ACTION OF CHEMICAL SUBSTANCES IN THE SOIL. 327 



3°. By adding to the soil those substances which are fitted to become 

 the food of plants. This is what we do in strictly manuring the soil — 

 though we are as yet unable in many cases to say whether that which 

 we add promotes vegetation by actually feeding the plant and entering 

 into its substance — or only by preparing food for it. There is reason to 

 believe, however, that many substances, such as potash, soda, &c., act 

 in several capacities, — now preparing food for the plant in the soil, now 

 bearing it into the living circulation, and now actually entering into the 

 perfecf substance of the growing vegetable. In order to steer clear of 

 the difficulty which this circumstance throws in the way of an exact 

 classification of the chemical substances applied to the soil, I shall con- 

 sider generally under the name oi manures^ all those substances which are 

 usually applied to the land for the purpose of promoting vegetable growth ; 

 whether those substances be supposed to do so directly by feeding the 

 plants, or only indirectly, by preparing their food, or by conveying it into 

 their circulation. 



Manures, then, in this sense, are either simple, like common salt and 

 nitrate of soda, or they are mixed, like farm-yard manure and the nu- 

 merous artificial manures now on sale. Or, again, they consist of sub- 

 stances of mineral, of vegetable, or of animal origin. The latter is the 

 more natural, and is by far the most useful, classificati9n. We shall, 

 therefore, consider the various substances employed in improving the 

 soil — or what is in substance the same thing, in promoting vegetation,— 

 in the following order : — 



1°. Mineral manures — including those substances, whether simple or 

 mixed, which are of mineral origin, or which consist entirely of inor- 

 ganic or mineral matter. Under this head the use of lime and of the 

 ashes of plants will fall to be considered. 



2°. Vegetable manures. — These are all of natural origin, and are all 

 mixtures of organic and inorganic matter. 



3°. Animal manures, which are also mixtures, but, owing to their im- 

 mediate origin, dilTer remarkably in constitution from vegetable sub- 

 stances. 



§ 1. Of mineral manures. 



Mineral manures may be conveniently considered under the two heads 

 of saline and earthy manures. 



A. SALINE MANURES. 



1^. Carbonate of potash. — This substance, in the form either of crude 

 potash or of the pearl-ash of the shops, has hitherto been considered too 

 nigh in price to admit of its extensive application in the culture of tlie 

 land. 



2°. Carbonate of soda. — This remark, however, does not apply to 

 the carbonate of soda (corhmon soda of the shops), 'which is sufficiently 

 low in price (dfill a ton) to allow of its being applied with advantage 

 under many circumstances. In the case of grass-lands, which are over- 

 run with moss — of such as abound largely in vegetable matter or in 

 noxious sulphate of iron — a weak solution applied with a water-cart 

 might be expected to produce good results. It might be applied in the 

 sai«e way to fields of sprouting corn, or in fine powder as a top-dressing 



