The improvement of the land, therefore, by the planting of trees, depends in part upon the quantity of _organic_ food which the trees can extract from the air, and afterwards drop in the form of leaves upon the soil, and in part upon the kind and quantity of _inorganic_ matter which the roots can bring up from beneath, and in like manner strew upon the surface. The quantity and quality of the latter will, in a great measure, determine the kind of grasses which will spring up, and the consequent value of the pasture in the feeding of stock. In the larch districts of the Duke of Atholl, the most abundant grasses that spring up are said to be the holcus mollis and the holcus lanatus, (the _creeping_ and the _meadow_ soft-grasses.) 2. _Laying down to grass._—On this point two facts seem to be pretty generally acknowledged: _First_, that land laid down to artificial grasses for one, two, three, or more years, is in some degree rested or recruited, and is fitted for the better production of after-corn crops. Letting it lie a year or two longer in grass, therefore, is one of the received modes of bringing back to a sound condition a soil that has been exhausted by injudicious cropping. _Second_, that land thus laid down with artificial grasses deteriorates more or less after two or three years, and only by slow degrees acquires a thick sward of rich and nourishing herbage. Hence the opinion, that grass-land improves in quality the longer it is permitted to lie,—the unwillingness to plough up old pasture,—and the comparatively high rents which, in some parts of the country, old grass lands are known to yield. Granting that grass lands do thus _generally_ increase in value, _three_ important facts must be borne in mind before we attempt to assign the cause of this improvement, or the circumstances under which it is likely to take place for the longest time and to the greatest extent. 1. The value of the grass in any given spot may increase for an indefinite period—but it will never improve beyond a certain extent—it will necessarily be limited, as all other crops are, by the quality of the land. Hence the mere laying down to grass will not make _all_ land _good_, however long it may lie. The extensive commons, heaths, and wastes, which have been in grass from the most remote times, are evidence of this. They have in most cases yielded so poor a herbage as to have been considered unworthy of being enclosed as a permanent pasture. 2. Some grass lands will retain the good condition they thus slowly acquire for a very long period, and _without manuring_, in the same way, and upon the same principle, that some rich corn lands have yielded successive crops for 100 years without manure. The rich grass lands of England, and especially of Ireland, many of which have been in pasture from time immemorial, without, it is said, receiving any return for all they have yielded, are illustrations of this fact.