PRINCIPLES OP AGRICULTURE. 143 



211. Now. by following out this system, and ascertaining 

 what Plants form the most abundant, and at the same time the 

 most nutritious excretions, and what are the others which are 

 moat benefited by these, keeping in view, also, the nature of 

 the mineral ingredients they may respectively require, the agri- 

 culturist may hereafter be able to dispense almost entirely with 

 artificial manure, as he has already done with the fallowing 

 system ; for he will only have to adapt his rotation to the par- 

 ticular soil, and then the excretions of one plant will serve as a 

 manure to the other. Among the most useful for this purpose 

 may be mentioned Lucern, which is remarkable for the extensive 

 ramification of its roots, and the strong development of its leaves, 

 and which requires but a small proportion of inorganic matter. 

 This plant produces an abundant secretion from its roots, which, 

 in the course of several years adds considerably to the quantity 

 of vegetable matter in the soil, whilst its leaves serve as nutri- 

 tious food for cattle. 



212. The subject of the foregoing Chapter has been treated 

 in more detail than may seem consistent with the plan of this 

 work ; because it is, of all departments of Vegetable Physiology, 

 the one of most importance to the well-being of Man. It can 

 scarcely be doubted that, by improvements in the art of Agri- 

 culture, the quantity of food for man and beast produced in this 

 country, and the amount of those valuable articles which are 

 cultivated in the various colonies of Great Britain, may be 

 greatly increased. But these improvements cannot be carried 

 into advantageous operation, until correct ideas on the subject 

 have been generally diffused among those, who are concerned in 

 the work ; for, unless the principles on which they are founded 

 are properly understood, it is more than probable, that loss 

 instead of gain will result, from the attempt to introduce them. 



213. These principles may be thus recapitulated. 



I. The soil should be of such a character as to afford a steady 

 supply of moisture to the roots, and to allow the air to penetrate 

 it freely ; if it does not possess these requisites, it should be im- 

 proved by tempering (. 176 8). 



II. In order to produce that luxuriant growth of plants whicli 



