farm crops remove considerable 

 amounts of potash; and since many soils, particularly 

 those composed largely of sand, are not rich in this ele- 

 ment, potash becomes a very important constituent of 

 manures. 



In the early history of the country, wood-ashes were 

 an important, and practically the only, source of potash 

 for manurial purposes, aside from yard manure and vege- 

 table wastes. At the present time by far the most 



