[Market-farming] Seed Starting Mix

Steve Diver steved at ncat.org
Mon Sep 12 16:23:49 EDT 2005

Pounds of fertilizer per cubic yard is a
common rate of application for potting mixes.

There are 27 cu. ft. per cubic yard.

One cu. ft. equals 7.48 gallons.

A quick calculations shows that 15 gallons
of media is roughly 2 cu. ft.

Interestingly, a common nursery mix quotient
uses 120% of "potting media" components
to arrive at the known volume of "potting mix"
needed, because the ingredients settle 20% when
mixed and mingled together.

To determine porosity of the mix -- to understand
if the ratio of media components is adequate or
that it needs to be adjusted-- a water volumetric test
is performed, accompanied by calcuations. 

An eyeball observation test of water drainage may
be satisfactory, to confirm the mix has adequate
drainage and aeration... that it's not too tight with
poor drainage and aeration.

One of the Bioform dry formulations is 5-3-4.


One strategy is to look into conventional horticulture
literature and find an NPK fertilizer with a comparable
analysis of 5-3-4, and simply use the weight to volume
rate already suggested.

The other strategy is to eyeball the volume you
have in front of you, and dump enough fertilizer
into the mix that "looks" right.

How much volume does a five pound bag of Bioform
fill up.... a quart, a half gallon?  

Maybe you only need a pint of Bioform per 2 cu. feet
of mix, or maybe the plants will respond to a quart.... 
but a half gallon is probably "too" much.

Ultimately, you don't know until you grow out
a crop and evaluate plant growth and plant health.

Evaluating a graduated fertilizer series is better than
using one rate, alone, to understand the optimum
rate of application.  

E.g., use 2.5 lbs, 5 lbs, 7.5 lbs, and 10 lbs
per volume of potting mix (though, for 2 cu feet
I'm guessing the 2.5 and 5 lb rates will be adequate).

You can do a fertilizer series on a much smaller
volume of potting mix and evaluate germination
rate, germination percentage, and potential
phytotoxicity in a bioassay grow-out trial, using
seeds and seedlings of wheat, rye, cress, sunflower,
buckwheat, tomatoes, cucumber....  if you 
really want to grasp the weight to volume rate
of application with respect to fertilizer boost vs.
fertilizer burn.

Hope that's helpful,

Steve Diver

 >I'm making some seed starting mix and I want some advice about how much
 >Bioform fertilizer to add. I have mixed 5 gallons of sand, 5 of broken
 >down leaves and 5 of soil. Now I want to add fertilizer but do not know
 >how much to add. Should I add anything else? Can anyone help?
 >Marty Kraft
 >Kansas City

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