296 THEORIES OF FERTILISER ACTION [chap. 



was grown and the unmanured plots averaged 24-2 

 bushels per acre, a relatively much higher yield than 

 the Swedes had shown — yet barley had been repeatedly 

 grown on the field in the years immediately before it 

 was brought under experiment. 



As it stands at present Whitney's theory must be 

 regarded as lacking the necessary experimental founda- 

 tion ; no convincing evidence has been produced of the 

 fundamental fact of the excretion of toxic substances 

 from plants past the autotrophic seedling stage, nor is 

 there direct proof of the initial supposition that all soils 

 give rise to soil solutions sufficiently rich in the elements 

 of plant food to nourish a full crop, did not some other 

 factor come into play. If, however, we give the theory 

 a wider form, and instead of excretions from the plant 

 understand debris of any kind left behind by the plant 

 and the results of bacterial action upon it, we may 

 thereby obtain a clue to certain phenomena at present 

 imperfectly understood. The value of a rotation of 

 crops is undoubted and in the main is explicable by the 

 opportunity it affords of cleaning the ground, the 

 freedom from any accumulation of weeds, insect, or 

 fungoid pests associated with a particular crop, and to 

 the successive tillage of different layers of the soil, but 

 for many crops there remains a certain beneficial effect 

 from a rotation beyond the factors enumerated. 



The Rothamsted experiments have shown that 

 wheat can be grown continuously upon the same land 

 for more than fifty years, and that the yield when 

 proper fertilisers are applied remains as large in the 

 later as in the earlier years of the series ; any decline 

 that is taking place is hardly outside the limits of 

 seasonal variation and can easily be accounted for by 

 the difficulties of tillage and the increase of one or two 

 troublesome weeds. Mangolds, again, in the Rothamsted 



