CROPS. 43 



bably capable of feeding on some compounds of nitrogen 

 and carbon which are comparatively useless to other crops, 

 and hence the facility with which they acquire nitrogen 

 from the soil. A deeply rooted crop like red clover collects 

 nitrogenous compounds from the subsoil, and accumulates 

 nitrogen at the surface in the form of a crop. 



The particular food supply of a leguminous crop be- 

 comes exhausted by repeated cropping, and the land is 

 said to be "clover" or "bean sick;" 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. Gypsum is also valuable, though to a less 

 extent. 



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. 



The turnip and mangel crop differ in several respects. 

 Turnips and swedes draw their food chiefly from the sur- 

 face 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 

 powder of appropriating the combined phosphoric acid of 

 the soil; fresh applications of phosphatic manures thus 

 always produce a marked effect on this crop. 



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, 



