376 NORMAN 



nitrate, do not enter into exchange with clays, because they are unable to 

 replace lattice hydroxyls. 



Uptake mechanisms in plants. Up to this point the discussion has dealt 

 primarily with the forms in which the major plant nutrients are present in 

 soils, and hardly at all with the mechanism by which these nutrient ions are 

 taken up by plants. The assumption has been made that, if an essential 

 nutrient ion is present in the root zone, it can readily enter the plant. The 

 mechanism of nutrient uptake has turned out to be a much more difficult 

 problem than was thought earlier to be the case, and although much progress 

 has been made recently, especially through the use of radioisotopic tracers, 

 explanations wholly acceptable to all investigators still remain to be developed. 

 For reasons of simplicity uptake experiments are almost invariably carried 

 out in simple salt solutions, and frequently at ionic concentrations consider- 

 ably greater than encountered in the soil solution. Such experiments, though 

 informative up to a point, are likely to be incomplete. Furthermore, experi- 

 ments on uptake carried out with whole plants inevitably also became trans- 

 port experiments in which it is difficult to separate those processes involved 

 in primary entry from those involved in subsequent movement of the ions 

 that have been taken up. 



There are certain generalizations about the uptake of ions by plants that 

 should be made. 



1. Ion uptake is selective, but very imperfectly so. Ions are not taken up 

 strictly in proportion to the relative concentrations present in the root environ- 

 ment, nor is uptake limited to those ions that are essential for plant growth. 

 Non-essential ions can be taken up in substantial amounts and may compete 

 with the uptake of essential ions. 



2. Essential ion uptake is not related to the immediate or total quantitative 

 requirement of the plant. The amount which enters is greatly influenced by 

 the supply in the environment, not by the needs of the plant. There may be 

 uptake greatly in excess of the requirement for optimum growth. This phe- 

 nomenon is known as "luxury consumption." Luxury consumption is, how- 

 ever, a relative matter. In practice the supply of one or more critical elements 

 may limit growth; the remainder then may be in the zone of luxury con- 

 sumption. If the supply of the limiting nutrient is later increased, the cir- 

 cumstance with respect to those formerly in excess may be quickly changed. 



3. Ion uptake by different species of plants given the same nutrient en- 

 vironment is often very different quantitatively. This fact has to be accom- 

 modated in any theory of the mechanism of uptake. 



4. Continued ion uptake and accumulation is a property only of living 

 and respiring roots. If added to a nutrient solution, certain metabolic in- 

 hibitors, which halt cellular respiratory processes, will prevent ion uptake, as 

 will withdrawal of oxygen or depression of temperature. 



5. The mechanisms of uptake of cations and anions are not identical, nor 



