STATISTICAL-ANALYSIS OF REDUCTION IN TENSILE-STRENGTH OF COTTON SHIPSAS A MEASURE OF SOIL MICROBIAL ACTIVITY

The cotton strip assay (CSA) is an established technique for measuring soil microbial activity. The technique involves burying cotton strips and measuring their tensile strength after a certain time. This gives a measure of the rotting rate, R, of the cotton strips. R is then a m easure of soil microbial activity. This paper examines properties of t he technique and indicates how the assay can be optimised. Humidity co nditioning of the cotton strips before measuring their tensile strengt h reduced the within and between day variance and enabled the distribu tion of the tensile strength measurements to approximate normality. Th e test data came from a three-way factorial experiment (two soils, two temperatures, three moisture levels). The cotton strips were buried i n the soil for intervals of time ranging up to 6 weeks. This enabled t he rate of loss of cotton tensile strength with time to be studied und er a range of conditions. An inverse cubic model accounted for greater than 90% of the total variation within each treatment combination. Th is offers support for summarising the decomposition process by a singl e parameter R. The approximate variance of the decomposition rate was estimated from a function incorporating the variance of tensile streng th and the differential of the function for the rate of decomposition, R, with respect to tensile strength. This variance function has a min imum when the measured strength is approximately 2/3 that of the origi nal strength. The estimates of R are almost unbiased and relatively ro bust against the cotton strips being left in the soil for more or less than the optimal time. We conclude that the rotting rate R should be measured using the inverse cubic equation, and that the cotton strips should be left in the soil until their strength has been reduced to ab out 2/3. (C) 1997 Published by Elsevier Science B.V.