50 



dilutions used were not high enough to materially re- 

 duce the toxicity toward root development. With 50 

 per cent soil extract and 50 per cent distilled water, 

 or with 25 per cent soil extract and 75 per cent dis- 

 tilled water, the solutions were toxic enough to serious- 

 ly reduce root growth. On the other hand, with ap- 

 proximately 50 per cent or 75 per cent of the manga- 

 nese precipitated, good root growth was obtained. Re- 

 duced toxicity of the cultures in which a part of the 

 manganese precipitated is probably due to the anti- 

 dotal action of the potassium salts formed in the cul- 

 ture medium. See Table VI for the data, and Plate IV, 

 figs. 1 and 2 for the photographs of these cultures. 



Table VI. — The Effect of Precipitation and of Dilution 



on the Toxicity of a Soil Extract Containing 15.8 



p. p. 777. of Al., and 80.0 /;. p. m. of Mn. in 



Solution. 



Culture Medium 



Av. air dry 



weight of 



Roots 



Tops 



Tap water 



Distilled water _ 



Soil extract 



Soil extract plus 

 Soil extract plus 

 Soil extract jjIus 

 Soil extract plus 

 Soil extract plus 

 Soil extract plus 

 Soil extract 

 Soil extract 

 Soil extract 



0.6 

 1.2 

 1.8 

 2.4 

 3.0 



cc. 

 cc. 

 cc. 

 cc. 

 cc. 



N/1 

 N/1 

 N /I 

 N/1 

 N/1 



NaOH 

 NaOH 

 NaOH 

 NaOH 

 NaOH 



Second leaching 



CaO 

 75 per cent, dist. water 

 50 per cent, dist. water 

 25 per cent, dist. water 



25 per cent 

 50 per cent 

 75 per cent 



.137 

 .109 

 .088 

 .145 

 .157 

 .162 

 .144 

 .179 

 .170 

 .097 

 .093 

 .119 

 .154 



.234 

 .189 

 .222 

 !317 

 .269 

 .293 

 .259 

 .317 

 .318 

 .279 

 .276 

 .317 

 .267 



Even though a good top growth was obtained, there 

 was practicallv no root development in the untreated 

 soil extract. The addition of 0.6 cc. N/1 NaOH re- 

 duced the toxicity slightly; but this amount was not 

 as effective as 1.2 cc. Beyond this point, in- 

 creased amounts of alkali were not proportionately ef- 

 fective. Tests made on these culture solutions after 

 the plants were taken out, showed that only a part of 

 the manganese had been precipitated. This experiment 

 strongly indicates the ])rotective action of the sodium 

 and the calcium salts formed with a part of the acid 

 radicle formerlv held bv manganese and aluminum. 

 Considermg both the precipitation and the dilution 



