426 THE SOIL AND GRASSES CONTINUALLY CHANGE. 



c. This, agairi; is owing to a new change which has come over the 

 soil. It has become, in some degree, exhausted of those substances 

 which are necessary to the growth of the more valuable grasses — 

 less nutritive species, therefore, and such as are less willingly eaten 

 by cattle, take their place. 



Such is the almost universal process of change which old grass fields 

 undergo, whether they be regularly mown or constantly pastured only 

 — provided they are left entirely to themselves. If mown they begin 

 to Ikil the sooner, but even when pastured they can be kept in a state 

 of full productiveness only by repeated top-dressings, especially of sa- 

 line manure — that is, by adding to the soil those substances which are 

 necessary to the growth of the valuable grasses, and of which it suf- 

 fers a yearly and unavoidable loss. Hence, the rich grass lands of 

 our fathers are found now in too many cases to yield a herbage of 

 little value. Hence, also, in nearly all countries, one of the first steps 

 of an improving agriculture is to plough out the old and failing pas- 

 tures, and either to convert them permanently into arable fields, or after 

 a few years' cropping and manuring, again to lay them down to grass. 



But when thus ploughed out. the surface soil upon old grass land is 

 found to have undergone a remarkable alteration. Wh^n sown with 

 grass seeds, it may have been a stiff, more or less grey, blue, or yellow 

 clay — when ploughed out it is a rich, brown, generally light and fria- 

 ble vegetable mould. Or when laid down it may have been a pale- 

 colored, red, or yellow sand or loam. In this case the surface soil 

 is still, when turned up, of a rich brown colour — it is lighter only and 

 more sandy than in the former case, and rests upon a subsoil of sand 

 or loam instead of one of clay. It is from the production of this change 

 that the improvement caused by laying down to grass principally re- 

 sults. In what does this change consist ? and how is it effected ? 



If the surface soil upon stiff clay lands, which have lain long in grass, 

 be chemically examined, it will be found to be not only much richer 

 in organic matter, but often also poorer in alumina than the soil which 

 formed the surface when the grass seeds were first sown upon it. 

 The brown mould which forms on lighter lands will exhibit similar 

 differences when compared with the soil on which it rests ; but the 

 proportion of alumina in the latter being originally .small, the diffe- 

 rence in respect to this constituent will not be so perceptible. 



The effect of this change on the surface soil is in all cases to make 

 it more rich in those substances which cultivated plants require, and 

 therefore more fertile in corn. But strong clay lands derive the fur- 

 ther important benefit of being rendered more loose and friable, and 

 thus more easily and more economically cultivated. 



The mode in which this change is brought about is as folloAVs : — 



1°. The roots, in penetrating, open and loosen the subjacent stiff clay. 

 Diffusing themselves every where, they gradually raise, by increasing 

 the bulk of, the surface soil. The latter is thus converted into a mix- 

 ture of clay and decayed roots, which is of a dark colour, and is necessa- 

 rily more loose and friable than the original or subjacent unmixed clay. 



2°. But this admixture of roots effects the chemical composition as 

 well as the state of aggregation of the soil. The roots and stems of 

 the grasses contain much inorganic — earthy ^ind saline — matter (Lee. 



