476 NATURE AND PROPERTIES OF SOILS 



Pota sh is essential to starch formation, either in photo- 

 synthesis or in translocation, and is necessary in the develop- 

 ment of chlorophyll. It is important to cereals in grain for- 

 mation, giving plump heavy kernels. As with phosphorus, it 

 may be present in large quantities in the soil and yet exert 

 no harmful effect on the crop. While potassium and sodium 

 are similar in a chemical way, sodium cannot take the place 

 of potash in plant nutrition. Where there is an insufficiency 

 of potash, however, sodium seems in some way, either directly 

 or indirectly, to be useful. 1 



268. The element in the "minimum." — In connection 

 with the obvious importance of utilizing, for any particular 

 soil and crop, a fertilizer well balanced as to the three primary 

 elements, two queries naturally arise. These are: (1) What 

 are the proper proportions of nitrogen, phosphoric acid, and 

 potash to apply under given conditions? (2) What would 

 be the effect if any one of these should not be present in suffi- 

 cient quantity as to make it equal in function to the others? 



The first query cannot be disposed of until the question of 

 fertilizer mixtures has been considered. The second, how- 

 ever, is not affected by so many factors, and is more clearly 

 a question of the function of the elements concerned and is 

 logically discussed at this point. 



Any element that exists in relatively small amounts as com- 

 pared with the other important nutrient constituents natur- 

 ally becomes the controlling factor in plant development. 

 Any reoluction or increase in this element will cause a_co rre- 

 sponding reduction or increase in the crop yie ld. This ele- 

 ment, then, is said to be " in the minimu m." In fertilizer 

 practice, ideal conditions would exist if no constituent func- 

 tioned as a decided minimum and the entire influence of each 

 single element was fully utilized. In other words, the fertil- 

 izer would be balanced as to its relationship to normal plant 



J Hartwell, B. L., and Damon, S. C, The Value of Sodium when 

 Potassium is Insufficient; E. I. Agr. Exp. Sta., Bui. 177, 1919. 



