66 BARRINGTON MOORE Vol. Ill, No. I 



Humus has long been recognized as extremely important; in fact, the 

 accumulation of humus in sandy soils has been considered sufficient to enable 

 such soils to support the same kind of forest as that growing on the more 

 fertile soils of the same climatic region. But, prior to Hesselman's work, 

 the value of the humus had been attributed very largely, if not entirely, to 

 its effect upon the water-holding capacity of the soil. It is known to render 

 heavy soils lighter and better aired, and to make sandy soils less subject to 

 rapid percolation and more retentive of moisture. Its influence has been 

 considered almost wholly from the point of view of its effect upon the physical 

 properties of the soil. 



Soil reaction, acidity or alkalinity, is known to be of much importance, 

 but most attention has been given to degree of acidity, with comparatively 

 little study of alkalinity except for toxic salts in arid regions. It would be 

 interesting to know the effect of alkalinity upon tree growth and whether 

 certain trees are kept off of moderately alkaline situations by competition — 

 or because they are unable to grow there. 



Experiment 



An experiment was carried out in the summer of 1919 at Mt. Kisco, 

 about 30 miles north of New York, to determine as far as possible (i) the 

 influence of humus on tree growth, aside from its effect on the moisture- 

 holding capacity of the soil, and (2) the influence alkalinity produced by 

 slaked burnt lime (calcium hydrate) . 



From the point of view of fire protection alone, the influence of humus 

 upon tree growth is important. Ground fires, such as those which fre- 

 quently, often intentionally, run through hardwood forests and longleaf pine 

 forests, destroy a certain amount of humus (3). 



Lime is considered beneficial, so that any injury from lime should be due 

 chiefly to alkalinity, although this is not absolutely certain, because it is prob- 

 able that some trees do not thrive on lime soils which are neutral. The 

 major part of the influence should, however, be due to the reaction. 



In studying the influence of humus three soils were used, representing 

 two extremes, absence of humus and pure humus, and an intermediate con- 

 dition, a mixture of sand and humus. The first soil was pure sand, a rather 

 sterile, fine, grey, water-worn, glacial sand, predominantly quartz, taken from 

 about three feet below the surface of a glacial esker. The physical prop- 

 erties may be summed up by stating that the wilting coefficient, on the basis 

 of air-dry weight, was .85 percent.^ The reaction was neutral. 



The pure humus, the opposite extreme of the pure sand, was a dark brown 

 leaf mold not completely decomposed. It was collected from between crev- 

 ices in a rock at the base of a cliff to insure a minimum of mineral soil. The 



2 The wilting coefficient figures throughout this paper are approximations, but serve 

 to indicate differences in the physical properties of the various soils. 



