In many parts of this island, the value of land has been much augmented by the application of marl. Treat ing of this article in a practical way, it may be divided into shell-marl and earth-marl. Shell-marl is composed of animal shells dissolved ; earth-marl is a fossil. The of the latter is various ; white, black, blue, red, and its hardness is as Carious as its colour ; being sometimes soft and ductile like clay, sometimes hard and solid, like stone, and sometimes it is extended into thin beds, like slate. Shell-mart is easily distin guished by the shells, which always appear in it ; but the similarity, betwixt earth-marl and many other fossil substances, renders it difficult to distinguish them.
Shell-marl is very different in its nature from clayey and stone marls, and, from its effects upon the soil, is commonly classed among the animal manures. The Rey. Alr Dickson states, That it does not dissolve with water, as the other marls do. It sucks it up, and swells with it like a sponge. It is a much stronger attractor of acids than they." Dr llome says, that it takes six times more of acids to saturate it, than any of the other marls, which he had met with. But the greatest ence, betwixt the shell-marl and the other marls, consists in this ; the shell-marl contains oils. It is uncertain, if the other marls contain any oils ; but this kind, it is said, contains them in great plenty, This marl, it would seem, from the qualities which it possesses, promotes vegetation in all the diffe;ent ways. It increases the food of plants ; it communicates to the soil a power of attracting this !nod from the air ; it enlar ges the pasture of plants ; and it prepares tile vegetable food for entering their roots.
The Shelly sand, often found deposited in beds in the crevices and level parts of the sea coasts, is another substance capable of being employed, both as a manure and stimulant, not only on account of its containing cal careous matter in greater or less proportions, but also from the mixture of animal and vegetable substances that are found in it. The portion of calcareous matter, contained in these substances, must vary according to circumstances ; but, when the quantity is any way large, and in a reduced or attenuated state, the quality is so much the more valuable. On that account, the quantity, which ought to he applied to the soil, must be regulat ed entirely by the extent of calcareous matter, or found upon trial, to be contained in the article, which, as already said, is very variable.
The clayey and stone marls are distinguished by their colours ; viz. white, black, blue, and red. The white,
being of a soft crumbly nature, is considered to be the best for pasture land ; and the blue, which is more com pact and firm, for corn land. In the districts where marl is much used, these distinctions of management are attended to, though either of the kinds may be em ployed with advantage, if the following rules are ad hered to.
If marl is of the blue kind, or of ally kind that is com pact and firm, lay it tip(,11 the land early in the season, so as the weather medow it down before the last plough ; and, if on pasture land, let it also be early ,aid on, and spread very tuin, breaking any lumps afterwards which are not completely separated by tilt list spread ing. If mail i, of the white, or a,,v of the loose or c•um bling sorts, it need not he laid on so cal ly ; because those varieties break and dissolve almost. as 50011 as exposed to the weather.
There are many kinds of impure and mixed marls, such as sandy, clayey, loamy, and stony marls, accord ing as these varieties of soil are incorporated or mixed with the principal These sorts, of course, arc inferior to tile pure marls ; but the stony kind is considered to he the best, bee ause .ts efficacy is more lasting, though the fat and crumbling kinds enrich or operate more speedily. The hard marls, however, in every case, operate fur the greatest length of time, and are often followed with bad consequences to the soil, un less good management, with regard to cropping, is ex ercised during the period of their operation. After be ing long eXCeSSIN ly fruniul and productive, the soil will gradually h tome so sterile and barren as scarcely to be worth cultivating ; in which case, the greatest exertion can hardly procure a return of fertility. In this respect, the effect of ove•-cropping land, that has been marled, is precisely the same as takes place with lime. An uncommon exertion is made, occasioning a proportionable debility, though, were good husbandry studiously practised, the exertion would neither be so excessive, in the first instance, nor the after-conse quences so mischievous. In instances, land has been reduced so much, as to be thought little better than useless, by the effects of lime and marl. Both, however, are excellent agents in forwarding agriculture, though often their agency has been misapplied, and used for mischievous pqrposes. Under a correct rotation of cropping, and with a suitable supply of dung, neither lime nor marl is injurious. Reverse these circum stances. and the contrary effect must necessarily be pro duced.