Analyses of Ashes of Plants. 



135 



food, both mineral and vegetable, for the use of other crops. 

 They are continually employed, abstracting by their leaves from 

 the air the constituents of which their own vegetable substance is 

 made up. They never cease to collect by their roots, and to 

 bring to the surface the mineral matters which are essential to 

 their own growth, and to that of the crops which follow them — 

 and both these forms of matter, the organic or vegetable, and the 

 inorganic or mineral are (when the crop has been consumed by 

 sheep) in great part left in the soil in the condition most favour- 

 able for the purposes of the succeeding plants. It is chiefly, 

 however, to the accumulation of vegetable matter, that we are to 

 attribute the influence of the root crop generally in improving 

 the soil ; for although the turnip does certainly add to the avail- 

 able stock of mineral ingredients of the surface soil, its own growth 

 is very mainly dependent upon the sufficiency of their supply, 

 and to obtain a very liberal return we must be proportionately 

 liberal in our grants of the materials which are indispensable to 

 the construction of the crop. 



Root crops, from the great development of their gas-collecting 

 leaves, are comparatively independent of the soil for vegetable 

 nourishment. They may in reality add to, rather than take from 

 the quantity of vegetable matter in the soil, even when entirely 

 removed — for land has been found after several years' cropping 

 with turnips, all the produce being carried off, absolutely richer in 

 organic matter than it was at first, the plant having returned to 

 the soil more than it had taken from it.* How much more is 

 this the case when a large portion of the organic matter, after 

 passing through the body of the sheep, is returned in a highly 

 comminuted state to the land. 



It cannot, however, be too often insisted upon, that, whilst we 

 may fully restore by a green or root crop all the organic wealth of 

 which the soil has been deprived by the grain of a crop of corn, 

 we have no such resource for a renewal of its mineral ingre- 

 dients. The phosphoric acid, the potash, and the magnesia of a 

 plant must be obtained from the soil, and the soil alone. But 

 even in this respect much may be done ; it may be safely pre- 



* See Dr. Daubeny's ' Memoir on the Rotation of Crops.' So 

 little is known with regard to the excrementitious matter of plants that we 

 do not lay much stress upon this argument. There is little doubt that in 

 the circulation of the vegetable juices in plants there is a continual ejec- 

 tion into the soil of matters not required in the economy of the plant ; but 

 whether the amount thus voided can at any time exceed that which is 

 taken in by the roots we have scarcely sufficient evidence to decide. It 

 is extremely likely that in broad-leaved plants of rapid growth this result 

 may occur ; but of the increase of organic matter in a soil where the green 

 or root crop is eaten off or ploughed in, there is not a shadow of a doubt. 



