TILE AND PIPE DRAINING. 109 



substance ; the former, it is stated, containing nearly 90 per cent., 

 the latter varying from 70 to 80 per cent. Water, filtering 

 through a soil, opens its pores to the admission of air, which is 

 most essential to the growth of the plant, or perhaps, more 

 properly speaking, to the fertility of the soil. Humboldt 

 observed that argillaceous soils and humus deprived the air of 

 its oxygen. He satisfactorily ascertained that earth taken from 

 the galleries of mines at Salzburg only became fertile after 

 having been exposed to the atmosphere for a considerable length 

 of time. These observations established the necessity of the 

 presence of oxygen in the interstices of the soil, or, as he then 

 said, and as may still be maintained, the utility of a previous 

 oxidation of the soil. All our agricultural facts, indeed, confirm 

 this view of the necessity of air in the interstices of the soil that 

 is destined for the growth of vegetables. When, by ploughing 

 very deeply, for example, we bring up a portion of the subsoil 

 into the arable layer, in order to increase its thickness, we 

 always lessen the fertility of the ground for a time : in spite of 

 the action of manures, and of any treatment we may adopt, a 

 certain time must elapse before the subsoil can produce an advan 

 tageous effect : it is absolutely necessary that it have been 

 exposed to the atmospheric influences : and it is then only that 

 deep ploughing, which gives the arable layer a greater thickness, 

 pays completely for the expense it has occasioned.* 



Water contributes, in the next place, when filtering gradually 

 through the soil, to dissolve the manures, and prepare them to 

 assist in the growth of the plants in some cases, for the elements 

 of these manures to be taken up by the plants. But water m 

 too great abundance destroys these manures, and carries them 

 away. Rain water, falling upon the surface, when the tempera 

 ture of the air is higher than that of the soil, contributes to 

 increase this temperature of the soil. Water, when stagnant in 

 a soil, diminishes its temperature. The extreme wetness of a 

 soil renders it difficult to be worked ; impedes the sowing or 

 planting ; often destroys the seed and the crop ; occasions it 

 to become poached or inaccessible to animals ; and in many 

 other ways may be said to make the cultivation of such soils 

 hopeless. 



* Boussingault, p. 286. 

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