Quantitative Distribution, Forest Soil Microorganisms in a Yellow Poplar Plantation

Abstract

Since living microbes such as fungi, bacteria, and actinomycetes exert a profound influence upon the genetical development of soil profiles, they also exert an indirect effect upon the growth of forest trees. This is accomplished by altering the physical, chemical, and microbiological characteristics of the soil profile. Although the quantity, or numbers of organisms in any given soil, are only relative indices of forest soil fertility, these quantitative counts may offer clues as to the effect of other site factors. Lohnis and Fred (1923) came to the conclusion that a determination of bacterial numbers in the soil is worthless as an attempt in interpreting soil phenomena. However, later investigators (Waksman, 1927; Waksman and Starkey, 1931) have clearly shown that studies oln the numbers of organisms are important as well as indicative, when used in conjunction with other edaphic and climatic factors. In comparing numbers of bacteria, no single variable such as season of the year can be considered alone without reference to moisture content, acidity, and other factors, as shown by Waksman (1927). Studies of soil organisms in both natural and planted forest stands by Shipman (1947) have demonstrated that forest site quality is indirectly related to soil organism populations. Soil, whether in a natural state or once in cultivation, cannot be considered uniform in its spatial distribution of soil microorganisms. While the qualitative distri-bution of soil organisms remains much the same, there are very marked differences in the quantitative relationships of a given soil area (Waksman and Starkey, 1931). It is important to study these numerical variations, as expressed by horizon differentiation, whether visible natural horizons are present or not. In evaluating existing forest sites for actual or potential growth, or in appraising the value of any land to produce a crop, one cannot ignore the soil inhabitants and their effect upon the final crop. This microbial effect may be direct, as in the case of soil pathogens, or it may be indirect by altering the physical and chemical characteristics of the crop substrate. A field and laboratory study was undertaken to determine the total numbers, horizonal distribution, and seasonal fluctuation of certain forest soil organisms under a combination of environmental factors.