428 WHY ARTIFICIAL PASTURES DETERIORATE. 



clined to attribute to the slow but constant labour of these insects^ tho 

 entire formation of the fertile surface soils over large tracts of country. 

 (^•Geological Transactions.") 



I have directed your attention to these causes chiefly in explana- 

 tion of the changes which by long lying in grass the surface of our 

 stitf clay lands is found to undergo. But they apply equally to other 

 soils also — the only difference being that, in the case of such as are 

 already light and open, the change of texture is not so great, and 

 therefore does not so generally arrest the attention. 



Upon this subject I may trouble you further with two practical re- 

 marks : 



1°. That the richest old grass lands — those which have remained 

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

 soils (the Oxford and Lias clays, Lee. XL, § 8). This is owing to 

 the fact that such soils naturally contain, and by their comparative 

 impermeability re-tain, a larger store of those inorganic substances on 

 which the valuable grasses live. When the surface soil becomes de- 

 ficient 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 obedience to which all pas- 

 tured lands, when left to nature, must ultimately become exhausted. 

 They must eventually become poorer ; but in their case the deterio- 

 ration will be slower and more distant, and by judicious top-dressings 

 may be still longer protracted. 



2°. The natural changes which the surface soil undergoes, and es- 

 pecially upon clay lands when laid down to grass, explain why it is so 

 difficult to procure, by means of artifical grasses, a sward equal to that 

 wliich grows naturally upon old pasture lands. As the soil changes 

 upon our artificial pastures, it becomes better fitted to nourish other spe- 

 cies of grass than those which we have sown. These naturally spring 

 up, therefore, and cover the soil. But these intruders are themselves 

 not destined to be permanent possessors of the land. The soil under- 

 goes a further change, and new species again appear upon it. We can- 

 not tell how often diiferent kinds of grass thus succeed each other upon 

 the soil, but we know that the final rich sward which covers a grass field 

 when it has reached its most valuable condition, is the result of a long 

 series of natural changes which time only can bring about. 



The soil of an old pasture field, which has been ploughed up, is made 

 to undergo an important change both in texture and in chemical con- 

 stitution, before it is again laid down to grass. The same grasses, 

 therefore, which previously covered it will no longer flourish, even 

 when they are sown. Hence the unwillingness felt by practical men 

 to plough up their old pastures — but hence, also, the benefit which 

 results from the breaking up of such as are old, worn out, or covered 

 with unwholesome grasses. When again converted into pasture land, 

 new races appear, and a more nourishing sward is produced.* 



* For an exccUent article on the superior fes<Ung qualities of recent artificial erasseg 

 over many oM pasture lands, by Mr. BosweJl, of Kingcaussiftj see the Quarterly Jojirnai 

 if Agriculture, "N., p. 783. 



