Determination of water activity in soil/compost

Peter Herman herman at
Fri Jan 12 10:14:57 EST 1996

K N and P J Harris (ecoli at wrote:
: Soil water activity.
: Get back to me on <P.J.Harris at> and I will put your enquiry 
: to our best soil physicist. Basically soil scientists do not work on 
: water activity terms but matric and osmotic potentials. there is no real 
: reason why there should not be common ground (and even better - 
: conversion factors).

Alas, it isn't an easy conversion and those of us who talk to both 
microbiologists (water activity users) and ecologists/soil scientists 
(psi users) tend to follow the Farenheit Centigrade model and just think 
about the 2 scales independantly  ie plants wilt at -15 bar or 0.90 Aw.

The conversion is Psi = RT ln Aw/V

where R and T are gas constant and temp Kelvin and V is partial molar 
volume.  Stolp's microbial ecology book in the Cambridge series gives a 
good treatment of this and a table of Aw and Psi of various environments


R. Peter Herman				email	Peter.Herman at
Sveriges Lantbruksuniversitet		Phone:	+46 18 67 12 20
Inst. f. Markvetenskap			Fax:	+46 18 67 27 95
S750 07 Uppsala, Sweden		

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