TOXIC EXUDATES AND SOIL TOXINS. 153 



of the one or the other depends upon soil conditions, composition, 

 drainage, plants, etc., all of which are affected by tilling, cultivation, 

 draining, liming, fertilizing, and rotation. 



Shorey (1913) has carried further the study of organic soil con- 

 stituents, and has isolated the following compounds from widely 

 separated soils : oxalic, succinic, saccharic, and acrylic acids, lysine, ade- 

 nine, choline, trimethylamine, salicylic aldehyde, mannite, rhamnose, 

 trithiobenzaldehyde, nucleic acid, and an unidentified aldehyde. 

 This brings the number of compounds isolated to 35, of which 13 are 

 organic acids, 9 organic bases, 3 sugars, 2 aldehydes, and 2 alcohols. 



Schreiner and Reed (1909) have confirmed the results of Molisch 

 (1888), Czapek (1896), and Raciborski (1905) as to the oxidizing 

 power of the roots of growing plants, finding this to be greatest in the 

 root-hair region. The oxidizing power is greater when plants are 

 grown in an extract of productive soil than in one of an unpro- 

 ductive soil. Oxidation was usually favored by adding an absorbing 

 agent to the extract, as well as by the addition of nitrates, phosphates, 

 and calcium salts. Toxic organic substances in solution were ex- 

 tremely injurious to the oxidizing power, which was able to reduce 

 the toxicity, however, especially in the presence of nitrates. Oxida- 

 tion by roots is due largely if not entirely to the acitivity of a peroxi- 

 dase produced by them. This enzyme is most active in neutral or 

 slightly alkaline solutions, and its action may be inhibited by acids 

 as well as by putrefaction processes. Oxidation by roots is of agri- 

 cultural interest, since the promotion of oxidation is an important 

 factor in tillage and cultivation. 



Schreiner and Sullivan (1910) have studied oxidation in the soil 

 and conclude that it plays an important part in the organic and in- 

 organic changes that occur. It appears not to be enzymotic, but the 

 result of interaction between inorganic constituents and certain kinds 

 of organic matter. It may also be brought about by organic matter 

 in a state of autoxidation and by inorganic oxygen-carriers, such as 

 manganese and iron. Oxidation was increased by the addition of 

 salts of manganese, iron, aluminum, calcium, and magnesium. Some 

 kinds of organic matter, such as dihydroxystearic acid, inhibit soil 

 oxidation, but in general a plentiful supply of organic material pro- 

 motes oxidation. Excessive oxidation, however, is harmful to vege- 

 tation. The oxidative power of the soil is regarded as a sympton of 

 its condition, so that whatever decreases oxidation tends also to bring 

 about the conditions that decrease growth, while the factors that 

 favor oxidation are those that promote productivity. 



Sullivan and Reid (1912) have shown that soils possess the power 

 to decompose hydrogen peroxid, and that this is greater in soil than 

 in subsoil and in strong than in weak soils. In general, the catalytic 

 power of soils seems to be due not to an enzyme, such as catalase, 

 but rather to the separate or joint activity of the inorganic and 



