MANURES. 33 



discrimination, lest the humus of the soil be unduly dimin- 

 ished. Heavy clays, or soils rich in humus, are those most 

 benefited by burnt lime. In reclaiming peat bogs lime is 

 of the highest value. The acid humic matter of the peat 

 is neutralised by the lime, and the nitrogen held in com- 

 bination is converted into ammonia and nitric acid, and 

 thus made available to a crop. 



The general effect of lime is to render available the 

 plant food already in the soil, without itself supplying 

 any significant amount ; liming cannot, therefore, be suc- 

 cessfully repeated except at considerable intervals. 



Potassium Salts. — These salts are now obtained from 

 Stassfurt and Leopoldshall in large quantities ; they form 

 a thick deposit overlying an enormous mass of rock* salt. 

 The commonest potassium salt employed as manure is 

 kainit ; it consists of chloride of potassium, sulphate of 

 magnesium, and water, with frequently chloride of mag- 

 nesium and common salt in addition. Kainit will contain 

 13 to 14 per cent, of potash. Calcined kainit contains 

 less water, and some magnesia in place of the chloride of 

 magnesium; it will contain 15 to 17 per cent, of potash. 



Wood ashes may also be employed as a potash manure ; 

 they will contain between 5 and 15 per cent, of potash. 

 The ash of young boughs is richer than that from full- 

 sized timber. 



Potash manures produce their gi-eatest effect on pasture ; 

 clover and turnips may also be benefited by their use. 

 Many soils are naturally well furnished with potash, on 

 these soils potash manures are almost without effect. 



Common Salt.— Chloride of sodium supplies no essential 



