TOXIC EXUDATES AND SOIL TOXINS. 157 



taining phosphoric acid and potash in the same proportions as the solutions 

 from the completely manured plots. Another set of the soil solutions was 

 boiled before use, since boiling had been reputed to destroy the toxin and 

 would at any rate kill off any bacteria that might be factors in the result. 

 Lastly in another set the solutions were evaporated, the residue ignited and 

 dissolved afresh in a minimum quantity of hydrochloric acid, then diluted 

 to the original volume. 



"In this series boiUng was without effect, whether the solutions contained 

 added nutrients or not; the residue left on evaporation, after ignition and 

 re-solution, gave generally lower results, in some cases to a marked degree. 

 The soil solutions from completely manured plots gave higher yields than the 

 artificial solutions of corresponding strength. In order to ascertain whether 

 the results were limited in any way by the nature of the plant (it might be 

 objected as regards Series I that barley and wheat are so closely akin as to 

 excrete the same toxin) the experiments in Series II were repeated with 

 sunflowers, white lupins, and buckwheat. 



"These plants are far from being so suitable for experiment as barley, 

 and the results are somewhat erratic (e. g., white lupins gave almost their 

 maximum yield in the solution from the unmanured plot, indicating that 

 growth had been mainly sustained on the original food-store in the seed), 

 but they in no way indicate the presence of a toxin in the soil solutions which 

 depresses the growth of barley, but ex hypothesi is without effect on plants 

 of another order. Finally in Series III, both barley and peas grew as freely 

 in the soil solutions from the completely manured plots and in the solutions 

 from the incompletely manured plots after repair of the deficiency by adding 

 salts, as in the artificial solutions made up with pure salts. Indeed the 

 superiority, though hardly large enough to be significant, lay with the plants 

 grown in the soil solutions. Thus the experiment yielded no evidence of the 

 existence in soils on which a particular plant had been growing for 60 years 

 and upwards, of a soluble 'toxin' having a depressing effect upon the growth of 

 that plant." 



Lyon and Bizzell (1913 : 38) have conducted a series of experi- 

 ments on the stimulating influence of plants on each other. These 

 were made by planting primary plants or crops, followed by secon- 

 dary ones at a somewhat later time. In the case of greenhouse soil 

 and of nutrient solutions in crushed quartz, the yield of the primary 

 crop in mixtures was greater than when it was grow^n alone, in just 

 the same number of cases as it was less. When the primary crops 

 and mixtures were grown on field soil, the yield of the primary crop 

 and the mixture w^as greater in 11 cases and less in 4. Moreover, it 

 was found that the so-called stimulus is stronger during the early 

 part than during the later part of the life of the plant. This is indi- 

 cated by the field experiment, in which nearly all of the primary crop 

 harvested at bloom gave a larger yield in combination than alone, 

 while similar mixtures allowed to mature, gave opposite results. 



Bottomley (1914 : 531) has found that "bacterized" peat acts as 

 a stimulant to growth, and that phosphototungstic and silver frac- 

 tions derived from it show the effect of the accessory food factors of 

 Hopkins and to some extent of the vitamines of Funk. He concludes 

 that the nutrition of a plant may depend upon the presence of these 



