THE ROTHAMSTED EXPERIMENTS 357 



on the phosphorus plots with wheat and turnips, and practically 

 the same yields of barley, as the legume system, clearly indicating 

 that where the soil contains a fair supply of nitrogen in proportion 

 to its phosphorus content the legume crops add little if any nitro- 

 gen to the soil in excess of what they take from the soil, when the 

 regular legume crops are all removed. Ultimately, however, with 

 the continued reduction of the absolute or relative supply of nitro- 

 gen, in comparison with other essential elements, a point is reached 

 below which the legumes leave in the roots and stubble more nitro- 

 gen than they have taken from the soil. In soils practically devoid 

 of available nitrogen only legumes can be grown, and their total 

 content of nitrogen must, of course, be taken from the air. 



It is evident that nitrogen has become so depleted in the unfer- 

 tilized land that the legume residues are now furnishing the wheat 

 crop with some nitrogen taken from the air, but this effect does not 

 extend to the turnips or barley crop. On the other hand, where an 

 abundant supply of minerals makes possible the production of 

 large crops of legumes, the atmospheric nitrogen stored in the 

 legume crop residues (or possibly gathered subsequently as sug- 

 gested elsewhere) not only maintains the yield of wheat but mark- 

 edly affects both the turnips and the barley, although the yield of 

 barley is steadily decreasing. 



As a general average on unfertilized land the wheat after clover 

 or beans has yielded about 10 per cent less than after fallow; but 

 the clover residues have increased the yield of wheat by 18 per cent 

 on the mineral plots and by 13 per cent on the plots receiving 

 minerals and nitrogen, compared with the fallow system; whereas 

 the wheat yields after beans have averaged less than after fallow 

 on all plots. These results are in accord with the data already 

 given, showing that the roots and stubble of annual legumes, such 

 as cowpeas and soy beans, contain much less nitrogen and organic 

 matter than the roots and residues of red clover, alfalfa, and sweet 

 clover. 



(5) The fallow system is unquestionably very exhaustive of the 

 soil's supply of nitrogen. During the first twenty years the fallow 

 system produced as an average larger crops than the legume 

 system, but the decrease in yield under the fallow system has in 

 most cases been more marked than under the legume system. 



