ROTATION OF CROPS. 195 



found that the growth of comparatively small crops of 

 wheat, even with a fair share of weeds, for 15 or 20 years 

 on rich prairie land has not exhausted its fertility. There 

 will besides, for some time to come, be more rich prairie land 

 to bring under the plough. Upon the whole, it seems prob- 

 able that, with the improved methods which should result 

 from increased density of population, and with the increased 

 areas brought under cultivation, it will be longer than is 

 sometimes supposed before the capability of the United 

 States of production for export will be materially dimin- 

 ished. Obviously, somewhat similar arguments are, mutatis 

 mutandis, applicable to Canada. As, however, the resources 

 of the rest of the world, taken as a whole, show no signs of 

 diminution, it may be a question how far the range of prices 

 will affect the production in any particular country. 



SECTION V.— ROTATION OF CROPS. 

 Introduction and Historical Sketch. 



In the preceding sections attention has been devoted to 

 the consideration of the influence of exhaustion, manures, 

 and variations of season, on the amounts of produce, and on 

 the composition, of certain individual and typical crops when 

 each is grown separately year after year on the same land. 

 In this way there have been discussed the characteristic re- 

 quirements and results of growth of various cereal crops as 

 representatives of the natural order Graminese ; of various 

 root-crops of the orders Cruciferae and Chenopodiacese ; and 

 lastly, of various Leguminous crops. 



Our subject now is the — Rotation of Crops. The mere 

 numerical results of the field experiments made at Rotham- 

 sted on rotation have been recorded in the annual ' Memor- 

 anda ' ; but the first systematic discussion, either of them or 

 of the laboratory investigations undertaken in connection 

 with them, is that given in this paper, in this volume, and in 

 the Journal of the Royal Agricultural Society of England 

 (December 31, 1894) ; and although the present communi- 

 cation embodies a good deal of detail, and a somewhat com- 

 prehensive consideration of it, there still remains much which 

 could not be included within the limits of this paper. 



The practice of Rotation is admitted to be the foundation importance 

 of the improvements in our own agriculture which have taken °/ rotatwn - 

 place during this and a considerable part of the last century. 

 It is of great importance, therefore, carefully to consider, both 



