THE NUTRITION OF THE ROOT 



65 



§ 10. How can we improve our fields so as to obtain the 

 best possible crops ? 



In practice, fields may be grouped, according to their consti- 

 tution, into those with a sandy soil, those with a lime soil, and 

 those with a clay soil. Let us examine them in turn. 



The characteristic of a sandy soil is that it can be saturated 

 with a relatively small amount of water. The smaller the 

 grains of sand, the less porous is the soil, and the greater is 

 its power of retaining in its lower layers a small supply of 

 water and of condensing the atmospheric moisture at its sur- 

 face. Powdery sand is harmful on account of its density (im- 

 perviousness). Its great power of absorbing heat in its upper 

 layers from the direct rays of the sun is of advantage only 

 in cold situations, usually, however, harmful. Its power of 

 absorbing oxygen is small. To improve fields of this kind, a 

 large amount of stable manure is necessary, as the rain-water 

 is then retained in lai-ge quantities, and at the same time the 

 requirements of the plants in this direction are reduced. For 

 experiments have proved that in a well-manured soil the roots 

 need less water for the production of a given amount of vege- 



