THE DEAINAGE OF GEAS8 LAND. n 



surface soil becomes freed from water, it draws up and re-absorbs 

 moisture from below ; and it is especially when the soil becomes 

 dry, and its particles are disintegrated, that it possesses this 

 power. The water which is thus brought from the subsoil con- 

 tains some of the mineral constituents from the formation below 

 which further aid the growth of plants. This fact accounts for 

 the widely different grasses which are to be seen in old pastures 

 on surface soils that appear to be identical. The mineral con- 

 stituents in a state of solution are brought up by the water 

 from considerable depths, and by this means, amongst others, 

 the geological stratum asserts its influence upon the herbage 

 growing on the surface. 



It is a mistake to suppose that the rainfall goes direct to 

 the drains and is at once expelled from the land. On the contrary, 

 the rain sinks into the land until it meets and mixes with the 

 subsoil water, and the drains do not begin to run until the subsoil 

 water rises above their level. While water, however small the 

 quantity, is flowing in a drain-pipe, and probably long after it 

 has ceased to flow, it may be taken for granted that the subsoil 

 is saturated with moisture up to the level of the drains. The 

 rise and fall of the subsoil water are therefore determined by the 

 level of the drain rather than by the surface of the soil, as it 

 would be in "an undrained state. Thus, in well-drained land, the 

 atmosphere is being continually carried into the soil by rain, 

 and forced into it by atmospheric pressure as the subsoil water 

 falls to a lower level, and the air is expelled when the water rises. 

 A water-logged surface is not only injurious to plant-life because 

 there is too much moisture in it and too httle warmth, but be- 

 cause neither rain nor atmospheric air can enter from above, 

 nor mineral constituents be drawn from below. Drainage sets 

 all these natural forces in motion, and they open the soil and 

 disintegrate its particles for the benefit of the plant-hfe upon it. 



the earthy particles ; the outer surface of the root-fibres here may be supposed to form 

 the one, the porous earthy particles the other wall of a capillary vessel, the connection 

 between them being effected by an extremely fine layer of water.' 



