FORMATION OF LEAFMOLD COVILLE. 335 



brown leaf by rain to the black mellow leaf mold in which all traces 

 of leaf structure have disappeared. When freshly fallen the leaves 

 show 0.4 normal acidity.^ Those not familiar with the chemical expres- 

 sion " normal acidity " may perhaps most readily understand the term 

 by reference to ordinary lemon juice, which has very nearly normal 

 acidity in the chemical sense. Fresh oak leaves may be conceived 

 therefore as having about one-third the acidity of lemon juice, gram 

 to cubic centimeter. From a soil standpoint such a degree of acidity 

 is exceedingly high. Probably no tree or flowering plant could live 

 if its roots were imbedded in a soil as acid as this. A correct ap- 

 preciation of the excessive acidity of freshly fallen leaves enables 

 one to understand why it is that the leaves of our lawn trees, if 

 pllowed to lie and leach upon the grass, either injure or destroy it. 

 On such neglected lawns the turf grows thin, mossy, and starved. 



From the height of their initial acidity it is a long descending 

 course through the various stages of leaf decomposition to the point 

 of chemical neutrality, and then upward a lesser distance on the 

 hill of alkalinit}^, in the black leaf mold stage. 



In order to ascertain the rate of decomposition in leaves of various 

 kinds, observations were begun in the autumn of 1909 on leaves of 

 silver maple (Acer saccharmmn) , sugar maple {Acer saccharum), 

 red oak {Quercus rubra), and scrub pine {Pinus virginiana) , ex- 

 posed to the weather in barrels and in concrete pits. In one experi- 

 ment a mass of trodden silver maple leaves 2 feet in depth, with an 

 initial acidity of 0.92 normal, was reduced in a single year to a 3-inch 

 layer of black mold containing only a few fragments of leaf skeletons 

 and giving an alkaline reaction. In these experiments sugar maple 

 leaves have shown a slower rate of decomposition than those of silver 

 maple, while red oak leaves still show an acidity of 0.010 normal after 

 three years of exposure, and leaves of Virginia pine an acidity of 

 0.055 normal under the same conditions. 



The alkalinity of leafmold is due chiefly to the lime it contains, 

 the lime content expressed in terms of calcium oxid often reaching 

 2 to 3 per cent of the dry weight. One sample had a lime content of 

 3.55 per cent. Many of the soils that result directly and exclusively 

 from the decomposition of limestone have a lower percentage of lime 

 than this. An alkaline leafmold containing 2 to 3 per cent of lime is 

 properly regarded as a highly calcareous soil. Yet such a deposit may 

 be formed in a region where the underlying soil is distinctly non- 

 calcareous, the lime content of the soil being only a small fraction 

 of 1 per cent and the soil reaction being acid. 



1 In the acidity determinations, made by Mr. J. P. Breazeale, phenolphthalein was used 

 as an indicator, tlie carbon dioxid liaving been first boiled off. For a full description 

 of the method followed, see Coville, 1910, p. 27, Experiments in blueberry culture. Bul- 

 letin 193, Bureau of Plant Industry, U. S. Department of Agriculture. 



