THE SOIL 57 



Oxygen and enriched with carbon dioxide. The air is not 

 stagnant, but undergoes constant renewal by diffusion from the 

 air above. The gases sucked out from soil vary considerably 

 in composition ; the oxygen may be anything between 10 and 

 20 per cent., the carbon dioxide from 1 to 10 per cent., while 

 the nitrogen usually differs very little in amount from that in 

 the atmosphere — about 78 per cent. The amount of carbon 

 dioxide is greater and of oxygen less during the summer and 

 autumn than in the winter or spring. 



The Water in a Soil. — This is normally present as a 

 liquid film enclosing the particles composing the soil, and 

 contains, in dilute solution, the soluble matter of the soil, and 

 also dissolved gases. Its origin is usually rain, and it therefore 

 retains any dissolved substances — chlorides, sulphates, <fec. — 

 which the rain contained. The actual composition of the water 

 in a soil must necessarily vary greatly, according to the amount 

 of rain which has recently fallen and other circumstances. 



Of the rain which falls a large proportion sinks into the soil 

 by the action both of gravitation and surface pressure. Some 

 of this runs off in the drains, carrying with it a proportion of 

 dissolved matter ; the rest remains in the interstices, and of this 

 a proportion is brought up to the surface by surface pressure, 

 and is there evaporated. It consequently becomes more con- 

 centrated, and, in dry weather particularly, the water in the 

 upper portion of a soil may contain very many times the 

 amount of dissolved substances which is found in drainage 

 water. As the liquid becomes more concentrated, doubtless 

 many of its constituents are absorbed by the soil. Evapora- 

 tion from the top layers thus causes the soil water, by the 

 surface pressure phenomenon already described, to bring up 

 into the upper parts of the soil considerable quantities of plant 

 food and other dissolved matter. In extreme cases in some- 

 what arid regions, the soil may become so charged with dis- 

 solved substances from this cause that it is unfit for plant 

 growth. Such soils are known as " alkali " or " brak " soils. 



