Phosphorus 281 



of lime carbonate are formed to answer the needs of 

 the plants and of the bacteria. 



SOIL BACTERIA IN RELATION TO PHOSPHATES AND OTHER 

 COMPOUNDS OF PHOSPHORUS 



The phosphoric acid in the soil exists there mainly 

 as phosphate of lime, the so-called tricalcic or insoluble 

 phosphate of lime. As a matter of fact, tricalcic phos- 

 phate is not entirely insoluble, but, rather, only very 

 slightly soluble. It takes a very small amount of it to 

 form a saturated solution, one which will hold no more 

 of it under any given conditions. Extremely dilute as is 

 this solution in soil-moisture, it is still capable of sup- 

 plying all the needs of the growing crop for phosphoric 

 acid in all productive soils. 



As the plants withdraw the phosphoric acid from 

 this solution, more is added from the undissolved re- 

 serves in the soil. In other words, there is the tendency 

 to reestablish a saturated solution. It happens, how- 

 ever, that not all soils possess the same power of replacing 

 the phosphate withdrawn from the soil-moisture by 

 the roots. Some soils apparently possess the ability 

 of replacing it rapidly, others of replacing it but slowly, 

 a fact of evident importance in the growing of crops, 

 since it is only the soluble plant-food that serves as the 

 immediate nourishment of crops. 



Bacterial activities and phosphorus. — The variable 

 ability of different soils to furnish soluble phosphates to 

 the crop cannot be directly correlated with the amount 

 of total phosphoric acid in the soil. There are soils con- 

 taining fairly large amounts of phosphoric acid that yield 



