44 THE LAND WE LIVE ON 



have any effect of increasing the crops, do so by altering 

 the physical texture of the soil. 



But other investigators, like R. D. Hall, the present 

 director of the celebrated Rothamstead Experimental 

 Station, clearly demonstrate from numerous and con- 

 tinuous manuring experiments that the views of the Ameri- 

 can scientists cannot be considered as generally applicable. 

 A good many factors unquestionably combine to produce 



fertility, and the duty of every farmer is to maintain, and if 

 possible increase, the fertility of the soil, so as to get 



maximum crops from his ground- 

 Not only the actual amounts of available mineral 

 matters in the soil are of importance, but also the proportion 

 between them, and it has been shown quite recently by 

 Japanese scientists, Loew, Aso, Daikuhara, and others, 

 who have done a large amount of experimental work, that 

 the ratio of Lime and Magnesia are of particular importance. 

 Whitney's theory that soil becomes unfertile by tne 

 accumulation of toxic substances excreted by the roots of 

 crops, and that the fertilizer act, not as a direct plantfood, 

 but by destroying these substances and putting them out 

 of action, has been supported by the results of investigations 

 carried out in India. F. Fletcher proved by field experi- 

 ments that tliere is an actual excretion of alkaloidic sub- 

 stances by tlie roots of plants, which are toxic both to the 

 parent plant and to other species. The sensitiveness of 

 crops to the excreted toxins varies considerably. The 

 results are of great importance with regard to the rotation 

 of crops, and explain the advantages of certain rotations, 

 showing for example why cotton grows feebly near sorghum, 

 yet thrives at least as well, if not better, after sorghum than 

 after cotton. 



Each crop fouls the soil for a succession of the same 

 variet}^ The toxic substances can be precipitated by 

 mineral manures, and also by certain vegetable refuse 

 (leaves, etc.) containing tannic acid. Even before Fletcher 

 published the results of his investigations several African 

 Chemists showed that the sterility of certain soils was caused 

 by the presence of toxic organic substances, the effects 

 of v/hich could be corrected by the use of stable manure, 

 green manure, leaves of sumach, oak, etc., tannic acid and 



