DRAINAGE, LEACHING IN SOIL INVESTIGATED

Press, Volume XCVIII, Issue 28938, 4 July 1959, Page 9

DRAINAGE, LEACHING IN SOIL INVESTIGATED

The visitor to the Winchmore irrigation research station who chances to wander a few yards from the station headquarters may cotne across a curious sort of earthwork, reminiscent of a trench or gun emplacement of war days.

It is, however, a lysimeter which enables the extent to which natural rainfall and irrigation water becomes drainage water in a 3ft tall core of soil and the leaching action of that water to be determined.

The lysimeter consists of a concrete cylinder 4ft in diameter and. 3ft 4in high which has been sunk into the ground by digging round it so that a core of completly undisturbed earth is enclosed within it When it has been sunk to the required depth a half inch thick galvanised steel plate is jacked underneath it from an adjacent excavation or pit There are perforations in the centre of the plate through which drainage drips into a shallow funnel and so into a collecting tank in front of the cylinder. When tests have been completed to ensure that sets of lysimeters are operating similarly a concrete wall is built in front of them and soil is filled in around them so except that the lip of the Cylinder protrudes four inches above the surface to stop run-off the surface of the lysimeter merges with the ground around it. Mr D. S. Rickard, a chemist

who is in charge of the laboratory at Winchmore, says that water which reaches the 3ft level would in all probability be expected to go further down and be lost to plants, though in some cases root systems penetrated deeper. Drainage records for two of the lysimeters have been kept -since the 1953-54 season. One is subject to irrigation treatment and one is not.

Water starts to drain off When the soil reaches field capacity which is the Maximum amount of water it can retain. An interesting point is that when field capacity is reached it has been found that the excess water drains off quite quickly. .

Thus when 3.06 inches of water were applied between noon and 3.15 p.m. the drainage which amounted to 0.63 inches began at 2.30 p.m. and finished at 4.30 p.m. Preliminary studies of the composition of . the drainage water have not indicated that there are serious losses of niitrients through leaching. In the case of calcium, the water applied may contain slightly more calcium than the drainage water. There is , virtually no loss of phosphorus and no significant movement of potassium or magnesium.

Overseas it is sometimes said that by inserting a plate in a clay loam type of soil the profile may be broken, but Mr Rickard says that he does not think that this will change the drainage pattern in free draining shingle and gravel like that at Wipchmore.