IMPROVEMEIs^T OP SOILS. 29 



the permeability, as well as on the composition of a soil, 

 its fertility is thus very materially dependent ; and land 

 drainage, eitlier natural or artificial, is essential to it. 



When there is fn excess of water in a soil, and no pro- 

 yision exists for withdrawing it, the interstitial canals 

 become completely filled, to the exclusion of the necessary 

 amount of air on which the activity of the soil, considered 

 as a laboratory for the provision of plant food depends. 

 When the soil is drained, the superfluous water flows off 

 through the air canals, and only so much moisture is re- 

 tained as can be absorbed by the minuter pores within 

 the small particles of earth ; and as there is, then, free 

 communication through these canals between the pores 

 and the drains, it is evident that the water will all be 

 withdrawn from the soil except that which is held by 

 capillary attraction. Thus the rain, which falls upon 

 and is absorbed by the surface-ground, percolates towards 

 the drainage level, flushmg every crevice and canal in 

 its descent, leaving behind it the nutritive ingredients 

 which it carries in suspension or in solution, and on 

 which the plants can feed as it passes by their roots, or 

 which the soil, acting as a filter, extracts and appro- 

 priates. 



The object of land-drainage is not merely to render wet 

 ground sufficiently dry for tillage operations, but so to 

 regulate the passage of moisture that, while the soil 

 shall have every facility for absorbing the necessary 

 quantity, stagnation, and the consequent starvation of 

 the plants, shall be prevented. Almost all lands require 

 it, in whole or in part, and there are few fields which 

 can be economically cultivated without draining some 

 portion of them. 



The causes which wet the soil of any field requiring 

 drainaore must first be considered ; when these are under- 

 stood, it will be easy to decide upon the best means of 

 providing a remedy. But in this consideration the strata 



