178 Mineral Nutrition of Plants 



nitrogen comparable with that of some other elements, though small 

 amounts of nitrate and ammonia may be added in rainfall. The nitrate 

 supply to the plants depends entirely on the pattern of release of 

 ammonia from organic nitrogen in the soil and, as indicated above, 

 this ordinarily means the release of ammonia from microbial tissues 

 or microbial residues. The nitrification process proper — that is, the 

 oxidation of ammonia through nitrite to nitrate — is entirely secondary 

 in importance. Without ammonia liberation there can be no nitrate. 



A major unsolved problem in soil microbiology and biochemistry is 

 why the release of ammonia from the organic nitrogen complexes of 

 microbial origin is so slow. The soil is indeed a "frugal custodian" of 

 nitrogen. It is fortunate for the farmer that this is the case, so that in 

 any one season only a small fraction of the nitrogen reserves becomes 

 available to support the growth of the crop. An acre of prairie soil may 

 contain many thousands of pounds of organic nitrogen, yet the amount 

 becoming available in a single season may be only 150-200 pounds. 

 Various theories have been advanced to account for the relative un- 

 availability of the soil nitrogen. Interactions with lignin residues or 

 with clay colloids have been suggested, yet none seems entirely satis- 

 factory. 



In humid regions where leaching occurs, the rate of release — or 

 mineralization — of soil nitrogen may be an important factor in de- 

 termining crop yields. If at all times there is sufficient nitrogen to meet 

 crop requirements, the soil may be judged well suited for any particular 

 crop. If the pattern of release does not fit the particular nitrogen re- 

 quirements of the crop for optimum growth, supplementary fertilizer 

 nitrogen may have to be supplied even though the total amount re- 

 leased during the season may well be above the needs of that crop. This 

 may not infrequently be the case with early planted or quickly grow- 

 ing crops. For example, it is not unusual in the corn belt to see nitrogen 

 deficiency symptoms in small grain in the spring or in young corn in 

 a cool, wet period. Such conditions arise because microbial activities are 

 temporarily limited by the low temperature or inadequate aeration. 



Rotational systems involving the return of crop residues may con- 

 siderably alter the pattern of nitrogen mineralization and the amount 

 of nitrogen released during subsequent seasons. More attention has 



