74 BOAKD OP AGRICULTURE. 



fallow crops, we have many of the advantages of the bare fallow, 

 and connected with it a very considerable addition to the organic 

 matter of the soil. This has been satisfactorily shown by experience, 

 and where from inability to purchase fertilizers, or from distance, or 

 any other cause they cannot be obtained or employed at remunera- 

 ting rates, green manuring (the term by which this operation is 

 familiarly called) is strongly recommended. If it were true, as was 

 once believed, that plants obtain all their nourishment from the soil, 

 'the known effects of green manuring could not be accounted for. 

 But such is not the fact. Plants derive their nourishment partly 

 from the soil in which they are grown by means of their roots, and 

 in part, and sometimes in very large proportion, from the atmos- 

 phere by the action of their leaves. These sustain to the plant a 

 relation similar to that of both stomach and lungs to the animal, and 

 not only do they digest and assimilate all the inorganic or mineral 

 substances which enter into their composition, dissolved from the 

 soil and brought up in the 'sap, but they take in largely from the 

 atmosphere of carbon, which is always present in it in the form of 

 carbonic acid, and which goes to make up the bulk of the plant ; 

 nor is this the only element of growth which is thus obtained. Nitro- 

 gen, which, if not more absolutely necessary than any other, is 

 certainly the one which in practice is found most difficult to supply, 

 is to a limited extent thus derived by the plant, and so much as is 

 thus obtained is safely laid up for the use of future crops. But it 

 is not alone by the leaves that a gain is effected. By the action of 

 the roots, which in some instances, as in clover, run very deep, in 

 bringing up from the subsoil food which otherwise would lie there 

 beyond the reach of many cultivated plants, we effect another posi- 

 tive gain ; and when the crop is turned under, we enrich the surface 

 soil not only by the addition of all the organic matter obtained from 

 the air, but by the addition of both vegetable and mineral substances 

 brought up from below. In the case of clover, a portion of the crop 

 is sometimes in practice fed off by cattle whose droppings return 

 directly to the soil nearly an equivalent for their food, and the 

 remainder being turned under, together with the manure, the gain 

 is nearly as great. 



We see something very similar to green manuring in the opera- 

 tions of nature, and on a grand scale in the case of forests, where 



