38 FIRST PRINCIPLES OF AGRICULTURE. 



richer or poorer. If left to themselves, the constituents 

 rendered soluble by air, moisture, and chemical action, 

 as well as the finer particles of earth, are carried by 

 rains in greater or less amounts into the streams and 

 brooks. Certain of the soil constituents are, however, 

 less liable to be lost through drainage than others; that 

 is, soils do not exert the same retentive power for all 

 constituents. 



The constituents of the greatest interest to the farmer 

 are nitrogen, phosphoric acid, potash, and lime. Of 

 these, nitrogen and lime form certain compounds that 

 are extremely soluble and freely movable in the soil; 

 drainage waters are seldom free from traces of nitrates, 

 and of chlorides of sodium and calcium (lime). 

 . On the other hand, ammonia, a compound containing 

 nitrogen, and phosphoric acid and potash are seldom found 

 under natural conditions in any considerable amounts in 

 drainage waters. For these the soil possesses a strong 

 retentive power, though they are not held so strongly as 

 to be unavailable to plants. 



This power of soils is not only important in showing 

 the probable loss or gain of fertility in uncultivated soils, 

 but has a wide bearing upon their possible improvement. 

 Phosphoric acid and potash particularly, when added to 

 soils, are fixed, and remain until removed by the plants. 



Clay, humus, and lime are the ingredients in soils 

 which exert the greatest influence in retaining the solu- 

 ble phosphates, potash salts, and ammonia compounds. 



Absorptive Properties of Soils. — The property 

 which a soil possesses of breaking up such compounds, 

 and holding fast to the essential elements, is both physi- 



