THE INFLUENCE OF IRRIGATION 221 



there may be produced from 50 to 100 pounds of nitric acid in 

 an acre- foot of soil during a season. This, when converted into 

 a nitrate, becomes a valuable supply of food to the grov^ing plant, 

 and, as w^e have seen, the quantity produced is dependent upon 

 the water applied. This quantity of acid would liberate appre- 

 ciable quantities of phosphorus and potassium in the soil, which is 

 therefore rendered available to the growing plant. 



'Other Acids. — The production of other acids by bacteria is 

 dependent upon optimum moisture content, and they obey the 

 same laws. It is found that the quantity of carbonic acid pro- 

 duced in twenty-four hours in a good aerable soil supplied with 

 the optimum amount of moisture is enormous. Some results indi- 

 cated that this at times may be as much as sixty-seven pounds per 

 acjre. The resulting carbonated waters react with some tricalcium 

 phosphate of the soil with the formation of more readily soluble 

 acid phosphates. Moreover, tricalcium phosphate is four times as 

 soluble in water charged with carbonic acid as it is in pure water. 

 This means that sixty-seven pounds of carbon dioxid is capable of 

 transforming 236 pounds of tricalcium phosphate into 280 

 pounds of the soluble diacid phosphate, provided all of the carbon 

 dioxid is utilized for this purpose. Although the greater part of 

 the carbonic acid would be used in other ways, yet this shows the 

 tremendous potential solvent powers of bacteria. 



Potassium occurs in soil mainly as silicates and is rendered 

 soluble by the nitrous, nitric sulfuric, acetic, lactic, butyric, and 

 carbonic acids generated by the bacteria. Carbon dioxid will even 

 react with the insoluble potassium silicates of the soil, liberating 

 the potassium as the soluble potassium carbonate. 



Salts Carried from Soil by Water. — ^Where only sufficient 

 water is added to a soil to moisten or even saturate it and none 

 drains from the soil, the plant-foot remains in the soil to be 

 utilized by the growing plant. But when more water is added 

 than the soil can hold, it passes through the soil and carries with 

 it the fertility. The magnitude of this factor is exemplified by 

 the enormous quantities of the various salts found in the waters 

 of some lakes and oceans. 



