92 THE CHEMISTRY OF THE FARM 



been collected from the subsoil by the roots of the crop 

 and returned to the surface soil as animal manure. 

 The latter includes the accumulated receipts from the 

 atmosphere and subsoil during the three years, minus 

 the quantity lost by drainage and that assimilated by 

 the animals. The accumulated nitrogen will be chiefly 

 in the form of grass roots and stems, and humus. 

 When such land is ploughed up, the vegetable matter 

 and humus are oxidised, and gradually yield their nitro- 

 gen as nitric acid; the ash constituents which they 

 contained are at the same time liberated, and become 

 once more available as plant food. 



Such a mode of cropping has several advantages 

 over a bare fallow : (1) The land is turned to profit- 

 able 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 vegetable matter, and conse- 

 quently 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 

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

 at Rothamsted, which gained nitrogen during thirty- 

 three years at the rate of about 52 lbs. per acre per 

 annum. This land was regularly manured with farmyard 

 ^nd ^^rtificial mai^ure. Taking into account the quantity 



