198 CROPS GROWN IN ROTATION 



which must in the main be set down to the mineral dressings 

 received three years earlier in the rotation. Where clover or 

 beans are grown the crop mounts up to nearly 38 bushels per 

 acre, or to the maximum grown even on the plots receiving 

 nitrogen as well as minerals, so thoroughly have the leguminous 

 plants done their work of accumulating nitrogen for the 

 succeeding crop of wheat. 



The application of the nitrogen (141 Ib. in the shape of 

 rape cake and ammonium-salts) to the Swedes has nearly 

 doubled the crop, the average during the last seven courses 

 having been 18 tons, as against less than 10 tons with the 

 minerals only. Dependent as the Swede crop has been shown 

 to be upon the minerals, the soil of the plots receiving no 

 nitrogenous manure has been so far depleted that nitrification 

 of the reserves of humus still remaining in the soil is not able 

 alone to supply enough available nitrogen for the needs of the 

 crop, as is shown by the increased yield produced by a direct 

 application of nitrogenous manure. 



The effect of the nitrogen applied to the Swedes is still very 

 palpable in the barley crop, the yield of which is about 40 per 

 cent, larger on the completely manured plots than on the plots 

 receiving no nitrogen. Coming to the leguminous crop, the 

 nitrogen has no effect ; the clover is not better, and the 

 beans are very distinctly worse where it has been applied. 

 This affords very strong evidence of the extent to which 

 leguminous plants are able to feed themselves with nitrogen 

 from the atmosphere and become independent of nitrogen in the 

 soil, an excess of which may even be injurious to their growth. 



The manner in which accumulations of nitrogen in the soil 

 interfere with the growth of leguminous plants is curiously 

 seen in another way on this field : one of the commonest 

 weeds among both the barley and the wheat on the unmanured 

 plots and on those receiving only minerals is the little legumi- 

 nous Black Medick (Medicago lupulina), which often almost 

 entirely covers the surface of the ground towards harvest time. 

 This plant, however, is much less abundant, and indeed is 



