ACIDITY 323 



plant. One interesting phase of the problem is the relation 

 between the acidity of the soil and the kind of plant that grows 

 in it. There is a tradition among farmers that moss or sorrel, 

 the red-flowered Rumex acetosella, or yellow-flowered Oxalis 

 stricta in a field is a sure sign that the soil is "sour," but the 

 studies of Wherry involving many tests of hydrogen-ion concen- 

 tration show that these plants do not correlate with high acidity 

 at all. The soil reaction may be alkaline, to pH 8, even where 

 the growth of sorrel is luxuriant. These plants indicate sterility, 

 not acidity. However, other plants, e.g., bluets (Houstonia 

 coerulea), are very rare on neutral soil and almost always appear 

 in a field where the rain has leached out most of the surface 

 alkalies and an acid condition is beginning to develop. 



Chlorosis of plants, a disease involving loss of chlorophyll and 

 resulting discoloration of the leaves from the normal green to a 

 sickly yellow, has long been known to be due to a deficiency in 

 iron, which functions in the plant as a catalyst in the manufactur- 

 ing of chlorophyll. To supply this deficiency, plants were 

 sprayed with iron chloride, and the disease disappeared. But 

 it was later found that there is usually sufficient iron in the soil 

 and even in the pale yellow leaves themselves — indeed, chlorotic 

 leaves sometimes have more iron in them than normal ones. 

 Apparently, therefore, some factor other than iron deficiency is, 

 or may be, the cause, yet spraying with iron chloride does help. 

 It was observed that chlorotic plants usually grow upon alkaline 

 soil. Nowhere is this better seen than in the Crimea, where 

 the soil is highly alkaline, and chlorosis in fruit trees is very 

 common. The natural conclusion is that alkalinity and not iron 

 deficiency is usually the cause of chlorosis. Adding acid, and 

 not iron, to the soil or leaves is, therefore, the correct thing to do. 

 Addition of acid makes the iron available to the plant. Spraying 

 with iron chloride does not add needed iron but simply makes 

 the leaves more acid and thus makes the iron, already present in 

 sufficient quantity, available. While this is perfectly true, it is 

 not the whole story. 



Chlorosis of plants may occur on neutral soil where the disease 

 is a pure case of iron deficiency. On the other hand, there may 

 be an absence of chlorosis on alkaline soil (where plants are 

 usually chlorotic). The healthy plant succeeds here because the 

 iron is available not as a salt like iron chloride, as is ordinarily 



