CHAPTER XI 

 SURPLUSES AND DEFICIENCIES 



Though much has been written on the function of individual mineral 

 constituents, it is questionable whether definite roles can be assigned 

 to them, except in so far as they enter into the composition of specific 

 organic compounds that have known functions. Thus magnesium is a 

 component of the chlorophyll molecule, which is essential for photosyn- 

 thesis. It is important, nevertheless, to know the effects attending a 

 surplus or a deficiency of one or more mineral elements, so that the symp- 

 toms may be recognized and the condition corrected. However, patho- 

 logical conditions found to follow an excess or deficiency of any one 

 element do not necessarily indicate a direct relation of the element to 

 the symptoms. Thus, though a deficiency of iron is known to produce 

 chlorosis, a disordered condition in which chlorophyll does not develop, 

 iron does not occur in the chlorophyll molecule. 



From the considerations in the previous chapters, it follows that 

 either a surplus or a deficiency of any soil element may affect the plant 

 by disturbing the balance between its various constituents. A defi- 

 ciency of an element may also affect the plant when that element is a 

 limiting factor of growth. In all cases, a surplus or a deficiency must be 

 understood to mean an amount greater or less than that which is utilized 

 along with the other elements of the soil. The effect of a surplus of any 

 essential soil constituent must be upon the balance or equilibrium of the 

 plant. There may be no effect, since the plant may adjust itself to a 

 surplus which is merely tolerated. There is much evidence that the 

 quantities of potassium and calcium in plant tissues are often much in 

 excess of the amounts used in metabolism. The same undoubtedly 

 holds for other essential and many non-essential elements such as sodium, 

 chlorine, aluminum and sihcon. On the other hand, distinct pathological 

 conditions may ensue which lead eventually to the death of the plant. 

 Likewise elements which are not essential to the nutrition of the plant 

 may be tolerated or they may produce disturbances, the effects of which 

 may be either to stimulate assimilation or to induce pathological con- 

 ditions and eventually death. As a general physiological theorem, it 

 may be stated that any substance which is toxic in certain amounts is 

 stimulating in smaller amounts. 



SURPLUSES 



The evidences for the existence of pathological conditions due to the 

 absorption of a surplus of some soil nutrient are practically limited to the 

 cases of nitrogen and magnesium. 



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