152 AERATION AND AIR-CONTENT. 



will do much to destroy them, by favoring the decay of organic matter 

 through the activities of soil organisms and the processes of oxidation. 



Jensen (1907 : 872) has tried the effect of tree seedlings on the 

 growth of wheat in paraffined wire pots, in which the water-content 

 was maintained by frequent watering. The results showed a de- 

 crease in the green weight of the wheat grown in the pots with the 

 tree seedlings. It is pointed out that this can not be due to shade, 

 to water-content, or to the nutrient content, and it is assumed that 

 the retarding effect is due to substances excreted by the tree roots. 

 The conclusion is reached that tree seedlings of the tulip-tree, dog- 

 wood, maple, cherry, and pine retard the growth of wheat, when the 

 roots of the latter are in close physical relation with the tree roots. 

 The retardation differs with the species and is greatest while the tree 

 seedlings are most active physiologically. The final conclusion is to 

 the effect that the injurious action of trees upon wheat appears to 

 be due to the excretion of substances by the tree roots, which are toxic 

 to the growth of wheat. 



Schreiner and Shorey (1909) isolated four organic compounds from 

 soil, of which two, picoline carboxylic acid and dihydroxystearic acid, 

 were found to be harmful to wheat seedlings, the second in all con- 

 centrations, and the first in that of 100 parts per million. The results 

 were stated to furnish simple tangible proof that injurious organic 

 compounds exist in unproductive soils and to lay the foundation for 

 the rational study and improvement of unfavorable conditions. In 

 further studies of dihydroxystearic acid, Shreiner and Skinner (1910) 

 have found that it retards the growth of wheat plants when present 

 in solution in pure distilled water at the rate of 50 parts per million. 

 It was likewise harmful in the presence of nutrient and fertilizer salts 

 in all ratios of P 2 3 NH3, and K 2 0, but least harmful in the ratios best 

 suited to plant growth. The direct effect of the acid is to darken the 

 root-tips, stunt root development, and inhibit strongly the oxidizing 

 power of roots. The fertilizer combinations that tend to increase 

 root oxidation are also those that minimize the harmful effects. 

 Schreiner and Lathrop (1911) have examined 60 soils from 18 States, 

 and have found dihydroxystearic acid in half of the 35 soils classed as 

 poor and in but 2 of the 25 good soils. This acid is regarded as a 

 direct factor in the low productivity of soils by virtue of its harmful 

 effect on growing crops, or as an indirect factor, serving to indicate 

 other harmful compounds or conditions. 



Schreiner and Skinner (1912) have isolated a number of nitroge- 

 nous constituents from the soil, and tested their effect upon wheat 

 seedlings. The majority of these, such as nucleic acid, xanthine, 

 guanine, creatine, choline, etc., exert a beneficial effect and are able 

 to replace nitrate in its effect, while others, such as picoline carboxylic 

 acid and guanidine are harmful. It is thus clear that the soil con- 

 tains both beneficial and harmful compounds, and the predominance 



