156 SOILS AND FERTILIZERS 



198. Forms in which nitrogen is absorbed by plants. — 

 The utilization of atmospheric nitrogen by leguminous plants 

 and by a few others that have nodule-bearing roots has been 

 established beyond question ; but the extent to which this 

 form of nitrogen may be utilized by other plants, or the kinds 

 of plants that have the ability to use it, are subjects on which 

 opinions differ. It is sufficient to say that such plants as 

 red clover, alfalfa, peas, beans, vetch, and so on, are able to 

 use atmospheric nitrogen. It must be remembered, however, 

 that they also use nitrogen that is in the soil itself and that 

 they may remove large quantities of this material. 



199. Nitrates as plant-food material. — Most upland 

 plants used in agriculture appear to absorb most of their 

 nitrogen in the form of nitrates. This it will be remem- 

 bered is the final form in which nitrogen appears when ni- 

 trogenous substances undergo normal decomposition in soil. 

 The nitrogen of the various nitrogen carrying fertilizers is 

 finally converted into nitrate in the soil. 



200. Absorption of ammonia by agricultural plants. — 

 Ammonia is rarely found in soils, except when they are 

 saturated with water. Plants like rice, that grow on water- 

 covered soil, can utilize ammonia ; in fact, rice has been found 

 to make a better growth on ammonium compounds than on 

 nitrates. This is a case in which the plant has evidently 

 adapted itself to its surroundings, for upland rice presumably 

 uses nitrate nitrogen. However, some dry land plants can 

 also use ammonia. It has been found, for instance, that 

 peas obtained nitrogen as readily .from ammonium salts as 

 from sodium nitrate. On the other hand wheat plants, while 

 able to secure some nitrogen from ammonia, have been found 

 to grow much better when they could obtain nitrates. 



201. Direct utilization of organic nitrogen by crops. — 

 One of the early beliefs in regard to plant nutrition was 

 that organic matter was directly absorbed by plants and that 



