NUTRITIVE FUNCTIONS OF SALTS 111 



This table shows us that seeds are relatively high in magnesium 

 and phosphorus while deficient in silicon and calcium. Fruits 

 are rich in potassium and iron. Stems and leaves are especially 

 rich in calcium, but stems in addition contain much silicon and 

 leaves, much iron. Roots and storage organs are richer than 

 other organs in their potassium content. 



These differences are correlated with the amount of potash, 

 nitrogen, and phosphorus removed from the soil, as the following 

 table illustrates (adapted from III. Ag. Exp. Sta. Bui. No. 123 

 and the N. Y. Exp. Sta. Bui. No. 265) : 



n roTi Yield Amount in 'pounds removed per acre 



P Nitrogen Phosphoric acid Potash 



42.6 7.2 7.8 



25.6 5.9 41.50 



48.0 6.0 47.7 



15.0 2.4 21.0 



42.0 8.7 60.0 



2. The nature of the soil. The following table (after Duggar) 

 shows how soils may vary in their content of the critical elements: 



These figures indicate the number of pounds per acre in the upper 

 seven inches of soil, but the total amount of an element present 

 is not a true index of its nutrient value. The salts of the essential 

 elements must not only be present but they must be present in 

 an available form. A chemical analysis is thus not a true guide to 

 the fertility of the soil. Iron is more available in the ferric than 

 in the ferrous condition, although it may be used in the ferrous 

 state and even in such compounds as potassium ferrocyanide. 

 The availability of the various forms of nitrogen has been dis- 

 cussed in the previous chapter. Sulphur is used only in the form 

 of sulphates and phosphorus is more available in the phosphate 

 form than in any other. Duggar (1923) has made some interest- 

 ing nutritional studies using salts which are only very slightly 

 soluble and which thus gradually furnish the plant with the nu- 

 trient elements. The roots of plants secrete carbonic acid and 



