SOIL EXTRACT IN SOIL MICROBIOLOGY

Abstract

Soil extract contains unidentified growth factors not present in other materials. In this study the extract was considered critically in so far as various treatments affect numbers of bacteria developing on soil-extract agar. Dilution of the extract to 1/10th concentration produced lower counts in most soils. Calcium sulphate and calcium carbonate, used as precipitants, failed to produce more or clearer extracts. Unheated extract provided the nutritional requirements for a considerable fraction of the bacteria in soil, but not for as large a fraction as did heated extract. Dried extract produced lower counts than did freshly prepared extract. The addition of 0.02% dipotassium acid phosphate to soil extract increased counts significantly. The use of the proportion of soil and water that gave the amount of extract desired, without the addition of water after filtration, produced the maximum amount of extractives from the amount of soil used.