Is there a test to get a percentage of organic matter in a soil
sample? Anyone know how is this typically done in a lab? I was
wondering if the sample is weighed, burned off, and re-weighed to
calculate a percentage? I guess compost made from leaves, twigs, peat
moss, grass would be close to 100% "organic matter." Sand, stones,
clay, perlite, vermiculite, water, nitrogen salts would be "inorganic
matter?" What I am hearing is that good soil must have greater than
5% organic matter.
Many labs test for this......its just a matter of finding one locally.
The lab Victoria referenced is an excellent one and does go about it
differently, using chemical extraction rather than burning, which is
However, I tend to agree with Bill on this matter. It is difficult to
overapply organic matter as it continues to breakdown and gets
converted to other, essential elements. It simply doesn't last very
long :-) And few soils tend to have an overabundance anyway, so
routine applications of a compost or organic mulch of your choice or
growing cover crops is generally advised for most home gardens and
especially those that focus on harvested, edible cropping.
Yes. The sample should be taken with surface organic material scraped
aside. Sample should be taken from a hole 8" to 12" deep (the material
from that hole is your sample). For organic analysis, the sample would
first need to be dried and weighed, then aerated and held at a temp
greater than 455F for several hours, and then reweighed.
If you put a similar sample in a large jar with water and shake your
sample until it forms a slurry, the sand will fall out after five min.
or so, the silt after 20 min., and the clay after 24 hours (descending
particle size). The height of each band in comparison to the total
sedimentation will give you the approximate composition of your soil.
Good soil will be 20 - 30% clay, 30-50% silt, 30 - 50% sand, and 5 - 10%
There is more about microorganisms and their interactions with plants
but I've already gone beyond what you asked.
One cautionary note though, beware chemical fertilizers. This doesn't
come from ridged ideology but from the facts that numero-uno: chemical
fertilizers, used at suggested rates, kill off soil organisms, which
leads to less top soil, which leads to the use of more chemferts to
maintain production, and numero-two-o: the translocation of the nitrogen
to the plants now rapidly growing leaves (nitrogen rich, tender leaves)
makes them a target for garden insects.
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