ROTATION OF CROPS. 51 



finally sown with wheat. In the case of a clay soil, this 

 treatment would probably lead to the following results : — 



1. An improvement in the mechanical texture of the soil. 



2. The disintegration of some of the mineral silicates, 

 whereby potash and other necessary ash constituents of 

 plants would be liberated and made available for vegeta- 

 tion. 3. The absorption of ammonia from the atmosphere 

 by the soil. 4. The receipt of both ammonia and nitric 

 acid from the air in the form of rain. 5. The oxidation of 

 ammonia, and of the vegetable remains in the soil, nitric 

 acid being produced. 



The production of nitric acid is probably the most 

 important result of a bare fallow. In soils at Rothamsted 

 left as bare fallow, there has been found at the end of 

 summer 34 — 55 lb. of nitrogen per acre in the form of 

 nitric acid in the first 20 inches from the surface. Sup- 

 posing the season of fallow is a fairly dry one, the increase 

 in the available nitrogenous food will probably enable the 

 soil to produce twice as much wheat as it could do without 

 this treatment. If, however, the soil is exposed to heavy 

 rain, the nitrates produced will be more or less washed 

 out, and the benefit of the fallow greatly diminished. 

 Bare falloAV can be used systematically with advantage 

 only on clay soils having a considerable absorptive power 

 for ammonia, and in a tolerably dry climate ; under other 

 circumstances a continuance of the practice must issue 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 



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