j 4 o SCIENCE PROGRESS 



in many cases be complicated by another factor because the 

 relative infertility of a soil is often communicated to its aqueous 

 extract. Thus two soils of Cecil clay, very similar in physical 

 characteristics, show marked differences in fertility, and their 

 aqueous extracts showed like differences. But we have seen 

 that the soil solution is similar in composition in all soils 

 and direct analysis proved these two aqueous extracts to be 

 no exceptions to the rule. Hence the infertility of the poor 

 soil was not due to any deficiency of food-stuffs, but to some 

 toxic substance. Numerous other cases have been investigated, 

 and wherever the relative infertility of a soil is transmitted 

 to the aqueous extract it is supposed that some toxin is present. 

 It is further considered that the toxin is organic. 



So great was the toxicity of the solution that in some in- 

 stances it was an even poorer medium for plant growth than 

 distilled water. But Livingstone found [14] that its toxicity 

 could be lessened in various ways: by dilution, when indeed it 

 might actually increase plant growth ; and also by shaking with 

 calcium carbonate, precipitated ferric or aluminium hydroxides, 

 cotton wool, shredded filter paper or carbon black ; the solution 

 then became much improved as a culture medium. Further, 

 the addition of fertilisers, especially such as were known to 

 benefit the soil, also improved the solution : the organic matter 

 of dung and of green manure was very useful and had some 

 specific effect not shown by the inorganic constituents. In one 

 experiment wheat plants gave the following results : 



Amount of 

 Transpiration. 



I. Plants grown in soil extract alone . . 100 

 II. „ „ „ + dung extract . . 400 



III. „ „ „ + ash of dung extract ^ 



+ NaN0 3 equivalent to nitrogen lost W96 

 in preparing the ash . . . .J 



Weight of plants, tops only. 



Green. 

 IOO 

 213 



112 



Dry. 

 IOO 

 151 



IO9 



The value of the dung extract is clearly not due to its 

 ash ingredients or its nitrogen, because all these are present 

 in Series III. "That the organic matter is directly of use to 

 plants as nutrient material is not probable," say the authors ; 

 " it appears to be beneficial largely through some action on 

 the soil constituents." It is therefore considered that the in- 

 fertility of many soils is due to toxic constituents, and as these 

 are probably organic a double set of experiments was begun 

 by Schreiner and others [16-21]; the effect on plant growth 



