266 CITRUS FRUITS AND THEIR CULTURE. 



suffer in consequence. The soil should be kept loose and 

 friable during the period of greatest growth that the roots 

 may be freely supplied with air. 



If, in addition to this, the rootlets can reach and 

 penetrate every portion of the soil, growing here and 

 there at will, they then have every opportunity to come in 

 contact with and absorb the plant food in the soil. While 

 it is true that food materials in solution may move to- 

 wards the roots, still, in general, the roots must search 

 out and procure the plant food. How can they do this 

 important work if the soil is hard, compact and impene- 

 trable? 



The food materials in any soil are found either in 

 chemical substances present in the soil, or in organic com- 

 binations. If fertilizers are applied or added to the soil, 

 they, too, fall into either one or the other of these classes. 

 Most of these materials do not immediately yield up the 

 plant food which they contain, but they must be acted 

 upon by certain agents before their store of food becomes 

 available, i. e., so that the roots can absorb and the plants 

 use it. A large part, or practically all, of the plant food in 

 organic substances is liberated through the agency of mi- 

 croscopic plants called bacteria. That these may thrive 

 and multiply, plenty of air should be admitted to the soil. 

 and the soil should at the same time be warm and moist. 

 Cultivation goes a long way towards making the condi- 

 tions ideal for the growth and development of soil bac- 

 teria. The other class of agents is those which act chem- 

 ically. To this group belong the acids and other sub- 

 stances which are capable of breaking up the food-con- 

 taining materials in the soil. Some of these disintegra- 

 tors are present in the air, and are carried to the soil 



