208 SCIENTIFIC AGRICULTURE. 



of any of the soluble saline manures, being felt by the land for 

 more than ten years. 



The following mixture is said to be nearly equal in efficacy 

 for a year or two, to one ton of wood ashes. 



Crude potash, 60 pounds. 



Grystalized carbonate of soda, 60 " 



Sulphate of soda^ 20 



Common salt, 20 " 



160 



Leached ashes are nearly destitute of potash, and cannot, of 

 course, supply this substance to vegetation; they are said, 

 however, to be of service to oat crops in particular, and are 

 beneficial to clay soils. The ashes of coal, peat, turf, straw 

 and cane are also valuable as fertilizers, according to their 

 constitution and the crops to which they are applied. 



Crushed or pulverized rocks of various kinds could be used 

 with the same benefit and in the same cases, according to their 

 elementary composition, as other mineral manures: crushed 

 granite would furnish a considerable amount of potash ; it is 

 easily ground after being heated to a red heat. Crushed trap 

 contains much lime, and is a good manure : crushed lavas are 

 also valuable on most soils. 



Marl. The composition and other chemical characters of 

 marl have been described : it consists of lime, clay, and often 

 sand, shells, and other matters. The object and effect of 

 marling are similar to those of liming land. Marl should be 

 used according to its constitution ; clay marl should usually be 

 put on sandy soils, and lime or sandy marl on clay soils. The 

 best time for laying on marl is at the end of autumn, so that 

 it may be pulverized by frosts during the winter Boussin- 

 gault says, land which contains ten per cent, of carbonate of 

 lime can dispense with marl. 



The effect of marl is not unlimited, but, like lime, requires 



