198 THE BOOK OF NATURE STUDY 



of an arable field ; the difference is less seen in the sandy soil, 

 because it contains so little of the very finest stuff that can wash 

 through the soil. 



By repeating such experiments on a variety of soils we 

 may learn that they all consist fundamentally of mixtures of 

 clay and sand of various grades ; in clays and heavy soils the 

 finest particles predominate and there is very little coarse sand 

 to keep the soil open ; in the really barren sands the coarse sand 

 predominates, but there are many sandy soils, made up, however, 

 of the finest grades of sand, which are fertile enough. What a 

 farmer calls a good free working loam is generally found to consist 

 of a well balanced mixture of the finer grades of sand, bound to- 

 gether by clay and kept open by a sufficiency of coarse particles. 



Having thus learned something of the constitution of soils, it 

 becomes possible to begin to interpret their behaviour towards 

 water. Take five ordinary glass funnels, fit them with filter 

 papers or plug them with a little cotton wool, and pack on to 

 four of them fifty grams each of the dry powdered sandy, clay, 

 peaty, and alluvial soils respectively. Pour on to each an 

 equal volume of water, one hundred cubic centimetres will serve, 

 and catch in beakers the water that runs through ; note the 

 time taken, and the amount passing through in each case. In 

 the fifth funnel take another fifty grams of clay soil, but work 

 it up into a paste with a little of the 100 c.c. of water before 

 putting it on the filter, and then add the rest of the water. 

 The peat and clay soils retain the most water, the alluvial soil 

 coming between them and the sand; the percolation will be 

 slowest with the clay soil, while it will hardly take place at 

 all with the clay that had been first of all pulped up in a wet 

 condition. This shows how the rain falling on a sandy soil 

 finds its way downwards very quickly, little being retained 

 by the soil for the needs of the plant, so that a light sandy 

 soil easily suffers from drought ; it also shows how a peaty 

 soil retains a great deal of water while allowing the excess 

 to move through it pretty quickly. On a clay soil, however, 

 because it is made up of very fine particles, both a large pro- 

 portion of water is retained and the movements through the soil 

 are very slow ; moreover, the slowness of percolation is enormously 



