An approximate indirect evaluation of water retention relationship for small pots

The determination of the water retention relationship for substrates in small pot is not common especially when preparing mixture of peat with sand or perlite in different proportion. On the other side it is not easy to interpolate between primary materials for different mixing ratios. A theoretical development is presented considering a truncated cone pot (or a cylindrical one as a limit) and assuming a linear relationship for water retention. Applications to mixtures of peat and sand or peat and perlite with different ratios evidence strong differences of these two series due both to changes of mean porosity and changes in the slope of the water characteristics. Consequences on air content and from different pot shape are outlined.