182 LECTURES ON 



grounds from 16 bushels to an average (for ten years) of 34 bushels 

 per acre. Although he asserts that he has never known this pi n — 

 which differs from Tull's chiefly in the depth of tillage — to fail where 

 carried out according to his directions, it is easy to see that not every 

 soil will admit of its successful application, even independently of 

 considerations of cost. This method demands for its success that the 

 soil be so deep and so readily decomposable that the plant may find 

 its needful supplies in one-half the accustomed superficies, and there- 

 fore must possess physical properties that, under the treatment, are 

 in the highest degree favorable to vegetation. 



On large holdings the maintenance of such an amount of assimilable 

 food as constitutes the soil fertile, is often profitably accomplished by 

 the ancient practice of summer-fallow, which is the same thing for a 

 whole farm as the vacant strips in the Lois Weedon system are for 

 the wheat fields. A field is left void of crops, and is repeatedly 

 ploughed and harrowed during the whole of one summer, generally re- 

 ceiving the seed of some winter grain in the autumn. The fallow is 

 thus an extra period of rest for the soil — enables it to accumulate 

 within itself a store of fertility against future harvests, and is often 

 attended with collateral advantages that alone are sufficient to war- 

 rant its employment, viz., the destruction of weeds, insects, and the 

 improvement of the texture of the soil. 



In many situations these processes of tillage are so laborious or in- 

 effectual that recourse must be had to other operations to change* 

 radically the characters of the soil. 



Heavy clays, especially in a moist climate, are very difficult of 

 tillage from their peculiar physical qualities. In spring time they 

 become so exceedingly tenacious and compacted by the rains, that 

 they dry with extreme slowness.* While wet they resist any attempt 

 at pulverization, because if ploughed in that condition the plastic up- 

 turned masses harden in drying to intractable clods. It hence results 

 that heavy clays need to be tilled when they have arrived at a certain 

 stage of dryness, and then the operation of ploughing is exceedingly 

 laborious, while the full preparation of the seed-bed is brought late 

 into the season. As clay soils dry, the surface is baked into a crust 

 which impedes the circulation of water, and which, shrinking and 

 cracking apart in innumerable places, ruptures the rootlets of plants. 

 Is is especially difficult to induce a deep tilth in such soils, so that 

 during protracted drought the crops suffer greatly on them. 



When clays are not continuous in depth, but rest upon a gravelly 

 and open sub-soil; or when, by art; underground channels are pro- 

 vided for the removal of surplus water, these impediments to tillage 

 and to profitable culture are greatly lessened or entirely removed. 



Many soils of lighter character, and in wet climates, sandy soils 

 even, are remarkably benefited by artificial provision for the removal 

 of surplus or bottom water. 



It is but a few years since the introduction into general practice of 

 a system of drainage intended to effect this purpose took place in 

 Great Britain, James Smith, of Deanston, Scotland, led by an in- 

 ductive study of the evils, and the true means to be employed in the 



