HOW SOILS L08B MINERAL MA I 113 



cropped for many generations without exhausting its 

 total supply of phosphoric acid, but its supply of avail- 

 able phosphates may be exhausted with one or two 

 crops. The question, then, ii how musi the >n\ be 



d in order that its store of phosphates may bc- 

 eome available as food for cultivated crops. It is diffi- 

 cult to answer this question satisfactorily, for the 

 solubility of the soil phosphates is influenced by i 

 number of different conditions in the soil, none of 

 which is perfectly understood. Hut it is known that 

 the amount <>f soluble phosphoric acid in soils il in- 

 fluenced to a great extent by the amount of humus it 

 contain-. The so-called insoluble phosphates, though 

 almost insoluble in water, arc acted on bv even wmk 

 acids, and by them converted into compounds soluble 

 in water. Humus contain- 1 number of weak acids, 

 and they undoubtedly act on the insoluble phosphates 

 in the soil, convert ing them into forms suitable for 

 plant food. Here we have another illustration of the 

 VallM of soil bacteria, and the importance .»f careful 

 cultivation. Soil bacteria supply nitrogen for grow- 



Ing crops; they kite have much to do with supplying 

 the available phosphates. Some salti in the soil proba- 

 bly ntTeet the supply of soluble phosphates, but their 

 a< tiofi is not SO important as the SCtlOfl of humus. 



109. Potash in the Soil. — The supply of potash in 

 the soil I we bare already learned, principally 



from the decay of rocks called feldspars, in which the 



potash is combined with silica to form what tie called 

 potassium silicate* When these rocks decay and form 

 soils, some of the silicates combine with eJaj and form 

 8 



