22 



INTRODUCTORY 



[CI.AP 



reached this lower level, it will remain for an indefinite 

 period comparatively stationary, affected only by the 

 fluctuations due to season. At Rothamsted, for example, 

 wheat has now been grown year after year on the same 

 land for sixty-five seasons, and one plot has received 

 no manure throughout the whole period. In the first 



few years the crop declined steadily, but since then little 

 or no further drop has been seen. The yield remains 

 at about I2| bushels per acre for each successive ten 

 years' average, and has considerably overtopped that 

 amount during recent favourable seasons. This yield, 

 however, of 12^ bushels of corn per acre, is only about 

 a third of that obtained on the adjacent plots receiving 

 manure every year during the same period. 



These facts lead to a new point of view : it is not 

 merely the amount of this or that plant food present in 

 the soil which must be taken into account but also 

 their mode of combination. The material may be 

 present in the soil and soluble in the acid used for 

 analysis, but yet may be beyond the reach of the plant 



