66 THE CHEMISTEY OP THE FAEM. 



Such a mode of cropping has several advantages over 

 a bare fallow : 1. The land is turned to profitable use, 

 food being produced for the farm stock. 2. Both ash 

 constituents and nitrates are collected from the subsoil 

 and brought to the surface. 3. Nitrogen is acquired from 

 the atmosphere by the crop, as well as by the soil. This 

 is especially true if leguminous plants are grown. 4. The 

 nitrogen collected is kept in an insoluble form, as vege- 

 table matter, and consequently cannot be washed away, 

 but accumulates in the surface soil to a greater extent 

 than is possible in a bare fallow. 5. Humus is produced 

 in considerable quantity, the beneficial actions of which 

 have already been noticed. 



As an illustration of the accumulation of nitrogen in 

 the surface soil when land is laid down permanently in 

 pasture, we may refer to the arable land laid down to 

 grass at Rothamsted, which gained nitrogen during 33 

 years at the rate of about 52 Ibs. per acre per annum. 

 Taking into account the quantity of hay removed, the 

 greater part of this increase of nitrogen in the soil could 

 not be accounted for by the quantity applied as manure ; 

 it was probably to a considerable extent derived from the 

 atmosphere. 



Leguminous crops, as already mentioned, have a 

 special power of acquiring nitrogen from the atmosphere 

 by means of their root tubercles, and are hence of the 

 greatest value in a rotation. The accumulation of nitrogen 

 in the surface soil in the form of roots, stubble, and de- 

 cayed vegetable matter, is in the case of a good crop of 

 clover so considerable that the whole of the above ground 

 growth may be removed as hay, and the land yet remain 

 greatly enriched with nitrogen and in an excellent COD- 



