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Scotland to preserve or increase the fertility of the country, inwhich great advancement has been made, in the course of thelast thirty years, and the annual product of the country consi-derably increased.
Sect. III.
Of the Classes to luhich the various Substances employed as Manures•properly belong, their Properties and Effects.
Though all those who have treated of agricultural subjectshave attempted to arrange manures under different classes, it isa matter in which some difficulty seems always to have beenfound. Some have classed them according to the manner theyare supposed to operate; others according to the origin fromwhence they are derived. According to the first mode, it hasbeen very common to arrange all substances regarded as manuresunder two classes, viz. nourishing and stimulating. This arrange-ment seems to be very imperfect; for though we may form apretty distinct notion of any substance which nature has adaptedto the purpose of increasing the size of another, by being gradu-ally added to it, and if the latter be possessed of animal or ve-getable life, we may with propriety say it is nourished by theformer; yet not being instructed whether it is the soil which isstimulated, or the plants which grow upon it, and unable to dis-cover how this hidden operation can be conducted, we must findstimulation a mystery beyond the grasp of ordinary understand-ings. To arrange manures under the classes of natural and ar-tificial, is not less vague and unsatisfactory, not coming undereither of the above general modes of classification. The ingeni-ous Dr Dickson, in his work on Scotch Husbandry, * classesmanures as follows.
1st, Such as communicate to the soil the vegetable pabulumwhich they contain.
2d, Such as enable it to attract vegetable food from the at-mosphere.
3d, Such as give a more unlimited range to the roots of plantsin quest of food.
4th, Such as prepare the food lodged in the soil for enteringthe roots of plants.
This is a more specific classification of manures, respectingthe manner they are supposed to operate. But since the fanci-ful doctrine of salts and oils being attracted from the atmospherehas been exploded, it is doubtful if the atmosphere contains anymatter which substances used as manures attract and communi-cate in support of vegetation. The second class may thereforebe omitted; and, reversing the order, the classification, accordingto this view, might be as follows.
* Page 358, et seq.