206 MOVEMENTS OF SALTS IN THE SOIL 



understood if we take into account firstly, that the salts in a 

 soil are not equally distributed throughout it, but lie in bands, 

 the position of which may vary greatly ; and secondly, that 

 the pipes are fed by two distinct kinds of drainage water, 

 one reaching them directly through channels from the surface, 

 the other consisting of the discharge of the saturated soil. 



The first discharge from the drain-pipes after the commence- 

 ment of heavy rain will consist largely of water brought 

 directly from the surface through channels in the soil, and 

 water from this source will form a large part of the discharge 

 so long as rain continues to fall. When, however, rain has 

 ceased, this supply of water to the drain-pipes will soon dis- 

 appear. The pipe, however, continues to run in diminishing 

 quantity for several hours, the discharge being derived suc- 

 cessively from lower and lower layers of soil, till all the soil 

 lying above the pipe is exhausted, when the running comes 

 to an end. 



The first and last running of the drain-pipe thus consists 

 of water having a totally distinct source. The relative 

 strength of these two kinds of drainage water depends en- 

 tirely upon circumstances. When the surface soil is richest 

 in soluble salts as when active nitrification has taken place, 

 or a dressing of saline manure has been applied the first dis- 

 charge from the pipe may be much stronger than the last. 

 If, on the other hand, the surface soil has been more or less 

 washed out by previous rains, or the salts applied have 

 spread by diffusion, then the under soil becomes the part 

 containing most saline matter, and the discharge from the 

 drain-pipe consequently increases in strength as its flow 

 diminishes. This is, in fact, the condition most usually 

 present. The discharge during very heavy rain is generally 



