8 Hall, How does tJie Plant obtaiji its mitrimcnt ? 



manured plots of the barley fields 4A and 7/2. To the 

 solutions from the imperfectly manured plots (1,0, 2 A, 3 a), 

 phosphoric acid and potash were added in amounts 

 required to bring the proportion of these constituents up 

 to the standard, allowance being made for the small 

 amount already present derived from the soil. 



The results of this series were in strict conformity 

 with those of the preceding series. The evidence was 

 slight for the presence in the soil solutions, even in those 

 from the dunged plot, of other substances favourable to 

 growth. 



We may now consider how far these results bear on 

 the theory that crops leave behind in the soil specific 

 toxins which depress the growth of succeeding crops of 

 the same kind. In Series I. wheat and barley yielded 

 almost exactly the same weight of plant whether they 

 grew in solutions from the wheat or barley soils. (Table 1.) 

 As a rule, the wheat plants were a little heavier when 

 grown in the solutions from the barley soils than when 

 grown in solutions from the corresponding wheat soils 

 (3 compares v/ith 1,0, 1 1 with 2A, 7 with 4A, 2 with 7/2), but 

 the barley plants were similarly heavier in the solutions 

 from the barley soils. The ratio of root to shoot is very 

 close in the two sets. Again, wheat and barley grown in 

 the same solution yield weights agreeing within the range 

 of error of such experiments. These facts alone would 

 dismiss the hypothesis that the wheat soils contain any 

 soluble toxin injurious to wheat but not to barley, and 

 vice versa, notwithstanding the sixty years' repeated 

 growth of these crops in the same soils. In Series II. the 

 demonstration was pushed a stage further by including in 

 the comparison an artificial culture solution made from 

 pure salts, and containing phosphoric acid and potash in 

 the same proportions as the solutions from the completely 



