138 



SOIL CONDITIONS AND PLANT GROWTH 



was up to 4 per cent. A close study jointly by a botanist and 

 a chemist would carry the problem much nearer to a solu- 

 tion, 



' 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 conditions 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. 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 and called 

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



Table XL. — Analyses of the Organic Material Extracted by Alkalis 

 FROM Soil (often called Humus, Soluble Humus, Active Humus, 

 MATjkRE Noire, etc.). 



1 Chem. Zeit., 1920, 44, 551. The molecular weight was about 680. 



2 Biochemical Journ., 1907, 2, 458. For other studies see Mary Cunning- 

 ham and Chas. Doree, Trans. Chem. Soc, 1917, III, 589-608. 



