54 THE CHEMISTRY OF THE FARM. 



repeated cropping with them, and is said to be " clover 

 sick" or "bean sick." The origin of this barrenness 

 has not yet been satisfactorily explained ; it is generally 

 intensified by an attack from insects on the weakened 

 plant. No means of remedying this condition is known 

 save by the growth of other crops for a series of years. 



Potash manures have generally a very beneficial effect 

 upon leguminous crops; they fail, however, to cure 

 clover sickness. Farmyard manure, phosphates, gypsum 

 and lime are also serviceable. 



Root Crops. All these crops contain a large amount 

 both of nitrogen and ash constituents ; among the latter 

 potash greatly preponderates. Turnips contain more 

 sulphur than any other farm crop. 



Turnips and swedes draw their food chiefly from the 

 surface soil. Their power of taking up nitrogen from the 

 soil is distinctly greater than that of the cereal crops. 

 Turnips are also well able to supply themselves with potash 

 when growing in a fertile soil, but they have singularly 

 little power of appropriating the combined phosphoric 

 acid of the soil. On exhausted land it is generally im- 

 possible to obtain a crop without a supply of phosphates. 



Mangels have far deeper roots than turnips, and also a 

 longer period of growth. They have a great capacity for 

 drawing food from the soil, including both nitrogen, 

 potash, and phosphoric acid. When carted off the land 

 they are probably the most exhaustive crop that a farmer 

 can grow. As mangels have not the same difficulty that 

 turnips have of attacking the combined phosphoric acid 

 oi: the soil, phosphatic manures are, in their case, of much 

 less importance. Nitrate of sodium, when applied alone 



