SOIL GAINS IN NITROGEN 315 



centages of increase in the protein content and length of leaf in the 

 grasses grown with clover over grasses grown alone (Evans). 



Protein, Length of 



Kind of grass. n X 6.25. leaf. 



Timothy grown in lawn plat 18.89 21.27 



Timothy grown in field 7.68 19.41 



This, however, varies with the soil and there may be conditions 

 in which not only the protein content of the non-legume is slightly 

 reduced by the association with the legume, but that the percentage 

 of nitrogen in the legume may decrease as the proportion of non- 

 legume in the mixture increases as noted by Westgate. Even in 

 these cases the total nitrogen of the combined crops is usually 

 increased, provided the legumes are properly inoculated. 



Soil Gains in Nitrogen. The gains made by soil in nitrogen are 

 dependent upon a number of factors. 



(1) It is self-evident that the soil must be in good physical condi- 

 tion for maximum nitrogen gains. (2) The soil must contain the 

 elements essential to plant growth, with the exception of nitrogen. 

 The law of minimum holds rigidly in this case and the gains in nitro- 

 gen are limited by the limiting element of plant production other 

 than nitrogen. (3) Soils which contain sufficient available nitrogen 

 for the production of good crops gain little, if any, nitrogen from the 

 growth of legumes, for the plants together with the bacteria feed 

 first upon the combined nitrogen of the soil and only upon atmos- 

 pheric nitrogen when the soil nitrogen is exhausted. Soils may 

 contain an abundance of combined organic nitrogen which for some 

 reason is not rendered available and still gain from the turning under 

 of properly inoculated legumes. (4) The legume must be properly 

 inoculated; otherwise it obtains its nitrogen as do other plants. 

 (5) The soil must be a suitable home for the legume and bacteria, 

 that is, it must have a correct reaction, moisture, temperature, and 

 aeration for maximum nitrogen fixation. Hence, we can expect 

 to find a wide divergence in the results reported by investigators. 



Frank in 1891 found that soil which had been green-manured with 

 legumes showed an appreciable gain of nitrogen. And it is a well- 

 known fact that, in sand culture experiments in which the nitrogen 

 of the soil is very low much more nitrogen may be removed in the 

 legume crop than was found at first in the soil, and after the removal 

 of the crop the soil may have gained in nitrogen. But what would 

 happen in normally productive soil? The most reliable data now 

 existing are contributed by the Illinois Experiment Station and 

 indicate that tw r o-thirds of the nitrogen in legumes grown on soils of 

 normal productive power is obtained from the air. These figures were 

 obtained from the analysis of inoculated and uninoculated legumes 

 from like areas of normal soils and as a result of pot experiments. 

 Computed by these data a four-ton alfalfa crop adds 132 pounds, a 



