24 T. Lyttleton Lyon and James A. Bizzell 



soil on the unfertilized plats contains more nitrates than does the 

 uncropped soil. 



(2) In oat land the nitrates are never so high in the cropped as in the 

 uncropped soil. 



(3) While the maximum nitrate content in the uncropped soil occurred 

 at about the same dates in both years, that in the cropped soil varied with 

 the crops. 



(4) Under both crops the nitrate content was higher during the period 

 when the crop was making its greatest draft on the soil nitrogen than in 

 the later stages of growth, and this in spite of the fact that the nitrates in 

 the uncropped soil were increasmg while those in the cropped soil were 

 disappearing. 



These phenomena are explainable on the assumption that nitrification 

 is stimulated by some processes comiected ^dth the active gro'^i^h and 

 absorbing functions of the plant roots, and is inhibited by the conditions 

 accompanying the decreasing activities of the roots. 



Aside from the influence of cultivation and nitrogen absorption, the 

 reason for the large differences in the nitrates under these crops, as well 

 as under the corn, potatoes, alfalfa, and oats grown by King and 

 Whitson and by Stewart and Greaves, may be sought in the inherent 

 differences between plants of different species as regards their stimulating 

 or inhibiting effect on nitrate formation. Another factor that maj^ be 

 operative in determining the quantity- of nitrates is the habit of the 

 plant in respect to the form in which it absorbs nitrogen. It is con- 

 ceivable that a plant maj^ induce an active nitrification in the soil and 

 yet absorb its nitrogen largely in the form of organic nitrogen. 



The response of different plants to nitrogen in different forms indicates 

 a difference in their abilities to utilize this material when in certain com- 

 pounds. For instance, grass makes a quick and effective response to 

 sodium nitrate, and the nitrate content of the soil, unless heavily dressed 

 with nitrate fertilizer, is maintained at a verj^ low point. Oats are less 

 responsive to the nitrate fertilizer and do not reduce the nitrate content 

 of the soil so low. jMaize is but little affected by a nitrate fertilizer, while 

 it is greatly benefited by fresh applications of farm manure. The nitrates, 

 meanwhile, are present, apparenth' greatly in excess of the needs of the 

 plant. 



