Withholding P fertiliser for 21-26 years on sheep-grazed pasture led to an

exponential decline in exchangeable P despite expected zero or slightly positive P balances; on one of the two sites this was accompanied by a decrease in [P.

Perhaps the onus of proof, in distinguishing between an exponential decline and a humped soil production function, rests with the former, because one cannot rule out a humped function without shallow or exposed saprolite erosion rates depicting an exponential decline in soil production.

Furthermore, field evidence from a number of authors supports the theory for the exponential decline in bedrock lowering at depths greater than that where soil production is maximised.

Exponential decline has been found to closely approximate production decline from producing fields, although, as discussed by Nind [1981], this can lead to a slight negative bias in the resulting estimation of ultimate recovery and well life when using exponential decline models.

Given exponential decline, constant pumping costs, and data on (1 - R) [P.

With the assumption of exponential decline, we can write another expression for [q.

Specifically, he reports an

exponential decline in brightness with a half-life of about 113.

Simplistic assumptions were made about

exponential decline of emissions and dispersion over the first 10 days of the accident (13).