ADVANTAGES OF GEEEN CROPS. 65 



from 28 to 47 Ibs. of nitrogen in the form of nitrates per 

 annum. Bare fallow can be used with advantage only on 

 clay soils, and in a tolerably dry climate ; under other 

 circumstances the practice must result in a serious loss of 

 soil nitrogen. 



Green Crops. The most usual plan for bringing land 

 into condition for the growth of cereals is the cultivation 

 of green crops. These may be ploughed in, forming what 

 is termed green manuring ; or consumed on the land by 

 the farm stock ; or the crop may be removed, consumed 

 in cattle-sheds or in the farmyard, and the resulting 

 manure brought on to the land. The principle in every 

 case is that the constituents of the crop shall be returned 

 to the soil. 



Let us suppose that land is laid down with grass and 

 clover seeds, and after two or three years is ploughed up 

 and a cereal crop taken. Whilst the land is continuously 

 covered by vegetation the loss of nitric acid by drainage 

 will be reduced to a minimum. If the grass is fed off on 

 the land the surface soil will at the end of the three years 

 be considerably enriched both with ash constituents and 

 nitrogen. The former have 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 assimi- 

 lated by the animals. The accumulated nitrogen will be 

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

 When such land is ploughed up the vegetable matter and 

 humus are oxidised, and gradually yield their nitrogen as 

 nitric acid. 



6 



