154 



PLANT LIFE. 



frequently added artificially. ISotli manures and artificial 

 fertilizers (the latter consisting usual!)' of dried and ground 

 animal offalj are added cliielly for the purpose of supplying 

 compounds of nitrogen and jthosphorus. 



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

 rock parti( Ics are not in close contact, hut, on account of 

 their angular outline, leave sjiaces of greater or less size to lie 

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

 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, Ji\ arising from 

 rotit, i\ At 7. ,v, .v' tile hair has grown into contact with some of tlie soil particles, 7", 

 which are surroundecl by water ftlms (shaded by parallel lines). ^, u, t. The white 

 spaces are air bubbles, 5, 5', y, y'. Wlieii water enters tlie ttair at a, the thickness of 

 the film a, ^, t will be diminisiied, and some water will flow towards this point, re- 

 ducing all the other water films ilt the vicinity. I\Iore air enters from above. When 

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

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



to be naturally drained, cnnsidcrable ipiantities of this water 

 will disapiiear gradually, and the larger spaces between the 

 soil particles will l)coccu[iied partly by films of water adherent 

 to the soil grains, and ])artly b\' bubbles of air (fig. 171). 



199. Salts dissolved. — The \\'ater which thus filters 

 through tire soil dissobes and retains cerlain of its constitu- 

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



