SOIL CONDITIONS AND PLANT GROWTH 



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 1 ), 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 could 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 

 by boiling sugar with hydrochloric acid, on the singularly inadequate 

 ground that the product thus obtained is also a brown colloid. 

 Numerous analyses have been made both of the natural and the 

 synthetic " humic acid " some of which are given in Table XXXI. 2 



1 There is no agreement even on this point. Sven Oden (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. 73. 



2 Many partial analyses have been made. Cameron and Breazeale (66) in nineteen 

 samples obtained percentages of carbon varying from 33-3 to 50-1, whilst Hilgard (133) found 

 the nitrogen content to be : 



Westermann (302) has analysed humus from the Danish moors, and Gully (114) has studied 

 humus from South Bavarian moors. Many of the older analyses have been collected by 

 Wollny (319). 



