154 



PLANT LIFE. 



frequently added artificially. Both manures and artificial 

 fertilizers (the latter consisting usually of dried and ground 

 animal offal) are added chiefly for the purpose of supplying 

 compounds of nitrogen and phosphorus. 



198. Soil water. — No matter how fine the soil may be, the 

 rock particles are not in close contact, but, on account of 

 their angular outline, leave spaces of greater or less size to be 

 occupied by other materials. If a soil be examined immedi- 

 ately after a heavy rain-fall, these spaces will be found com- 

 pletely occupied by rain-water. If the soil be so situated as 



Fig. 171. — Diagram of a portion of soil penetrated by root hairs, h, h' , arising from 

 root, e. At z, s, s' the hair has grown into contact with some of the soil particles, 7 , 

 which are surrounded by water films (shaded by parallel lines), j3, a, t. The white 

 spaces are air bubbles, 5, 6', y, y'. When water enters the hair at a, the thickness of 

 the film a, /3, t will be diminished, and some water will flow towards this point, re- 

 ducing all the other water films in the vicinity. More air enters from above. When 

 rain falls, the reverse process occurs ; the films thicken, and the air may be entirely 

 driven out, to return as the surplus water drains away. — After Sachs. 



to be naturally drained, considerable quantities of this water 

 will disappear gradually, and the larger spaces between the 

 soil particles will be occupied partly by films of water adherent 

 to the soil grains, and partly by bubbles of air (fig. 171). 



199. Salts dissolved. — The water which thus filters 

 through the soil dissolves and retains certain of its constitu- 

 ents. As the rain passes through the atmosphere it also dis- 



