69 



mciit approximates a certain amount per acre? Again, 

 does sorrel invade acid soils because it grows well un- 

 der such conditions? White (22) has shown that lime 

 greatly benefits this plant, and that it is found on the 

 very sour plots 32 because other plants failed and there 

 is no com j)eti lion. Since sorrel is not an acid loving 

 plant, but still grows where clover and other crops fail, 

 apparently, it is the tolerance of manganese, more than 

 the tolerance of acidity which permits this plant to 

 grow under such conditions. 



In his discussion of the variations in yields from the 

 acid plots at Pennsylvania, White (21) says the vari- 

 ations may be due to "increasing amounts of organic 

 acids which in the absence of sufficient basic material 

 accumulate from year to year and exert an increas- 

 ing 'toxic' effect on plant growth. The resultant effect 

 may be physical, chemical, physiological, or bacterio- 

 logical." Studies on soils from Pennsylvania and from 

 Alabama indicate strongly that the toxicity of the soils 

 used is due chiefly to soluble manganese, with possibly 

 aluminum playing a small part. Precipitation of the 

 manganese from the water extract of these soils is all 

 that is necessarj^ to make such extracts very satisfac- 

 tory mediums for plant growth. Further, there is lit- 

 tle indication that the toxicity of the Alabama soil ex- 

 tracts is due to any form of organic matter. Extracts 

 from our soils have been ashed in several experiments 

 with little or no indication of benefit, except where the 

 ash was taken up with tap water. Nor has carbon 

 black effected any marked improvement in such ex- 

 tracts. Boiling in the open, or under a reflux condens- 

 er failed to reduce the toxicit3^ The distillate from 

 the soil extracts proves to be as good medium for plant 

 growth as ordinary distilled water. These tests are 

 deemed to be sufficient to show that the toxic body is 

 inorganic, rather than organic. Of the unusual inor- 

 ganic elements found, manganese was present in great- 

 est quantity, and is very probably the chief cause of 

 toxicity'. 



The data presented on the previous pages puts spe- 

 cial emphasis on the toxicity of manganese soluble in 

 the soil solution of acid soils. However, the possibility 



