62 



explanation as to the cause of the difference in toxicity. 

 These data in a large measure confirm the suggestion 

 that it is the relative amount of soluble manganese, 

 rather than the total, that determines the toxicity of a 

 soil, or of its extract. 



Considering in detail the extracts of the soil from 

 plot 32, a photograph of which is given in Plate V, fig. 

 1, it will be seen that the extract of the original soil 

 produced abnormal, bleached tops, but a fair develop- 

 ment of roots. However, the addition of a little caustic 

 potash to this extract removed the toxicity toward both 

 roots and tops. Incubated with sulphate of ammonia, 

 soil from plot 32 gave an extract that was extremely 

 toxic. Here again, the tops were almost white, and 

 made practically no growth. The roots grew but little 

 after being placed in the solutions, lateral root devel- 

 opment being almost completely suppressed. The 

 toxicity of this extract was completely removed 

 by the addition of caustic potash. In each case 

 the manganese precipitated from the extracts 

 was left in the tumblers in which the plants 

 grew. By a close inspection of the photograph on 

 Plate VI, fig. 1, the black precipitate may be seen in the 

 bottom of the tumblers, 3, 4, 6, and 7. The precipitated 

 manganese, with roots growing in it, is ver^^ clearly 

 shown in the photograph on Plate VI, fig. 2. Insoluble 

 manganese has no toxic effect whatever, since many 

 roots were growing normall3% even where several 

 inches of root were embedded in the precipitated man- 

 ganese. In so far as can be judged by the methods 

 used, it would appear that the infertility of the sour 

 plots in the Pennsylvania rotation experiments owe 

 their infertility, in a large measure, to the presence in 

 the soil solution of relatively large amounts of man- 

 ganese. 



Does Sulfofication Develop Soluble Manganese."^ 



When sulphur is added to an unsterilized soil and in- 

 cubated, a part of the sulphur is oxided to sulfuric acid. 

 The result of such oxidation is to increase the acidity 

 of acid soils. Ames and Boltz (1) found a decided 

 increase in the .water soluble aciditj^ of soils to which 

 sulphur had been added. Shedd (16) found that the 

 addition of large amounts of sulphur enormously in- 

 creased the acidity of the soil, even where lime had been 

 applied at the late of 4,000 pounds per acre. Where 

 large amounts of sulphur had been added, plants eith- 



