146 EXPERIMENTS WITH PLANTS 



ply is deficient, the soil becomes sour by the accumu- 

 lation of carbon dioxide and humous acids and the roots 

 are killed; at the same time poisonous substances are 

 formed which, if air were present, would be converted 

 into plant -food. Subsoils, when exposed to air, usually 

 change color, an indication of chemical action. 



The soil is not only a sponge, from which the plant 

 may obtain water, but also a storehouse of plant -food, 

 and a laboratory in which plant-food is prepared and 

 dissolved for the use of the plant. 



The amount of food in the soil depends partly on 

 the kind of rock from which it is derived, partly on its 

 "fixing" power, and partly on the kind and quantity 

 of plants which grow upon it. In all these respects 

 clay is superior to sand 1 ; the rocks from which it 

 comes are richer in food than the quartz from which 

 sand is formed: it has greater "fixing" power than 

 sand 1 and produces a greater growth of plants whose 

 decay enriches it still further: decay proceeds more 

 slowly in clay than in sandy soils (since it is colder 

 and wetter), and the products of decomposition are 

 more fully fixed by it. For these reasons (and also 

 because they contain more water) clay soils are richer 

 in plant- food than sandy 1 soils. The best soil is a 

 mixture of sand, clay and humus, which gives abun- 

 dant food and good tilth. 



1 The so-called "sand" of arid regions is as rich or richer than clay, since 

 it comes from rocks rich in plant-food and is not leached by frequent rains. 



