A Comparison of a Direct- and a Plate-counting Technique for the Quantitative Estimation of Soil Micro-organisms

Abstract

SUMMARY:Estimates of numbers of micro-organisms occurring in three differently manured soils, made by a direct-counting and plating technique, were compared. No correlation was found between the two methods and reasons for the large discrepancy between them are discussed. Contradictory information of the effect of external factors on soil micro-organisms can be given by the two different methods of counting.Estimates of the total numbers of bacteria in soil obtained by a direct-counting technique devised by Jones & Mollison (1948) are of the order of several thousands of millions/g. soil, while estimates by conventional plating methods are in tens of millionslg., even on the least selective media so far used. The same authors found a difference of the same kind between the quantity of fungal mycelium in soil as estimated by a direct count of number of pieces/g. as compared with colony plate counts, although in this comparison the former count is only 2-6 times greater than the latter.The effect of external factors such as soil moisture and temperature on the various groups of soil microflora has been investigated by several workers and, while certain correlations have been found, the results have been conflicting. The estimations of numbers of organisms were mainly based on plate counts. The present investigation was undertaken to compare direct counts with plate counts over a period of time, to determine how these varied with soil moisture and temperature and to show whether these two counting methods did, in their different ways, give the same sort of information about the soil population. The soils of three differently manured plots were sampled at intervals (Fig. 1) over the greater part of a year, and determinations made of the moisture content of these samples and of their bacterial, fungal and actinomycetal populations, by plating and a direct-counting technique. 'METHODSSoil sampZing. The soil samples were taken from three of the classical plots of Broadbalk field, which has grown wheat continuously for just over 100 years and whose plots have had different manurial treatments over that period.Plot 2 (farmyard manure), plot 3 (no manure) and plot 7 (complete minerals + ammonium sulphate) were sampled at intervals between January and December. Soil was collected by an auger to a depth of 4 in. in 12 areas on each plot; these were bulked and passed through a 3 mm. sieve. From these sieved samples the necessary amounts of soil were taken for the determination

Microbial colonization of retorted shale in field and laboratory studies

“…The limitations of determining bacterial or fungal cfu that are representative of microbial communities by plate counts have been examined by numerous researchers (Butterfield 1932, Egdell et al 1960, Gossling 1958, Jensen 1962, Skinner et al 1952, Weiringa 1958; the general consensus is that plate counts are by no means quantitative for the total microbial community and only represent a fraction of the total. Moreover, this fraction also does not represent an absolute value but a relative value dependent on well controlled and reproducible laboratory techniques, as well as the composition of the plating media.…”

“…The use of plate count techniques in estimating bacteria communities in soil have been critically evaluated by numerous researchers (Butterfield 1932, Skinner 1952, Weiringa 1958, Gossling 1958, Egdell et al 1960, Jensen 1962. A primary criticism concerns the inability to measure the total bacterial community in soil using a given growth medium because a given growth medium is selective for certain bacteria present.…”

Muramic Acid as a Measure of Microbial Biomass in Black Sea Sediments

“…Not only were the plate counts of classical microbial ecology underestimated by one to five orders of magnitude, but there were spiral-shaped organisms seen in direct counts of formaldehyde-treated dilutions that were not seen in plate counts. Other studies have shown no correlation between direct and plate counts (Skinner et al, 1952). Indeed whole groups of unusual shaped organisms have been seen to grow in special flattened capillary tubes implanted in soils and aqueous sediments.…”

Productivity, economics and soil fertility of soybean - safflower croppingsystem in response to nutrient management practices

“…Microbial biomass-C (MBC) and -N (MBN) were estimated by using chloroform fumigation incubation method as described in Horwath and Paul (1994). Soil microbiological properties and microbial counts were estimated by using standard methods described by Jones and Mollison (1948) and Skinner et al, (1952).…”