SOILS AND THEIR PROPERTIES 77 



faster than nitrogen. Those authors showed that in farm- 

 yard manure the ratio C to N equals 25 to i. In the top 

 nine inches of old pasture the ratio was 13 to i, but in the 

 subsoil 6 to i. Some of the American workers on the subject 

 have detected small traces of a variety of synthetic compounds, 

 but it is very difficult to decide whether these are important 

 or not. We have so many illustrations in living things of 

 the extraordinary potency of small traces that it does not 

 do to ignore little things, but until something more definite 

 is known it is not practicable in a conspectus of this character 

 to do much more than refer to the authors in the bibliography. 

 Available Plant Food. A very distinct advance was 

 made in soil analysis when Dr. Bernard Dyer introduced 

 his method of attacking soils by i per cent, citric acid 

 solution (see Bibliography). Dyer showed that for the 

 less exhausting crops 0*01 per cent, of phosphoric acid 

 or potash, soluble in i per cent, citric acid, represented 

 the margin between fertility and need of manure. The 

 method has also been found to apply to tropical soils. It 

 has been pointed out that the method of Dyer is purely 

 empirical and that if carried out under totally different 

 conditions different results will be obtained, but the strength 

 of Dyer's position lay in the fact that he correlated his method 

 with actual experiments at Rothamsted, and that his 

 conclusions have, in the main, been thoroughly well sub- 

 stantiated in most places where they have been tried. The 

 objections raised against his method are only general 

 objections to any single test ; so far as a single test is capable 

 of use at all, there are few single tests applicable to soils of 

 such general utility as the phosphoric acid and potash 

 soluble in i per cent, of citric acid. The relationship of 

 the soil to the soil water, to the plant, or to a I per cent, 

 solution of citric acid, are all cases of mass action. The state- 

 ment that repeated extractions with citric acid continue 

 to dissolve more and more phosphoric acid from the soil 

 is not a criticism of Dyer's method at all, but an explana- 

 tion of the reason of its success. It is just because citric 

 acid and carbonic acid and the plant in relation to the soil 



