520 



Mr. A. D. Hall 



[May 24, 



inactive at low temperatures, and the sugar was probably being 

 wholly taken by the Streptotlirix, etc., which are less affected by cold. 

 As these organisms must also obtain nitrogen, they were robbing 

 the barley of the small stock available in the soil, and so bringing 

 about the observed reduction of crop. A cliange was accordingly 

 made in the time of application of the sugar, which was put on as 

 soon as the barley had been harvested when the soil still retained its 

 summer heat, and the change was immediately followed by an increase 

 in the succeeding barley crops, as compared with the non-sugar plots, 

 that was as marked as the deficiency had been previously. This illus- 

 trates the many pitfalls which attend investigations in agricultural 

 science. Under laboratory conditions one can define the issue sharply, 

 but as soon as the experiments are extended to the open ground and 

 hving plant, so many extraneous and unsuspected factors come into 

 play, that what is popularly called a conflict between theory and 

 practice often becomes apparent. 



We may now take a more complex example from the Rothamsted 

 plots to illustrate what I have called the conservative systems of farm- 

 ing. One of the fields is farmed on a four-course rotation of turnips. 



barley, clover, wheat, but over half the field the clover is replaced by 

 a year's bare fallow. Further, if we confine our attention to the one 

 plot which never gets any nitrogen, but only mineral fertilizers, it is 

 again divided at right angles into plots from which the turnip crop is 

 wholly removed, and others on which it is returned, as so often occurs 

 in practice when the turnips are eaten off in aiiu by sheep. 



The above table shows the average yield on these plots and also 

 the changes in the nitrogen content of the soil at different dates. 



