6 FORESTRY IN NEW ENGLAND 



bonic acid of the air; the hydrogen and oxygen are derived chiefly 

 from the water in the soil; and the nitrogen from the ammonia 

 of the soil formed by nitrifying bacteria from organic decaying 

 matter. These other chemicals taken from the soil in salts and 

 oxides are contained in very small quantities. Thus iron, 

 though very important for the formation of chlorophyll in the 

 leaves, is present only in very small quantities, so the old remedy 

 of driving nails into a dying tree was of no value. 



The percentage of pure ash in the dry leaves of trees varies 

 from 2.3 per cent in white pine to 7.6 per cent in the ash tree, 

 and dry leaves have a higher percentage of minerals than wood. 

 This indicates the well-known fact that conifers are less demand- 

 ing as regards chemicals than broadleafs. For this reason the 

 soil improves under coniferous crops if a good canopy is main- 

 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 6| 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 



