io6 SOIL CONDITIONS AND PLANT GROWTH 



cause some peculiarities of behaviour on sandy soils, such as 

 the Bagshot beds, where there are thin partings of clay. 



A remarkable change sets in when clay is sufficiently 

 heated, and it permanently loses all its special properties. 



Several theories have been put forward to account for 

 the special properties of clay, and in particular its plasticity. 

 It has often been assumed that these properties are the 

 necessary result of the smallness of the particles, which brings 

 into prominence the surface forces. Rohland '^ attributes 

 plasticity to hydrated colloidal substances forming a gelatin- 

 ous film round the clay particles, and Le Chatelier^ to minute 

 flake-like particles. At present the evidence is insufficient 

 to allow of discrimination.^ 



These clay properties are of great importance to the fertility 

 of the soil, and no constituent is more necessary in proper 

 proportions, or more harmful in excess. Clay impedes the 

 movement of water in the soil and keeps it in the surface 

 layers within reach of the plant roots, thus making the soil 

 retentive of water. Excess of clay, however, interferes too 

 much with the water movements, making the soil water- 

 logged in wet weather and parched in dry seasons even though 

 the permanent water level is near the surface ; it also impedes 

 the movement of air to the roots and lowers the temperature 

 of the soil. The adhesive properties of clay cause the soil 

 particles to bind together into those aggregates on which 

 "tilth" depends; soil without clay would be very like a 

 sand heap. Here also, however, excess of clay does harm 

 and makes the soil so adhesive that it sticks to the tillage 

 implements and retards their movements ; it also tends to 

 form large clods unfavourable to vegetation. These effects 

 are intensified in wet weather ; the soil becomes sticky or 

 " poached" and must not be worked or the tilth is injured for 

 a long time. Another effect of a large amount of clay is to 

 make the soil shrink very much on drying, so that large 

 cracks appear in the fields in summer time. These harmful 



1 P. Rohland, Die Tone (Vienna), igio. 



^L,a silice et les silicates. Paris, 1914. 



^For discussion see A. B. Searle, British Clays, Shales, and Sands, 1912 



