114 THE SOIL. 



endowed with special organs to dissolve the food and 

 make it ready for absorption ; this preparation of the 

 nutriment is assigned by another law to the fruitful earth 

 itself, which in this respect discharges the functions per- 

 formed by the stomach and intestines of animals. The 

 arable soil decomposes all salts of potash, of ammonia, and 

 the soluble phosphates ; and the potash, ammonia, and 

 phosphoric acid always take the same form in the soil, no 

 matter from what salt they are derived. In performing 

 this function, the plant-bearmg earth constitutes for the 

 use of man and beast an immense purifying apparatus, 

 whereby it removes from the water all matters hurtful to 

 the health of animals, and all products resulting from the 

 decay and putrefaction of deceased generations of plants 

 and animals. 



The question how much of the several nutritive sub- 

 stances a soil must contain to yield remunerative crops is 

 of great importance, but its exact determination is beset 

 with vast difficulties. If, indeed, the nutritive power of 

 an arable soil depends upon the quantity of substances 

 held in physical combination in the ground, it is evident 

 that a chemical analysis, which cannot rigorously distm- 

 guish elements in chemical combination from those in 

 physical combination, must fail to afford any certain 

 conclusion in the matter. 



In comparing several equally productive soils, we often 

 find that they differ immensely in their chemical com- 

 position ; and that of two soils containmg, the one 80 to 

 90 per cent., the other only 20 per cent, of pebbles and 

 sand, the former will frequently yield better crops than 

 the latter. The case is possible, that a soil fruitful in itself 

 may not suffer any diminution of its fertility by being 

 mixed with half its volume of sand, but may actuaUy 

 become more productive, though it now contains, in every 



