SOIL ORGANISMS 437 



gators that the continuous growing of legumes, the tops being 

 removed as forage, does not always increase the nitrogen con- 

 tent of the soil to any greater extent than does a non-legumi- 

 nous crop. 



The results of Swanson^ are particularly striking in this 

 respect. This investigator sampled a number of fields in 

 Kansas that had grown alfalfa continuously for twenty or 

 fchirty years, at the same time obtaining soil from contiguous 

 native sod. In most eases the alfalfa soil was lower in 

 nitrogen than the sod. Lyon and Bizzell ^ found practically 

 the same content of nitrogen in contiguous alfalfa and 

 timothy soils after the crops had been growing six years. 

 The maize crop following the alfalfa was nevertheless much 

 greater than that after the timothy. Since the soil on which 

 a legume has been growing generally has a rather high nitrify- 

 ing capacity,^ the explanation seems to lie in the ready avail- 

 ability of the nitrogen in the soil which bore the legume, 

 rather than to the presence of an especially large amount. 



The amount of nitrogen fixed by the nodule organisms of 

 a leguminous crop is very uncertain. If the soil is acid, if 

 it contains alkali salts above a certain amount, or if nitrates 

 develop rapidly, nitrogen fixation is markedly retarded. Much 

 also depends on the virulence of the organisms, the character 

 of the legume, the presence of organic matter, and other im- 

 portant conditions. Hopkins ^ estimates that about one-third 



^Swanson, C. 0., The JSffeot of Prolonged Growing of Alfalfa on 

 tlie Nitrogen Content of the So%l; Jour. Amer. Soc. Agron., Vol. 9, 

 No. 7, ppi, 305-314, 1917. 



Swanson, C. 0., and Latshaw, W. L., Effect of Alfalfa on the Fer- 

 tility Elements of the Soil in Comparison with Grain Crops; Soil Sci., 

 Vol. VIII, No. 1, pp. 1-39, 1919. 



^Lyon, T. L., and Bizzell, J. A., Eccperiments Concerning the Top- 

 dressing of Timothy and Alfalfa; Cornell Agr. Exp. Sta., Bui. 339, 

 Tip. 136-139, 1913. 



^Lyon, T. L., Bizzell, J. A., and V^ilson, B. D., The Formation of 

 Nitrates in a Soil Followmg the Growth of Bed Clover and of Timothy; 

 Soil Sci., Vol. IX, No. 1, pp. 53-64, 1920. 



^Hopkins, 0. G., Soil Fertility and Fermanent Agriculture^ p. 223, 

 Boston, 1910. 



