Phil Busey Agronomy
Consulting Inc.


 

Measuring organic matter: Weight and volume converter

Return to Organic Matter

The Problem:  The first converter below calculates organic matter content by weight % of a soil mix that will result based on physical characteristics of the organic and sand sources and the volume mixing ratio. The second converter goes backwards to verify, based on soil test organic matter content by weight %, and the alleged volume mixing ratio and typical organic matter conents of organic sources, if there resulted in a plausible value for organic source bulk density.

The equations of these calculators are at the bottom of the page.

Whenever tests of root-zone mixes have less than 1.0% organic matter by weight it is likely that not enough organic source was used, or a poor quality source was used. Ideally organic matter should be derived from fibrous peat such as sphagnum and the resulting organic matter content for sand root-zone mixes should be between 1% and 5%, with values near 1.5%, to achieve adequate nutrient exchange capacity, available water capacity, and moderate stability. Higher organic matter content can be achieved with lower quality sources, so acceptable organic matter content cannot be evaluated by quantity alone without knowing the source. However, very low organic matter content is a clear indication of potential problems.

 Question:  What will be the estimated organic matter content (% weight) of a soil mix?
 Initial numbers in the blanks are provided as examples.  Replace with values you measure.
 1. Describe the organic source.
"The bulk density of the organic source was with an organic matter content of % ."
 Sample values for sphagnum peat are 0.08 to 0.25 for bulk density and 87% to 96% for organic matter
 Sample values for Florida muck are 0.26 to 0.73 for bulk density and 40% to 50% for organic matter
 2. Describe the sand source.
"The bulk density of the sand source was with an organic content of %."
 3. The mixing ratio was.
"The mixing ratio was % organic source by volume."
 4.   "The resulting mix should be % organic matter by weight."
 Question:  What is the estimated bulk density of the original organic source?
 1. What is organic matter content of the resulting soil mix (assumes no OM in the sand source, otherwise enter the difference in OM% between the sand and the mix)?
"The soil mix is % organic matter by weight."
 2. What was the bulk density of the sand source?
"The sand source was bulk density.
(And for simplification we assume the organic content of the sand is 0%)
 3. What was the organic matter content of the organic source?
"The organic source had % organic matter."
 If Florida muck is suspected as the organic source, a suggested value is 40% organic matter
 4. What was the mixing ratio?
"The mixing ratio was % organic source."
 5.   "The original organic source had bulk density of ."
 Comment: be suspicious of very low values, below 0.07, unless it is known for sure that sphagnum peat was used

The first conversion question, "What will be the estimated organic matter content (% weight) of a soil mix?" is answered by the equation for organic matter in an organic source-amended sand blend:
OM3 = (Db1 * r * OM1 + Db2 * (1-r) *OM2 ) / (Db1 *r + Db2 * (1-r))
where:
OM3 = organic matter content by weight of blend
OM1 = organic matter content by weight of organic source
OM2 = organic matter content by weight of sand
r = mixing ratio by volume
Db1 and Db2 are bulk densities or organic source and sand, respectively

The second conversion question, "What is the estimated bulk density of the original organic source?" assumes the sand has no organic matter (if it does, you can report OM3 as the difference between OM2 and OM3), and the above equation is simplified:
OM3 = (Db1 * r * OM1) / (Db1 *r + Db2 * (1-r))
and solved for Db1:
Db1 = (OM3 * Db2 * (1-r)) /( (OM1 – OM3) * r)