SURPLUSES AND DEFICIENCIES 199 



Sulphur. — As a result of sulphur deficiency, cell division is retarded 

 and fruit development is suppressed, '^^^ but the plant is able to develop 

 vegetatively to a limited extent. 



Iron. — A lack of iron produces the well-known condition of chlorosis 

 or yellows. This is not characteristic solely of iron want, for it may 

 result eventually from a lack of either nitrogen or magnesium, but the 

 effects of iron deficiency in producing chlorosis are more rapid than 

 those of nitrogen insufficiency and consequently more striking. When 

 iron is deficient, developing leaves are at first able to avail themselves 

 of iron in older tissues. Later the new leaves are green only at the tips 

 and eventually the newly developed leaves are entirely yellow. Chloro- 

 plasts develop in them, but they contain no chlorophyll. Recent 

 investigation^*^ has shown that organic compounds containing the pyrrol 

 ring, which appears in the structure of chlorophyll, correct the condition 

 of chlorosis produced by iron want, suggesting that iron may have 

 something to do with the formation of this ring. However, since iron is 

 just as essential for fungi and other parasitic plants which have no chloro- 

 phyll as for green plants, its importance cannot be limited to the part 

 it apparently plays in the synthesis of the pyrrol ring. 



Magnesium and Calcium. — A deficiency of magnesium^" reduces 

 fruit formation and eventually produces chlorosis. This is to be expected 

 since magnesium is a constituent of chlorophyll. Cell division in the 

 epidermis is also affected. 



A lack of calcium interferes with carbohydrate transportation and 

 utilization, but does not stop its manufacture. These disturbances 

 may be associated with the formation by calcium of insoluble salts with 

 substances which are products of carbohydrate utilization, as oxalic 

 acid. A lack of calcium would result in an accumulation of oxalic 

 acid, which is toxic in solution. This might be expected to interfere 

 with the processes of carbohj^drate utilization. Root growth is retarded 

 or stopped, an effect already mentioned as resulting from an excess 

 of magnesium. Hence some of the effects of calcium deficiency may 

 be associated with the resultant effect on the calcium-magnesium ratio. ^^^ 



Chlorine. — Though chlorine is not an essential element, some mention 

 should be made here of the effects of an absence of chlorine. There 

 are conditions in the field under which the best development occurs 

 only when chlorides are added to the soil."" Recent investigations show 

 that the effects of chlorides are markedly different on different plants, 

 but that in many cases they serve directly or indirectly as a fertilizer. ^^^ 



AN ANALYSIS OF THE FERTILIZER PROBLEM 



The data that have been presented on the factors affecting soil 

 productivity on the one hand and the metabolic processes going on within 

 the plant on the other, emphasize the incompleteness of the knowledge 



