The En\'ironmcnt of Plants 



141 



Fig. 79. Irrigated and unirrigated sugar cane, showing the value of suflSdent 

 water in the growing of this crop. 



SO filled with water as to exclude air from the roots. In order 

 that the roots may penetrate the soil readily, it should not be 

 too resistant ; but it should be compact enough to afford the 

 plant a firm anchorage. 



The soil must supply also certain indispensable chemical 

 elements used by plants in the manufacture of food. 

 These elements are potassium, calcium, magnesium, phos- 

 phorus, nitrogen, iron, and sulfur. In soils that contain 

 insufficient amounts of any of these substances the growth of 

 plants is hindered, and certain plants are excluded from such 

 a soil. One of the purposes in using chemical fertilizers is 

 to add to soils or to liberate in them sufficient quantities of 

 all the elements essential to the vigorous growth of crop 

 plants. It should be remembered that in addition to the 



