130 AERATION AND AIR-CONTENT. 



aeration is provided, the development of plants is essentially as 

 good in bog-water as in nutrient solution. The oxygen-content of 

 bog-water decreases and the carbon dioxid increases from the Carex 

 to the Chamcedaphne- Andromeda and the Larix-Picea stages. The 

 adjustment of the roots of bog-plants to the water-level is due to the 

 need of securing a sufficient supply of oxygen. In a later paper 

 (1921 : 50), he has found that the injury of cranberry vines as a 

 result of flooding is due to the lack of oxygen, and that injury is most 

 frequent during cloudy weather, when the oxygen-content is the 

 lowest. 



ACIDITY. 



While the scope of the present treatment does not permit a com- 

 prehensive account of the numerous studies of soil acidity, it seems 

 desirable to deal with the more recent investigations, because of the 

 light they throw upon acidity in bogs and upon soil toxins. Eco- 

 logical studies of acidity have been few, and the paucity of experi- 

 mental results makes it impossible to determine whether acidity is 

 a cause or merely a concomitant (Fernald, 1907; Coville, 1910, 

 1913; Sampson, 1912; Wherry, 1920). Practically all of the quan- 

 titative and experimental studies of acid soils have been made by 

 chemists, and the value of the results has been somewhat obscured 

 by the general lack of physiological and ecological knowledge. In 

 spite of their great divergence as to the causes of acidity, nearly 

 all of them are valuable in helping to determine quantitative rela- 

 tions, and some of them are of the first importance to ecological 

 investigation. 



The theory that soil acidity is due to the accumulation of insoluble 

 organic compounds, the so-called humic acids, has been generally 

 accepted until the last decade or two. Sprengel (1826) isolated a 

 substance from soil that he called humic acid, and Berzelius (1838) 

 obtained this acid and the related humin from the treatment of 

 soil with an acid. Mulder (1840) recognized 7 different organic 

 substances in the soil, namely, ulmic, humic, geic, apocrenic, and 

 crenic acids, regarded as successive steps in the decay of organic 

 matter in the soil, and humin and ulmin. Eggertz (1889) threw 

 doubt upon the existence of humic acids by showing their variability 

 of composition. Van Bemmeln (1888) contended that humic acids 

 were not definite compounds and that their formulae were without 

 value. He regarded humus substances as colloidal in nature and 

 the humates as adsorption compounds. This view has received 

 the support of many chemists, and it has apparently been con- 

 firmed for the acidity of sphagnum bogs by Baumann and Gully 

 (1910 : 47), who stated that no free humic acids are to be found in 

 peat-moss. They contend that the acidity of sphagnum and peat 

 soils generally is due to the colloids of the external walls of the sphag- 



