Water retained by various soil ingredients

Peter Taggart petersirises@gmail.com
Wed, 24 Oct 2012 10:57:50 PDT
I think that the basic principle for growing mixes is being glossed over in
this discussion; that  growing mediums consist of a THREE way ratio of AIR

Increasing the particle size will increase the proportion of air in the
mix, provided that the larger particles are a high enough proportion of the
ingredients so that they touch each other in order to have air gaps between
them, - the threshold for this to work is about 1/3.

To use moisture retaining particles will make a damper mix,  (I use leaf
mould for this job - it helps stop sudden dryness which causes premature
dormancy in many corms). Coarse leafmould, large particles of perlite,
larger pieces of composted bark, (?pumice)... will all raise the quantity
of air and water in the mix and reduce the proportion of substrate (and
reduce the weight).

Small  particles of moisture retaining ingredients will  increase the water
content, reduce the air content and increase the water even more as well as
the amount of substrate, and therefore the weight will go up.

Small particles of non water retaining ingredients such as fine sand, clay,
loam, rock dust.... will increase the water (due to capillary action) and
substrate content, make a denser and heavier mix, and reduce the air

Larger particles of non water retaining ingredients will make for a mix low
in water, high in air, low in substrate and low or high weight depending on
what the particles are made of (- usually gravel which is heavy).

considering this then take the ingredients you have and mix for the air :
water : substrate  proportions which are desired, remembering the other
properties of those ingredients such as weight, nutrient retention, PH...
and your personal growing conditions.

Peter (UK)

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