62 THE FERTILITY OF THE SOIL [CH. 



carbonate. There was a real danger of this, but it 

 was met by chalking and marling which from time 

 immemorial had been part of the agricultural practice 

 of these islands (see p. 80). In consequence soil 

 exhaustion could only set in through exhaustion of 

 the phosphates and potassium salts, and this was a 

 very slow process which, moreover, was still further 

 delayed by the practice of fertilising with wood 

 ashes, which supplied potassium salts derived from 

 the forest, and less frequently with salt (sodium salts 

 economising the consumption of potassium salts by 

 the plant). Thus the only weak point in the system 

 was the exhaustion of phosphate from the soil, but as 

 the yield of wheat was probably often under 10 bushels 

 per acre, which would only take out some 5 lbs. of 

 phosphoric acid (P2O5), and as the total population 

 was only small and sparsely scattered, the system was 

 for all practical purposes permanent. But the yield 

 was very poor. 



The next step up involved some very drastic 

 changes. Gradually the common arable fields and 

 pastures began to be enclosed and each man's holding 

 came into one piece. The process was slow and is 

 hardly complete even yet; the old strip farming of 

 common fields may still be seen in the Isle of Axholme, 

 Lincolnshire, while survivals of it may be detected in 

 many villages. For example, several arable common 



