SILVICS 7 



tained, as the minerals accumulate. It also illustrates the way 

 in which a forest soil is annually renewed by the return of the 

 leaves rich in minerals. 



Of all the elements taken from the soil nitrogen is the most 

 important. The chief source of nitrogen is the raw humus com- 

 posed of decayed leaves and wood. It has been found that dead 

 leaves mixed with soil absorb nitrogen from the atmosphere in 

 large quantities, probably through the assistance of bacteria, 

 mosses, and lichens. This capacity of the bacteria is reduced 

 by frost, so the supply of nitrogen is less in the open in places 

 exposed to frost. Some plants take in nitrogen from the air 

 through their leaves. About 10 pounds of nitrogen are carried 

 to the ground per acre annually by rain, and the litter of leaves 

 and twigs carries to the ground in a beech forest 40 pounds, in 

 spruce 28 pounds, and in pine 26 pounds of nitrogen per acre. 

 On the other hand, the annual consumption of nitrogen for wood 

 production is 9 pounds for beech per acre; 12 pounds for spruce 

 and fir; and 6J pounds for birch. An investigation in a planta- 

 tion of maritime pine on a sand dune 56 years after planting 

 produced, from a layer of soil 6 inches deep, 7 tons of organic 

 matter per acre with nitrogen 1.5 per cent, or 248 pounds of 

 nitrogen per acre. This is an annual accumulation of 4.5 pounds 

 per acre. In another plantation the average accumulation was 

 7.2 pounds per year. 1 



As regards the chemical composition of soil, sour, marshy soils 

 are unsuited to most species except Scotch and white pine and 

 spruce. These are about the only species that will thrive on 

 pure peat. Ash, maple, beech, and elm require a moderate 

 amount of lime in the soil, and oak, locust, European larch, and 

 Austrian pine thrive best on soils which contain some lime, 

 while chestnut seems to do better in a soil containing very little 

 lime. Some recent investigations in soil seem to show the com- 

 manding importance of lime as a controlling factor in tree growth. 

 Most trees are lime-loving to a certain extent, but an over- 

 abundance of lime in all cases is unfavorable. The hardwoods - 



1 See Forestry Quarterly, Vol. VI, p. 290, and Vol. VII, p. 192. 



