278 SOILS AND MANUEES 



matter is burned off. The twigs and young boughs are 

 the richest, and being useless as timber, they are often 

 burned for recovery of the potash. The operation was 

 formerly carried out in an iron pot hence the name 

 pot-ashes but is now usually done in pits^ both in 

 Canada and the United States. The ash contains 

 from 5 to 10 per cent, of potassium carbonate which 

 can be largely separated from the other ingredients by 

 lixiviation with water. The product obtained on re- 

 crystallisation is called crude potash, and contains from 

 50 to 60 per cent, of potassium carbonate. By a further 

 process of lixiviation, a purer substance containing up- 

 wards of 90 per cent, can be produced. This is called 

 pearl ash or American ashes. 



For manurial purposes it would be no advantage to 

 separate the potash from the other ingredients, but quite 

 the reverse. The ash includes usually from 2 to 4 per 

 cent, of phosphoric acid, and 30 to 40 per cent, of car- 

 bonate of lime, and both of these constituents, as well as 

 some of the potash, would be lost. 



The ashes of other plants and all kinds of vegetable 

 refuse may also be used as a source of potassium com- 

 pounds, and as potash manure. The ash of seaweed 

 has been used to a considerable extent in Scotland, under 

 the name of ''kelp." For manurial purposes, however, 

 it is an extremely wasteful process to burn the seaweed 

 or any other vegetable matter, as the nitrogen is thereby 

 dissipated and lost. If the material can be ploughed 

 into the soil direct, or can be reduced to suitable con- 

 dition by composting, it is much more economically 

 employed in that way. Apart from the plant food it 

 contains, the organic matter generally has a beneficial 

 effect on the physical properties of the soil. The influ- 

 ence of potassium carbonate, on the other hand, is highly 

 deleterious. It has a strongly alkaline reaction, causes 



