250 THE ROTHAMSTED EXPERIMENTS. 



Removal or It is true, also, that, under any specific rotation, there may 

 sumption °e deviations from the plan of retaining the whole of the root- 

 of crops. crop, the straw of the grain crops, and the leguminous fodder- 

 crops, on the farm, for the production of meat or milk, and, 

 coincidently, for that of manure to be returned to the land. 

 But it is also true that, when under the influence of special 

 local, or other demand — proximity to towns, easy railway or 

 other communication, and so on — the products which would 

 otherwise be retained on the farm are exported from it, the 

 import of town or other manures is generally an essential 

 condition of such practice. Indeed, this system of free sale 

 very frequently involves full compensation by purchased 

 manures of some kind. In our own country, such deviations 

 from the practice of merely selling grain and meat have been 

 much developed in recent years ; and they will doubtless con- 

 tinue to increase under the altered conditions of our agriculture, 

 dependent on very large imports of grain, increasing imports 

 of meat and other products of feeding, and very large imports 

 of cattle-food and other agricultural produce. Already much 

 more attention is being devoted to dairy products, not only 

 on grass farms, but on those that are mainly arable ; and there 

 will doubtless be some, but probably by no means so great 

 an extension as some suppose, in the production of other 

 smaller articles required by town populations. 

 Excep- It is further true, though the remark applies in a very 



tionairota- limited degree to our own country, that there are other devia- 

 tions which have more the character of exceptions to the 

 general rule of rotation, such as the introduction of flax, hemp, 

 tobacco, or other so-called industrial crops. But, in these 

 cases, as with potatoes, the growth involves special expenditure 

 for manure instead of conservation of it. Indeed, the induce- 

 ment is the high price of the product, rather than the main- 

 tenance, or the improvement, of the condition of the land for 

 future crops. 

 Self-sup- Still, as such deviations from regular rotation practice as 

 porting have been referred to, do, as has been said, generally involve 

 more or less, and frequently full, compensation by manure 

 from external sources, we may, in endeavouring to explain 

 the benefits which accrue from the practice of rotation, confine 

 attention, for the purposes of illustration, to what may be 

 called the self-supporting system, and to the simple four-course 

 one which has been selected for investigation at Eothamsted, 

 and from the results relating to which the illustrations which 

 Mineral nave been brought forward have been drawn. 

 constitu- It will be well first briefly to refer to the evidence relating 



ents ™ to some of the more important mineral constituents found in 

 crops. the different crops of the four-course rotation. 



