PREVIOUS DRYNESS OF THE SUBSOIL NECESSARY. 321 



end of a single season at the furthest, the under-soil may be as solid and 

 impermeable as ever. 



It is as the follower of the drain, therefore, in the course of improve- 

 ment, that the subsoil plough finds its most beneficial and most economi- 

 cal use. After land has been drained, the water may still too slowly 

 pass away, or the air may have too imperfect an entrance into the sub- 

 soil from which the drains have removed the vi'ater. In the former case, 

 the subsoil plough must be employed, in order that the drains may be- 

 come fully efficient; in the latter, that the under-layers may be opened 

 up to all the beneficial influences which the atmosphere is fitted to exert 

 upon them. In this respect it is an auxiliary to the drain. But as the 

 full eflect which the subsoil plough is capable of producing upon stiff" 

 and clayey subsoils, can only be obtained after they have been brought to 

 such a state of dryness that the sides of the cut or tear, which the plough 

 has made, will not again readily cohere, it is of importance that the 

 drains should be allowed a considerable time to operate before the use of 

 this plough is attempted. The expense of the process is comparatively 

 great, and this expense will be in a great measure thrown away upon 

 clay lands, which are undrained, or from which the water, either through 

 defective draining, or from the want of sufficient time, has not been able 

 fully to flow away. There are few kinds of clay land on which the ju- 

 dicious use of this valuable instrument will not prove both actually and 

 economically useful, thougli from the neglect of the above necessary pre- 

 caution, it has been found to fail in the hands of some. Such failures, 

 however, do not justify us in ascribing to some fancied defect in the in 

 strument, or in the theory upon which its use is recommended, what ne- 

 cessarily arose, and could have been predicted, from our own neglect of 

 an indispensable preliminary observation. The sanguine anticipations 

 of its inventor, Mr. Smith, of Deanston, may not be fully realized, yet 

 the value of the subsoil plough itself, and the benefits it is fitted to confer, 

 when rightly used, appear to me to be both theoretically and practically 

 established. 



§ 5. Of deep-ploughing and trenching. 



Deep-ploughing and trenching diflfer from ordinary and subsoil plough- 

 ing in this, — that their special object is to bring to the surface and to mix 

 with the upper-soil a portion of that which has lain long at a consider- 

 able depth, and has been more or less undisturbed. 



The benefit of such an admixture of fresh soil is in many localities un- 

 doubted, while in others the practical farmer is decidedly opposed to it. 

 On what principle does its beneficial action depend, and in what circum- 

 stances is it likely to be attended with disadvantage ? 



1°. It is known that when a heavy shower of rain falls it sinks into 

 the soil, and carries down with it such readily soluble substances as it 

 meets with on the surface. But other substances also, which are more 

 sparingly soluble, slowlj' and gradually find their way into the subsoil, 

 and there more or less permanently remain. Among these may be 

 reckoned gypsum, and especially those silicates of potash and soda al- 

 ready spoken of (page 206), as apparently so useful to corn-growing 

 plants. Such substances as these naturally accumulate beyond the 

 reach of the ordinary plough. Insoluble substances likewise slowly 



