$5^ CAUSES OF FERTILITY AND STERILITY [chaP. 



Marl containing carbonate of lime is always far more 

 valuable than clay; pure clay is so little friable, and 

 so sterile itself, that it effects an improvement only 

 slowly; marl not only ameliorates the texture but 

 adds at once a supply of carbonate of lime, potash 

 compounds, and in some cases phosphoric acid also. 

 Clay and marl both have a tendency to sink, and 

 eventually require renewing, but if well done will last 

 for thirty to fifty years, because the accumulation of 

 humus and fibrous root-remains, due to the increased 

 crops, itself binds the soil together. 



At the present day the need of marling or claying 

 on a small scale is often seen in old gardens, 

 particularly in old town gardens which are situated 

 upon gravel soils, initially very short of the finer soil 

 particles. The constant breaking of the surface by 

 cultivation, and the use of large quantities of stable 

 manure, which decays and leaves the soil open, result 

 in a continual washing down of the finest particles, until 

 the remaining soil loses all power of cohesion and of 

 resisting drought, falling into a dusty powder immedi- 

 ately on drying. A coating of clay in the early autumn, 

 or, better still, of good marl, is the only method of 

 giving consistency to such a soil and soon remedies 

 its worst defects, such as susceptibility to drought and 

 rapid fluctuations of temperature, and tendency to 

 produce soft vegetation, veiy liable to disease. 



Reclamation of Peat Land. 



One of the earliest methods of bringing peat land 

 in the Fens and similar districts into cultivation was, 

 to dry the land by means of open drains and break 

 up the surface with the breast plough ; the clods were 

 then gathered together, and burnt when dry, after- 

 wards the ashes were spread and a crop of rape taken. 



