THE PHYSICAL PEOPERTIES OF SOILS 67 



whole of the water is retained; no drainage takes place 

 at all, and the soil remains completely saturated. The 

 permanence of either of these conditions in the surface 

 soil would of course entirely preclude fertility, but the 

 rate of percolation in cultivated land may approach one 

 or other more or less closely. The naturally great resist- 

 ance of fine materials is increased in clays by the presence 

 of colloidal hydrates, which tend to block the passage of 

 the water below and to hinder the entrance of air above. 

 A shower of rain often temporarily arrests the flow of 

 the drainage by blocking the pores at the surface. A 

 contrary effect may be produced by expansion of the air 

 in the soil, consequent upon a diminution of the barometric 

 pressure or a rise of temperature. 



The amount of the drainage depends upon the rainfall. 

 The records which have been carefully kept at Kothamsted 

 since 1870 afford reliable information regarding the rela- 

 tion between the two at that place. The gauges by means 

 of which the percolation is measured consist of blocks 

 of natural soil in situ. They were constructed by under- 

 mining the soil at three different depths 20, 40 and 60 

 inches respectively and inserting perforated iron plates 

 to support it. This done, trenches were cut round the 

 blocks of soil and these were then isolated by means of 

 brick and cement walls. The soil taken out of the trenches 

 was then returned to the spaces outside the walls. The 

 percolating water is received in large zinc funnels, from 

 which it passes into the measuring cylinders. The sur- 

 face area of each gauge is exactly y^o part of an acre. 

 The soil is kept entirely free from weeds and other growths, 

 and the figures given in the table on p. 68 represent the 

 drainage from bare soils. 



The rainfall is measured by means of .a similar gauge 

 consisting of a rectangular zinc tray of the same surface 

 area jcfco acre constructed close to the drain gauges. 



F 2 



