CHAP, iv.] The Food of Trees 73 



sition of the air contained within the soil, in consequence of the 

 ramification of its roots it keeps the soil more open or porous than the 

 Spruce does. To this property, combined with the circumstance that it is 

 a shade-bearing species of tree, which yields a more nutritious and more 

 easily decomposible humus than the conifers, it owes its soil-improving 

 capacity and its great importance for mixed forests .' 



Where aeration is insufficient, the same practical effects take 

 place as in the case of inundations. The plants consume the 

 available supplies of oxygen within reach of their root-systems, 

 and then sicken and die off ultimately, although of course 

 different kinds of plants show great specific differences with 

 regard to their accommodative power in this respect. 



Nitrogen (N) is essential to the formation of the albuminoid 

 substances which play so important a part in relation to the 

 growth of plants. When organic matter decomposes, the 

 nitrogen becomes sooner or later transformed into ammonia 

 (NH 3 ), which again in turn becomes transformed with the aid 

 of a fungus (as Winogradski has proved) into nitric acid or 

 hydrogen nitrate (HNO 3 ). For the development of this fungus 

 (Bacillus}^ or main intermediate agency, a certain amount of 

 oxygen is requisite ; but when there is a deficiency of oxygen 

 in the air contained within the soil, nitrous oxide or nitrogen 

 monoxide (N 2 O) and free nitrogen are produced. This nitrifica- 

 tion is confined to soil within about 18 inches of the surface. 

 Other sources open to plants for obtaining the requisite supplies 

 of nitrogen are the ammonia contained in the atmospheric 

 precipitations, and also, but under special circumstances only, 

 the free nitrogen of the air. Observations have shown that 

 the summer showers contain a higher proportion of nitro- 

 genous combinations than those that fall in autumn or winter. 

 Although the older agricultural chemists, led by Boussingault, 

 and strongly supported in 1861 by the Rothamstead experiments 

 of Lawes and Gilbert, denied that plants could directly utilize the 

 free nitrogen contained in the air 1 , yet this was strenuously com- 



1 Compare Prince Kropotkin's article On Recent Science in the Nineteenth 

 Century for August 1893, pp. 205 et seq. 



