SOIL CONDITIONS AFFECTING PLANT GROWTH 57 



persistently held the view that jome may come from the soil 

 (p. 15). The physiological work has usually been done in 

 water cultures. Knudson ^ finds that saccharose, glucose, 

 maltose, and fructose are directly absorbed and utilised by 

 green maize, Canada field pea, timothy, radish, vetch, etc., 

 giving rise to a characteristic branched root system. While 

 these substances do not occur in the soil other soluble carbon 

 compounds are present, especially in glasshouse soils, and may 

 exert important effects. j 



There is considerable evidence, however, that by far the 

 larger portion of the carbon of the plant is taken up from the 

 atmosphere and not from the soil, but the phenomena are not 

 wholly independent of the soil. The amount of carbon dioxide 

 in the atmosphere is subject to slight variations which may 

 arise from variations in biochemical activity in the soil, and 

 may be a factor of importance in crop production. Brown and 

 Escombe (59«) found that the amount varied at Kew from 2*43 

 to 3 '60^ volumes per 10,000 volumes of air, the average being 

 2-94. Taking the jnonth of July as an example, the average 

 values were : — 



i8g8. 1899. igoo. 1901. 



CO2 in 10,000 volumes of air . 2*83 2*88 2*86 3'ii 



It is probable that the plant as a whole would respond to 

 variations of this order, making greater or less growth as the 

 amount of carbon dioxide rises or falls. ^ 



Nitrogen. — Of all the nitrogen compounds yet investigated 

 nitrates are the best, and, in natural conditions, probably the 

 only nitrogenous food for non-leguminous plants. The seed- 

 ling, still drawing its sustenance from the seed, lives on other 

 compounds : H. T. Brown (59<^) found that asparagine was the 

 most effective nutrient for the detached embryo of barley, 

 followed by other relatively simple substances like nitrates, 

 glutamic and aspartic acids, ammonium sulphate, etc., the more 

 complex substances being less useful. The experimental 



^Cornell Repts. (Ithaca, N.Y.), 1917, 747-813 (Memoir 9 of 1916). 



2 Only on one occasion was so high a number obtained. 



^ See E, Demoussy (83) and Otto Warburg, Biochem. Zeitsch., 1919, 100, 230, 



