PLANT FOOD AND PLANT GROWTH 43 



during the life of the plant; for some of the potassium that enters 

 the plant roots as nitrates, phosphate, or sulfate is afterward 

 found in the plant in organic salts, as tartrate (in grapes) , oxalate 

 (in sorrel), etc. There appears to be little or no evidence that any 

 living organic compounds of potassium or magnesium exist. 



It is the common belief that potassium has large influence over 

 the formation of carbohydrates; but the information is not suffi- 

 cient to determine whether this influence is direct or very indirect, 

 as in maintaining the general health of the plant by having some 

 absolutely necessary part in reactions involving the transference 

 of nitrogen or phosphorus from inorganic compounds to the living 

 organic combination. 



The potassium contained in plants is in large part very easily 

 removed by leaching with water, and hence peaty swamp soils 

 consisting largely of organic matter are frequently very deficient 

 in potassium. While potassium and magnesium are required by 

 plants in very considerable amounts, as stated, and as shown in 

 Table 2, yet, when measured by the average composition of the 

 earth's crust and by average crop requirements, the supply of these 

 two elements is very great. 



Calcium and iron. These elements are absolutely essential to 

 the normal growth and development of all agricultural plants, 

 but for the grain crops the amounts positively necessary are so 

 extremely small and the quantities present in the earth's crust are 

 so extremely large that it is rarely that either calcium or iron is 

 furnished to such plants in amounts insufficient to perform their 

 essential functions, except, of course, when they are artificially 

 withheld, as in investigational work. Legume plants are a very 

 marked exception, however, so far as calcium is concerned. 



Iron evidently has some important connection, direct or indi- 

 rect, with the formation of chlorophyll (the green coloring matter 

 of leaves) ; for, if iron is withheld from the plant, the leaves do not 

 become green, and if later iron is supplied, the chlorophyll soon 

 begins to develop. On the other hand, analysis has shown that the 

 chlorophyll itself does not contain iron, and the somewhat common 

 assumption that the green color of plants is due to the presence of 

 iron compounds of that color is incorrect. 



The iron held in nuclein compounds is not dissolved out by dilute 



