How To Prepare Soil For Plants

Soil Preparation
Marijuana grows best in a well-drained sandy soil or loam which is high in
nitrogen and potash, at least medium in phosphorous, and which contains little
or no clay. The pH should be between slightly acidic (6.5) and slightly
alkaline (7.5). If the pH is either too low or too high it will interfere with
nutrient uptake. The pH is measured on a scale of 0 to 14 with 7.0 assigned as
neutral. pH is a measure of the relative concentrations of hydroxonium ions
(H3O+) to hydroxide ions (OH2-). Several varieties of test kits and meters can
be purchased from garden stores which test the pH and nutrient contents of the
soil. Litmus paper (purple or blue) can be obtained from some chemists. Many
agricultural schools or colleges will test soil pH and nutrient contents for a
nominal fee.
Soil pH is raised to an acceptable level by adding slaked (hydrated) lime,
limestone and marl. There is no set formula we can give for raising the pH. At
a low pH it takes less time to raise the level by one point than it does when
the pH is nearer to neutral (7.0). Sandy soils require less lime than clay
soils to raise the pH. In general, if the soil tests acid, add 2 cups of
hydrated lime for each 50 lb bag of soil. On a more modest scale, this works
out at roughly one tablespoon for every 1 1/2 lbs of soil. Wet the soil
thoroughly after mixing. Re-test the soil in about 2 weeks, and repeat the
application until the pH is in an acceptable range. Soil that is too alkaline
is treated in the same way, but with aluminium sulphate at a rate of 1/2 cup
per 50 lbs of soil.
If you are digging up your soil, sift it well to remove stones and root clods.
Bake the soil in a 200 degree centigrade oven for twenty minutes in one-inch
layers, in a pressure cooker at 15 lbs/sq in pressure, or in a microwave on
full for ten minutes (make doubley sure there are no foreign particles in it).
This will destroy any weed seeds, insect eggs and disease organisms in the
soil.
It is, much, much, simpler to buy commercially prepared soils. These are
usually sterilised (but if the bag has been opened or holed, sterilize again
anyway) and have a good balance of nutrients. Ask for soil with a neutral pH.
Some nursery-men (and women) will sell you anything, so check the pH anyway
and reject any soil for "lime-hating" plants.
We have had good results from some brands using composted seaweed as an
ingredient.
The consistency and structure of the soil is important for healthy root
development, drainage and uniform water dispersion. The medium should not cake
when dry (this rules out John Innes No. 3) and should remain spongy or loose
when wet. Test the consistency,and adjust it if necessary.
To test the consistency of the soil, lightly moisten some and squeeze it in
your fist. The ball should crumble easily when touched. Soil consistency can
be adjusted by adding perlite, vermiculite, sand or kitty litter.
Perlite and vermiculite are inexpensive commercial products which are much
lighter than sand and are sterile. Vermiculite absorbs and holds water and air
in its fibre. Perlite traps moisture and air on its irregular surface much
like sand. Sphagnum or peat moss is often used to adjust soils, but should not
be used for marijuana as it tends to make the mix acidic. If you have alkaline
soil to start with though, you might get away with it.
Soils found deficient in nutrient content can be enriched by adding humus
(decayed organic matter) or fertilizers such as rose food, liquinure etc.
Humus is acidic and can alter the pH.
Soil-less mixtures are inexpensive and easy to prepare. They work well, are
neutral in pH and light in weight, but have absolutely no nutrient content.
They must be carefully fertilized and are not recommended for an inexperienced
grower. It is all too easy to over- or under-fertilize the plants.
Two tested formulas are:
1) One part perlite or sand to one part vermiculite and 1 tbsp of lime per
quart of mixture.
2) One part perlite or sand to one part Jiffy Mix and 1 tbsp of lime per quart
of mixture.
You can mix three parts of the soil-less mixture to one part cow manure, or
rely solely on soluble fertilizers when watering.
Simple Guidelines
Some of you who are not familiar with gardening may be a little over-whelmed
by all this talk of pH, nitrogen etc... so here is a simple uncomplicated
formula for those of you with no experience with plants:
Buy commercial soil. Avoid brands that have peat in their names, and not just
because peat is an over-exploited resource. It is very unlikely that a
commercial soil will be too alkaline for healthy growth, but it may well be
too acidic. The simplest way to assure yourself your soil is not too acidic,is
to put a piece of damp blue litmus paper (blue litmus is available from some
chemists and garden centres) under the surface of the soil. Wait a few minutes
and then take it out. It will not harm the soil - litmus comes from a lichen.
If the paper turns pink, the soil is acidic and lime must be added. Mix two
cups of slaked (hydrated) lime, from your garden store, to each 50 lbs of
soil. Don't add lime if the paper remains blue. If you find yourself asking
"is this paper pink, purple or just wet ?", then the soil is probably slightly
acidic and within an acceptable range for healthy growth.
Mix 10 lbs of natural, sterilized fertilizer (usually based on cow manure or
chickenshit) to each 50 lbs of soil. If you have some aversion to muck, or
don't like the smell (well rotted manure doesn't smell, by the way) then you
can use a soluble fertilizer when watering. We use a brand called
'phostrogen', which seems to work well. By volume, also mix in one part of
sand, perlite or vermiculite for every three parts of your soil. This helps
maintain the right soil texture for healthy root growth.
After potting and watering, the mixture should be re-tested in about a
fortnight if it tested acidic to begin with.
If it is still acidic, add hydrated lime by mixing one tbsp of lime per quart
of water, the first few times you water. Test your water supply by dipping a
piece of litmus in plain water to determine if it is influencing your tests.
To pot any of the mixtures, cover the drainage holes with a square of
newspaper or window screen to prevent soil from running out. Next, put in a
layer of sand, perlite, vermiculite or kitty litter about one inch deep to
ensure good drainage. Fill the pots to within 3/4 inch from the top of the pot
with the soil mixture. Water the pots until the soil is evenly moist, and
allow the pots to stand for a day or two so that bacteria necessary for
nutrient uptake can begin to grow and the fertilizers can start to dissolve
into the soil.