A NATIONAL PLAN FOR AMERICAN FORESTRY 311 



Forest litter is the layer of fallen leaves or needles, of dead branches, 

 down trunks, and other vegetable remains, which in varying depth is 

 found under the crowns of trees and brush species in every temperate- 

 zone forest. Through the gradual processes of decay and chemical 

 change, and through the agency of animals which trample or other- 

 wise disturb the surface of the ground, this litter is disintegrated into 

 humus. Percolating water then carries the fine particles of humus, 

 into the soil, where they are further broken down into nitrogenous 

 products by bacteria and other organisms. 



Forest litter exerts its influence in several ways. First and most 

 important, it contributes to the humus content of the soil. It is an 

 axiom in agriculture that humus, or organic matter, makes a heavy 

 soil lighter, and a light soil heavier, by causing the soil particles to 

 form crumbs. A crumb structure gives the maximum room for air 

 and water, both vital to plant growth. How powerful an effect 

 organic matter, although an unimportant fraction by weight in most 

 soils, has on the water-holding capacity of the soil is illustrated by 

 analysis in table 5, made by George Stewart of a granitic sand sup- 

 porting ponderosa pine in Idaho. About 200 samples of the soil were 

 taken to a depth of 4 inches, from openings, some large and some 

 small, in a virgin stand. The condition of the vegetation refers to 

 its value primarily as forage, and the deterioration is the result of 

 grazing. 



TABLE 5. Analysis of granitic sand soil under a ponderosa pine stand in Idaho 



1 In percentage of dry weight of soil. 



The ability of this soil to absorb water was nearly halved by its 

 loss of a very small quantity of organic matter. Inasmuch as the 

 soil of any watershed is the great underground reservoir replenished 

 from time to time by precipitation, but at all times draining into the 

 streams, its absorptive capacity is the great factor in sustained stream- 

 flow. Humus and the decaying roots of plants enormously increase 

 this capacity. 



A second major influence of forest litter is its promotion of water 

 percolation. If a soil is extremely shallow, or if precipitation is 

 unable to percolate into it rapidly, run-off must take place over the 

 surface from any but the lightest storms. If rain falls upon bare 

 soil it becomes muddied and carries fine material in suspension 

 downward into the minute interstices between the soil particles. 

 How promptly and completely muddy water will plug these pores 

 and slow the rate of percolation has been demonstrated by Lowder- 

 milk. 16 After establishing, over a period consisting of parts of 7 



16 Lowdermilk, W. C., "Influence of forest litter on run-off, percolation, and erosion." Jour. Forestry, 

 April 1930. 



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