RELATION OF PLANT GROWTH TO CONCENTRATION 73 



must also be present. Of these, iron is usually present in barely 

 appreciable concentration and more than this is not desirable, 

 or is even harmful for common crop plants. Under the normal 

 conditions for soils in humid areas, sulphur also is usually present 

 in scarcely more than appreciable quantities and there is no 

 positive evidence to show that higher concentrations are especially 

 desirable, though this may be the case for certain crops, such for 

 instance as the onion. Phosphorus is usually present to the 

 extent of 5 or 6 parts per million of phosphoric acid (P 2 O 5 ), 

 while it has repeatedly been shown that such crops as wheat 

 can thrive and make a good growth with a concentration a 

 tenth of this. It appears to be clear therefore that as far as 

 food supply is concerned there is normally an ample supply of 

 phosphorus in the soil solution; but it does not follow that 

 increasing the concentration of the solution if only temporarily 

 would not result in favorable effects upon growing plants. 



A consideration of the bases, however, introduces serious diffi- 

 culties, which will probably require much further research by 

 the plant physiologist as well as the soil chemist. It is impossible 

 as yet to determine the concentrations at which different plants 

 will not grow. It is even impossible to determine the concentra- 

 tions at which they will thrive best. It seems certain that differ- 

 ent crop plants require different amounts of these minerals, but 

 whether or not they require different concentrations of the con- 

 stituents in the nutrient solution for their several best growths 

 is yet not clearly shown. It now seems probable that to some 

 extent at least these basic mineral nutrients can replace one 

 another for the plant's metabolism. It has been shown in the 

 case of certain lower plant organisms that potassium can be 

 more or less successfully replaced by rubidium and caesium, and 

 in the case of some higher plants, possibly calcium, magnesium 

 and potassium can partially replace one another. 1 In spite of 

 the fact that sodium as well as potassium is a necessary con- 

 stituent for the metabolism of higher animals which feed upon 



1 For a more detailed discussion of this subject, and the functions of 

 the several ash constituents in plant nutrition, see : The physiology of 

 plants, by W. Pfeffer, translated by A. J. Ewart, vol. i, p. 410, et seq., 1900. 

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