80 THE FERTILITY OF THE SOIL [CH. 



soils are the loams, which consist of sand, silt and 

 clay together with calcium carbonate. Soils lacking 

 any of these constituents are usually less productive, 

 but the fertility limits are raised directly the lacking 

 constituent is supplied. 



Thus a sandy waste may be made productive after 

 addition of clay: and a dense clay soil may be 

 ameliorated by adding calcium carbonate, and to a 

 less extent by adding sand. These processes are 

 simple enough in principle but require considerable 

 human labour in practice, so that now-a-days they are 

 relatively costly. They are among the earliest im- 

 provements in our agriculture, being known and 

 practised by the Britons according to Pliny 1 . His 

 account of the process affords an interesting glimpse 

 of the agriculture of those far-off days. " The peoples 

 of Britain and Gaul have discovered another method 

 for nourishing the land. There is something they 

 call marl (marga). It contains a more condensed 

 richness, a sort of fatness of the land.... There were 

 formerly two kinds only, but several have lately been 

 put to use by clever men: the white, the red, the 

 columbina, argillaceous, tufa-like, and sandy are all 

 used now. Marl is two-fold in nature : hard or fatty ; 

 these can be distinguished by the touch. In like 

 manner it has a two-fold use; some kinds are used 

 for crops (fruges) only, others for herbage 

 1 Book 17, 6. 



