Book III. MANURES. 333 



and experience. It should always be kept in mind, that it is one thing to produce a 

 crop, and a different thing to grow crops with profit. 



2221. The principles of rotations of crops are thus laid down by Yvart and Ch. Pictet 

 (Cotirs complet cC Agriculture, articles Assolement, and Succession de Culture ; and Traiti 

 des Assolemens. Paris, 8vo) : 



Hhe first jrrinciple, or fundamental point, is, that every plant exhausts the soil. 

 The second, that all plants do not exhaust the soil equally. 



The third, that plants of different kinds do not exhaust the soil in the same manner. 

 The fourth, that all plants do not restore to the soil the same quantity, nor the same quality of 

 manure. 

 The fifth, that all plants are not equally favourable to the growth of weeds. 



2222. The following consequences are drawn from these fundamental principles : 



First. However well a soil may be prepared, it cannot long nourish crops of the same kind in succes- 

 sion, without becoming exhausted. 



Second. Every crop impoverishes a soil more or less, as more or less is restored to the soil by the plant 

 cultivated. 



Third. Perpendicular-rooting plants, and such as root horizontally, ought to succeed each other. 



Fourth. Plants of the same kind should not return too frequently in a rotation. 



Fifth. Two plants favourable to the growth of weeds, ought not to succeed each other. 



Sixth. Such plants as eminently exhaust the soil, as the grains and oil plants, should only be sown when 

 the land is in good heart. 



Seventh. In proportion as a soil is found to exhaust itself by successive crops, plants which are least ex- 

 hausting ought to be cultivated. 



2223. Influence of rotations in desty^oying insects. Olivier, member of the Institute of 

 France, has described all the insects, chiefly Tlpula and Jlfuscae, which live upon the 

 collar or crown of the roots of the cereal grasses, and he has shown that they multiply 

 themselves without end, when the same soil presents the same crop for several years in 

 succession, or even crops of analogous species. But when a crop intervenes on which 

 these insects cannot live, as beans or turnips after wheat or oats, then the whole race of 

 these insects perish from the field, for want of proper nourishment for their larvae, 

 (il/m. de la Societe Roi/cde et Centrale d'Agr. de Paris, vol. vii.) 



Chap. II. 

 Of Manures. 



2224. Every species of matter capable of promoting the grovoth of vegetables may be con- 

 sidered as manure. On examining the constituents of vegetables, we shall find that they 

 are composed of oxygen, hydrogen, carbon, and nitrogen, or azote, with a small propor- 

 tion of saline bodies. It is evident, therefore, that the substances employed as manure 

 should also be composed of these elements, for, unless they are, there will be a deficiency 

 in some of the elements in the vegetable itself; and it is probable that such deficiency 

 may prevent the formation of those substances within it, for which its peculiar organisa- 

 tion is contrived, and upon which its healthy existence depends. The elementary bodies 

 above enumerated are all contained in animal, and the first three in vegetable, matters. 

 Sometimes, though very seldom, vegetables contain a small quantity of nitrogen. As 

 certain salts are also constantly found to be present in healthy living vegetables, manures 

 or vegetable food may, consequently, be distinguished into animal, vegetable, and saline. 

 Kirwan, Dundonald, Darwin, and Davy, who produced the first chemical treatises on 

 soils, were also the first to treat chemically of manures. Of these, the latest in the order 

 of time is Sir H. Davy, from whose highly satisfactory work we shall extract the greater 

 part of this chapter. 



Sect. I. Of Manures of Animal and Vegetable Origin. 



2225. Decaying animal and vegetable substances constitute by far the most important 

 class of manures, or vegetable food, and may be considered as to the theory of their 

 operation, their specific kinds, and their preservation and application in practice. 



SuBSECT. 1. The Theory of the Operation of Manures of Animal and Vegetable Origin. 



2226. The rationale of organic manures is very satisfactorily given by Sir H. Davy, 

 who, after having proved that no solid substances can enter in that state into the plant, 

 explains the manner in which nourishment is derived from vegetable and animal sub- 

 stances. 



2227. Vegetable and animal substances deposited in the soil, as it is shown by universal 

 experience, are consumed during the process of vegetation ; and they can only nourish 

 the plant by affording solid matters capable of being dissolved by water, or gaseous sub- 

 stances capable of being absorbed by the fluids in the leaves of vegetables ; but such parts of 

 them as are rendered gaseous, and pass into the atmosphere, must produce a compara- 



