XVI GRASSES OF BRITAIN. 



server, but it is not generally true. Even if pastured by stock only 

 and never mown — the improvement will at length reach its limit or 

 highest point, and from this time the value of the sward will begin to 

 diminish. 



*' This, again, is owing to a new change which has come over 

 the soil. It has become, in some degree, exhausted of those sub- 

 stances which are necessary to the growth of the more valuable 

 grasses — less nutritive species, therefore, and such as are less will- 

 ingly 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 pas- 

 tured only — provided they are left entirely to themselves. If mown 

 they begin to fail the sooner, but even when pastured they can be 

 kept in a state of full productiveness only by repeated top -dressings, 

 especially of saline manure — that is, by adding to the soil those sub- 

 stances which are necessary to the growth of the valuable grasses, 

 and of which it suffers 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 faihng pastures, and either to convert them permanently 

 into arable fields, or, after a few years' cropping and manuring, again 

 to lay them down to grass." 



" That the richest old grass lands — those which have remained 

 longest in a fertile condition — are generally upon our strongest clay 

 soils. This is owing to the fact that such soils naturally contain, and 

 by their comparative impermeability re-tain, a larger store of those in- 

 organic substances on which the valuable grasses live. When the sur- 

 face soil becomes deficient in any of these, the roots descend further into 

 the subsoil and bring up a fresh supply. But these grass lands are 

 not on this account exempt from the law above explained, in obedi- 

 ence to which all pastured lands, when left to nature, must ultimately 

 become exhausted. They must eventually become poorer ; but in 

 their case the deterioration will be slower and more distant, and by 

 judicious top-dressings may be still longer protracted. 



« The natural changes which the surface soil undergoes, and 

 especially upon clay lands when laid down to grass, explain why it 



