322 CROP ROTATION 



Even where the legume was used in the system there had been 

 a decline in the yield. The yield of the turnips during the first 

 twenty years was 5264 pounds, the second 1723, and the third 

 only 967 pounds, thus showing a decrease of about five-sixths the 

 original in sixty years. 



The results with the barley are no better, for there is a drop 

 from the fair yield of 38 bushels per acre during the first period to 

 only 13.7 during the third. The wheat which followed the legume 

 in the rotation, and hence occupied the most favored place in the 

 system, shows a decrease of 5.3 bushels. Not even a good yield 

 has been maintained for the clover, for from 1850 to 1874 the 

 average yield was 4165 pounds, while from 1882 to 1906 the yield 

 was only 1246 pounds. In reality we find no greater decline in 

 the yields where fallow cultivation is practised. But both sys- 

 tems strongly testify to the fact that rotation is not maintaining 

 the productive powers of this soil. And the evidence is strong 

 that the legume gets no more nitrogen from the air than that 

 which is removed with the plant. Otherwise, we should expect 

 better results in the legume system than in the fallow system. 



Nitrogen Obtained from Atmosphere by Legumes. That the 

 alfalfa, when grown on fertile soil and the crop removed, does not 

 increase the nitrogen of the soil is seen from experiments con- 

 ducted by Dr. Hopkins at the University of Illinois. The experi- 

 ments were made possible by the fact that many of the Illinois 

 soils do not normally contain the symbiotic bacteria which make 

 it possible for the alfalfa to obtain nitrogen from the air. This 

 being the case, a field was taken which had not grown alfalfa and 

 which did not contain the symbiotic nitrogen-gathering bacteria. 

 This was planted to alfalfa, only one-half of it being inoculated 

 with the legume organism. To some of the plots w r ere added lime 

 and phosphorus to make sure that these were not the limiting fac- 

 tors. The results thus obtained are given below: 



Pounds in crop: 



Pounds, nitrogen 



Plot No. Treatment applied. Dry matter. Nitrogen. fixed by bacteria. 



la None 1180 21.81 \ 



lb Bacteria 2300 62.04 f 



2a Lime 1300 26.20 \., Q9 



26 Lime bacteria 2570 68.02 / 



3a Lime phosphorus .... 1740 35.40 1 ,> r c 



36 Lime phosphorus bacteria . 3290 89.05 / 



It is evident from these results that the alfalfa had obtained 

 from 40 to 53 pounds of nitrogen from the air, depending upon 

 the treatment. There was slightly more than one-third as much 

 nitrogen in the alfalfa crop from the uninoculated as in the inoculated. 

 Therefore, it is quite evident that the alfalfa in these plats had 

 obtained one-third of its nitrogen from the soil and two-thirds from 



