252 CAUSES OF FERTILITY AND STERILITY [chap 



soluble salts, particularly those of calcium, possess a 

 strong flocculating power, whereas the soluble alkalis — 

 the carbonates and hydrates of sodium, potassium, and 

 ammonium — are active deflocculators, causing the clay 

 particles to separate into their most fine-grained condition. 



It has long been recognised that large or frequent 

 dressings of nitrate of soda had an injurious action upon 

 the tilth of the soil, causing it to remain very wet, and 

 then to dry into hard, unkind clods. Since nitrate of 

 soda is very hygroscopic, the wetness induced in the 

 land was attributed to the absorption of moisture from 

 the atmosphere by the nitrate of soda, but when it is 

 considered what a very small proportion the water 

 absorbed by as much as 5 cwt. of nitrate of soda would 

 bear to the hundred tons which is the approximate 

 weight of an acre of soil an inch thick, it is obvious that 

 the difference in water content so induced would not be 

 sensible. 



Clay soils, in fact, which have been treated with 

 nitrate of soda, do not show any excess of water ; but 

 they are very much deflocculated, as may be ascertained 

 by comparing the appearance after standing of a jar of 

 distilled water rendered turbid by shaking up in it a 

 gram of the soil, with a second jar in which the water 

 has been shaken with a gram of the same soil in its 

 normal condition. But nitrate of soda itself possesses 

 flocculating powers even when concentrated, hence the 

 observed deflocculation can not be due to the direct action 

 of the fertiliser upon the clay. However it has been 

 found that when plants feed upon a nutrient solution 

 containing nitrate of soda, an excess of the nitric acid 

 is withdrawn by the plant, and part of the soda is left 

 in the medium combined with the carbon dioxide 

 secreted by the plant The existence of this soluble 

 alkali after the growth of the plant can be verified by 



