Microbial activity in soils depends on the status of the soil water, which
is expressed by pressure head (h) or water content (theta). There is no uni
que relationship between theta and h because moisture relations exhibit hys
teresis, For convenience microbial activity has usually been related to the
main drying curve but in general this will lead to bias in computer models
that aim to simulate microbial activity. This article aims to evaluate the
magnitude of the bias resulting from the use of computer simulation models
of mineralization coupled to a model that follows the hysteresis of water
relations in soil. Simulations of mineralization were found to accumulate a
bias of more than 15% of the mean annual mineralization by harvest. This b
ias was found to be most serious where soil is continually subject to sever
e wetting and drying cycles such as can be found in irrigated and tropical
agriculture.