440 NITROGEN METABOLISM 



nitrate, because the nitrogen of nitrate ions is reduced to other forms almost 

 as rapidly as it enters the plant. Under certain conditions, however, plants 

 accumulate relatively large quantities of nitrates in their tissues without any 

 toxic effects. Subsequently such accumulated nitrates may be utilized in the 

 nitrogen metabolism of the plant. Plants sometimes exhibit acute sj^mptoms 

 of nitrogen deficiency while they still contain considerable quantities of nitrates. 

 Although such plants have been able to absorb nitrates, metabolic conditions 

 within the plant have been such that they have been unable to utilize them 

 in the formation of nitrogenous organic compounds. 



As shown in the next section the first step in the utilization of nitrates 

 by plants is their reduction to nitrites. It seems probable, therefore, that 

 plants can utilize nitrites as a source of nitrogen, and this supposition has been 

 confirmed by solution culture experiments. However, nitrites are rarely if 

 ever an important source of nitrogen for plants in nature. 



Many species of plants when grown in sand or solution cultures under 

 suitable conditions develop as well or better when supplied with ammonium 

 salts as when supplied with nitrates. This is not surprising since the nitrogen 

 in ammonium compounds is in a highly reduced form similar to that found 

 in amino acids and related compounds. In certain types of soils it is probable 

 that ammonium compounds are the chief form in which nitrogen is available 

 to plants. This is apparently true of the acid podsolic soils of northern lati- 

 tudes and of many uncultivated soils in North Carolina and other southern 

 states. Such soils contain little nitrate, but considerable quantities of ammo- 

 nium compounds and plants growing in them apparently obtain their nitrogen 

 in the latter form. Unlike nitrate ions plants seldom accumulate appreciable 

 concentrations of ammonium ions. 



Even when ammonium fertilizers are applied to agricultural soils much 

 if not most of the absorption of nitrogen by plants occurs in the form of 

 nitrates. In such soils nitrification (see later) usually occurs very effec- 

 tively, resulting in rapid conversion of ammonium compounds to nitrates. 



As a result of the decay of organic remains there are present in most 

 soils at least small quantities of amino acids and other organic nitrogenous 

 compounds. There is considerable evidence that plants can absorb and utilize 

 such compounds in the synthesis of proteins. Under some conditions a con- 

 siderable proportion of the nitrogen used may be absorbed by plants in the 

 form of such compounds. 



Reduction of Nitrates in Plant Tissues. — Since in nitrates the nitrogen 

 is in a highly oxidized state ( — NO3) while in amino acids and other organic 

 compounds it is usually in a highly reduced state, it is evident that reduction 

 of nitrogen is one of the steps in the synthesis of amino acids and other 



