THE BIOLOGICAL CONDITIONS IN THE SOIL 245 



different crops were grown continuously on the same plots, 

 and the yields compared with those obtained when the same 

 crops were shifted from one plot to another, so that no crop 

 ever followed another of the same kind. No manure was 

 supplied. The results showed a gradual decrease in the yield 

 in almost every instance, and the decrease was generally 

 greater when the crop was repeated year after year on the 

 same plot than where it was shifted from one to another. 

 Nevertheless the difference between the yields in the two cases 

 was not sufficient to justify any assumption of the existence 

 of a toxin, except perhaps in the case of Euphorbia lathy ris ; 

 in the other seventeen cases it was attributed to the more 

 rapid removal from the continuous plots of the mineral 

 nutrients required by the plant. This explanation was 

 supported by analyses of the plant ash and of the soil — 

 analyses which led to the important distinction between 

 "available" and "unavailable" plant food. 



Pot experiments made by the writer at Rothamsted have 

 led to the same conclusions. Six crops of rye were grown in 

 succession in sand to which only nutrient salts were added so 

 as to maintain the food material at a constant amount. A 

 seventh crop was then taken and at the same time a crop was 

 grown on perfectly fresh sand, on which no crop had ever 

 grown before, but which was supplied with an equal amount 

 of the same nutrient salts. There was no significant difference 

 in the two crop yields. A similar experiment was made with 

 buckwheat, another with spinach, and a parallel series was 

 made in soil cultures. In all cases but one the result was the 

 same ; the 1910 weights were as follows : — 



Both sand and soil contained 2 per cent, of calcium carbonate. 



