70 SOIL CONDITIONS AND PLANT GROWTH 



4. Fen {Niedermoor in German, see (299)) formed from a calcicolous 

 vegetation (Phragmites, Cladium, Scirpus, Carex, etc.), in presence of 

 calcium carbonate and soluble mineral salts, showing no acid properties 

 and giving alkaline drainage waters. 



5. Carr, genetically related to the fen. 



Between fen and peat several transition forms have been described 

 by Weber (299) and also recognised in England. Some of our moors 

 are built up on older fens. 



Within each of these great classes several subdivisions are re- 

 cognised, but how far they arise from differences in the organic matter, 

 or from other differences, cannot yet be ascertained. Nor is it known 

 whether any of these classes is identical with the " humus " of grass or 

 arable land. There is no doubt that a close study jointly by a botanist 

 and a chemist would carry the problem much nearer to a solution. 



The observations indicate that the mixture we have agreed to call 

 humus does not vary erratically from field to field, but produces much 

 the same effects over any tract where similar soil and climatic condi- 

 tions prevail. The mixture changes when a new set of conditions 

 occurs, but its general character persists over a certain range and then 

 it merges into another type. 



How many such types are recognisable will not be known till many 

 more observations such as the above have been made, but as each type 

 is settled by the ecologist it becomes the business of the chemist to ex- 

 amine the mixture and endeavour to correlate its composition with 

 its properties. Two great divisions of the types can already be recog- 

 nised : a neutral group commonly spoken of as neutral humus or " mild 

 humus," and a group reacting like an acid (although Baumann (9, 10) 

 could find no evidence that it actually contained an acid ^), and called 

 " sour humus," acid humus, or by German writers Rohhumus. 



In order to understand much of the chemical work that has been 

 done, it is necessary to remember that the older chemists regarded 

 humus as a single definite substance, or as a mixture of two or three 

 definite substances. The favourite view was to consider neutral humus as 

 the calcium salt of "humic acid," which cpuld be extracted from the 

 soil by dilute alkalis after preliminary treatment with hydrochloric acid. 

 On acidifying this alkaline extract the " humic acid " came down as a 

 brown colloidal precipitate. Acid humus was the actual humic acid 

 itself It was further supposed that humic acid could be synthesised 



' There is no agreement even on this point. Sven Odfen {Ber., 1912, xlv., 651-660) has 

 recently made various physical measurements which in his opinion prove beyond doubt the 

 existence of a definite humic acid. See also footnote, p. 72. 



