THE FERTILITY OF SOILS IN THE TROPICS 367 



ing on other than natural causes, such as good or bad 

 cultivation, use or absence of manures and the like. 



Experience has shown, however, that an independent 

 person taking the field-sheet over the country will rarely 

 disagree with the opinion formed, and that in particular 

 there will be no difference of opinion that the good land 

 is not bad, and the bad not good. For contiguous 

 classes, good and medium, medium and bad, there is, of 

 course, no sharply defined boundary and opinion will 

 occasionally differ as to which class the land should be 

 assigned to. 



A check, however, was put 'from the outset on the 

 scale adopted by the field observers. It is well recog- 

 nized that the principal determining factor in the fertility 

 of the Delta land is the presence or absence of soluble 

 salts. If salts are absent good crops can be obtained, 

 if much salt is present it is impossible to grow crops. 

 Accordingly from the outset of the survey it was arranged 

 that frequent soil samples should be taken and that the 

 total soluble salts contained in these should be deter- 

 mined. 



Ihe laboratory work was in charge of Mr. Hughes. 

 The rule laid down was that on each I : 10,000 sheet five 

 samples should be taken from plots of each quality, good, 

 medium, bad and barren. When on a sheet there was 

 only a small area of a particular category the number 

 of samples was suitably reduced. The amount of salt 

 was determined by dissolving out the salt from 5 grams 

 of the sample in 100 c.c. of water and determining the 

 electric resistance of the solution in a suitable Wheat- 

 stone's bridge. A curve had previously been drawn 

 showing the relation between the electric resistance and 

 the salt content of solutions of different strengths and 

 from this curve the salt content of the sa'mples from the 

 field could be read off. In the case of very salty soils, the 

 solution has to be further diluted before making the 

 determination. 



The results of these analyses show that the fertility 

 survey is in fact a close approximation to a salt survey 

 by eye estimation. 



The mean salt-content of what we have called good 



