SYMBIOTIC NITROGEN FIXATION 



163 



secure large amounts of nitrogen from the latter. This power 

 of supplying nitrogen to non-legumes varies with diiferent 

 legumes. At times it may appear as an increased yield, whereas 

 at others it may appear as an increased proportion of nitrogen in 

 the dry matter of the non-legume or both. The following table 

 gives the percentages of increase in the protein content and length 

 of leaf in the grasses grown with clover over grasses grown alone 

 (Evans) : 



Kind of Grass 



Protein 

 n X 6.2$ 



Length of Leaf 



Timothy grown in lawn plat 

 Timothy grown in field 



18.89 

 7,68 



21.27 

 19.41 



This, however, varies with the soil, and there may be condi- 

 tions 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 propor- 

 tion of non-legume in the mixture increases. Even in these cases 

 the total nitrogen of the combined crops is usually increased, pro- 

 vided the legumes are properly inoculated. 



Soil Gains in. Nitrogen. — ^The gains made by soil in nitrogen 

 are dependent upon a number of factors. 



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

 condition for maximum nitrogen gains. (2) The soil must con- 

 tain the elements essential to plant growth, with the exception of 

 nitrogen. The law of minimum holds rigidly in this case and 

 the gains in nitrogen are determined 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 plant 

 together with the bacteria feed first upon the combined nitrogen 

 of the soil and only upon atmospheric nitrogen when the soil 

 nitrogen is exhausted. Soils may contain an abundance of com- 

 bined organic nitrogen which for some reason is not rendered 



