NUTRITION. 



127 



the upper layer of the soil of forests. To garden or field soils, 

 not naturally rich in organic matter, this is frequently sup- 

 plied artificially by adding manures and artificial fertilizers. 



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

 the rock particles are not in close contact, but, on ac- 

 count of their angular outline, leave spaces of greater or 

 less size to be occupied by other materials. If a soil be 

 examined immediately after a heavy rain-fall, these spaces 



Fig. 112.— Diagram of a portion of soil penetrated by root hairs, h, h' , arising from 

 root, e. At 2, s, s' the hair has grown into contact with some of the soil particles, T, 

 which are surrounded by water films ishaded by concentric lines), ^, a, t. Tlie white 

 spaces are air-bubbles, 6, 3', 7, -y'. When water enters the hair at a, the thichness of 

 the film a, p, 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. 



will be found completely occupied by rain-water. If the soil 

 be so situated as to be naturally drained, considerable quanti- 

 ties 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. 112). 



168. Salts dissolved. — The water which thus filters through 

 the soil dissolves and retains certain of its constituents. As 

 the rain passes through the atmosphere it also dissolves cer- 



