FIXED STATE OF FOOD OF PLANTS IN THE SOIL/ 119 



arable surface soil, to collect and retain all those nutri- 

 tive substances on which life depends. A plant is not, 

 like an animal, endowed with special organs to dissolve 

 the food and make it ready for absorption ; this prep- 

 aration of the nutriment is assigned by another law to 

 the fruitful earth itself, which in this respect discharges 

 the functions performed by the stomach and intestines 

 of animals. The arable soil decomposes all s.alts 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-bearing 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 distinguish elements in chemical combina- 

 tion 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 

 composition ; and that of two soils containing, 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 

 actually become more productive, though it now con- 

 tains, in every part of its transverse section, one-third 

 less nutritive matter than before. The reason is, that 

 by the addition of sand the food-affording surface of the 



