114 SOILS AND MANUEES 



properties of the soil. In fact, the calcium carbonate, 

 though it is a neutral salt, can act as a potential base and, 

 if present in sufficient quantity, will maintain the neutral 

 condition of the soil. 



An acid or alkaline reaction of the soil is easily detected 

 by pressing a piece of litmus paper lightly against the 

 fresh moist earth, but very often a slight acidity cannot 

 be detected after the soil has been air dried. 



Alkaline soils are of comparatively rare occurrence and 

 are practically unknown in this country, but in certain 

 parts of the Western States of America,where, owing to the 

 deficient rainfall, the soils are never thoroughly saturated, 

 and there is little or no drainage, alkaline substances 

 chiefly sodium carbonate accumulate and impart their 

 characteristic reaction to the soil. Many of these soils, 

 which would otherwise be capable of great fertility, are 

 rendered almost barren by the deleterious effects of the 

 alkali. The only complete remedy is to wash out the 

 alkaline salts by irrigation, but their effects can be miti- 

 gated by the addition of gypsum. This substance reacts 

 with sodium carbonate, forming neutral sodium sulphate 

 and calcium carbonate, as shown by the equation 



Na 2 C0 3 + CaS0 4 = Na 2 S0 4 + CaC0 3 



Sodium Calcium Sodium Calcium 



carbonate. sulphate. sulphate. carbonate. 



Except in the case of very open sandy soils, in which 

 organic matter is so rapidly oxidised that very little humus 

 is ever formed, there is a general tendency for the soils in 

 this country to become " sour " unless the acid substances, 

 formed by the decay of organic matter, are neutral- 

 ised by lime. If the soil does not naturally contain enough 

 calcium carbonate to effect this, frequent liming becomes 

 a paramount necessity. The ruddy brown deposit of ferric 

 hydrate commonly seen in the drainage waters from moor- 



