204 MOVEMENTS OF SALTS IN THE SOIL 



soil being thus deprived of soluble plant food ; intermittent 

 showers are far more beneficial. The injury due to heavy 

 rain will be felt to the greatest extent by shallow-rooted 

 plants. Deep soils are those most favourably circumstanced 

 for the conservation of soluble salts, the matters carried down 

 by rain from the surface having here the greatest opportunity 

 of rising again by diffusion when rain has ceased. 



Phenomena of Drainage Waters. In field soils the action 

 of rain in washing out the soluble salts near the surface is by 

 no means so extreme as we might suppose from laboratory 

 experiments made with powdered soils, yielding a uniform solid 

 mass when wet. A natural soil always contains numerous 

 channels and fissures ; the former produced by roots which 

 have afterwards decayed, or by the passage of worms ; the 

 latter by contraction during drought. Through these openings 

 a considerable proportion of the rain may reach the subsoil 

 without having done any considerable amount of work in 

 removing soluble matters. There are in consequence, at 

 a moderate distance below the surface, two distinct kinds 

 of drainage water ; one, the discharge from the general mass 

 of soil lying above ; the other consisting of water which has 

 come directly from the surface through the channels we have 

 just mentioned. The last-named discharge only occurs while 

 rain is actually falling, or for a short time afterwards. The 

 water from these two sources may have a very different 

 composition. In the case of the drainage waters yielded by 

 the heavy loam at Rothamsted, the distinction between these 

 two kinds of drainage water is very plainly marked. 



During March and April 1879 the quantity of nitrates was 

 daily determined in the drainage from the deepest drain-gauge 

 at Rothamsted, containing 5 ft. of a natural, undisturbed, field 



