Soilless Mixes & Amendments

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The basic component of soilless mixes is sphagnum peat moss. The various amendments added to each mix include , coir, vermiculite, dolomitic limestone, fertilizer, trace elements, perlite, wetting agents, gypsum,  rock wool, calcine clay, and pine bark. There are several manufacturers who blend these materials together to produce a range of special purpose mixes for greenhouses, nurseries, and gardeners.  These mixes are quite variable across North America because of shipping costs. That is, the constituents are variable in nature, and there use will be heaviest in the region of their origin. Some of the popular brands include Terra-Lite, Metro Mix, Sunshine, and Pro-Mix.  Each mix comes in several grades, the difference lying in the percentage of each of the amendments. This article is directed to recreational gardeners who are sowing seeds indoors or in a hobby greenhouse, are looking for the best seed sowing medium and the latest technique.

Germination mix, plug mix, and starter mix are the terms applied to mixes formulated for seed germination.  Other formulations include growing mix, rock wool mix, bark mix (high porosity), and bark mix (low porosity).  For your suitability testing of product, stick to those mixes labeled germination, plug, or starter.  These mixes usually include the peat moss/bark/coir mixture plus perlite, vermiculite, dolomitic limestone, and a wetting agent.  Some germination mixes will also include some fertilizer and trace elements.  Fertilizer and trace elements are not required for seed germination.  I prefer a mix without them, however I do use a weak fertilizer solution at initial sowing.  The characteristics of  a germination mix include a high water capacity (about 86%), an air capacity of 12 to 15%, and a Ph about 5.8    From this basic mix, you can custom make your own formulation.

Bark:

Southern pine bark and Northern softwoods are usually employed to increase bulk density and increase air space.  In addition, bark will slightly reduce the water holding capacity of the mix, and there is some evidence that bark imparts some disease resistance.  Bark is usually described as aged bark or as composted bark.  Insufficient aging or composting could result in excessive heating of the mix or draw excessive nitrogen, but these materials are usually tested thoroughly before use by reputible companies.  The fertilizer level of these mixes are usually amended with additional nitrogen.  I do not use bark mixes for seed germination, but find them attractive for potting-on in several cases.

Coir:

(pronounced "choir" as in church choir)  Coir dust is a by-product of the coconut fiber industry and is used in mixes to lessen the amount of peat required.  It has a high  air and water holding capacity.  Its nutrient-holding capacity is a bit less than peat, but it does contain significantly higher levels of soluble salts. The use of coir dust as a component of germination and plug mixes is dictated by the high cost of shipping peat down South.

Perlite:

Perlite is a natural heat expanded  volcanic mineral that is chemically inert, and will not alter the nutrient quality or the Ph of the material with which it is mixed. The particles provide excellent drainage, while trapping air and water on their irregular surfaces.  Perlite is usually sold in 4 cu. ft. bags for less than $10 by suppliers to the greenhouse and nursery industries, most of whom will also sell at retail.  Perlite is available in fine, medium, and coarse grades.  

The higher the total porosity of the mix, the greater the potential for providing water and air. Oxygen is essential to healthy root activity, but not more essential than moisture. Practical boundaries are 75% perlite/25% peat moss for lowest wet bulk density and 25% perlite/75% peat moss for highest wet bulk density. Finer grades of perlite decrease porosity of the mix, while coarse grades increase porosity.  For ease of transplanting tender seedlings, adequate levels of aeration and moisture, I have been using the coarse grade of perlite. It provides the additional benefit of maximum root oxegenation.

Added to your germination mix, perlite will improve drainage and may reduce the cost of the overall mix. Outdoors, perlite performs similarly to help hold nutrients from leaching in sandy soil and to improve aeration and drainage in heavy clay soils.  It can also be used as an insulating winter mulch if covered with about 2 inches of soil.

Vermiculite:

Vermiculite is a mica-type material that is mixed and heated to cause the accordion like particles to expand.  This expansion makes them capable of holding large amounts of plant available water and nutrients. As a seed covering, it helps maintain sufficient moisture at the growing medium surface for germination. Available in fine, medium, and coarse grades, I usually select the  medium grade, but rarely have use for this material except in potted plants. Vermiculite is usually sold in 4 cu. ft. bags for less than $12. Caution: Vermiculite is available at home centers and building supply companies as an additive for plaster and insulation in walls and ceilings. Do not use these products for soil conditioning. Construction grades of vermiculite do not absorb water well and are not handled properly during the manufacturing process to maintain a sterile product.  Make sure that the bag is labeled "horticultural."  There are three basic uses for vermiculite.  Added to your germination mix, it will improve water retention and may reduce the cost of the overall mix.  Finally, when added to sandy soil in the garden, water retention is greatly improved.  I prefer to use this material in potted plants only or in situations where seedling development time is long, as in bulb forming plants.

Wetting agents:

This is where the commercial mixes become distinguishable, one from another. Wetting agents are water absorbing polymers available in gel, liquid, or powder form.  The amount of wetting agent added to your germination mix determines the ability of the mix to absorb water from the bottom. Watering seedlings from the bottom is one of several techniques used to reduce the incidence of damping-off.  Therefore, I make bottom absorption my number one criteria in evaluating a commercial germination mix, and do not have to add wetting agents myself. They can be purchased to improve homebrew mixes, and make an excellent additive in gel form when repotting indoor plants. Peat and bark progressively lose their ability to retain and uniformly distribute water and chemicals to the rootzone. Wetting agents available from your distributor may include:

Soax made by Oasis Grower Products, 800-321-8286

PsiMatric made by Aquatrols, 800-257-7797 

Granite grit:

Sifted crushed granite is used as a feed additive to help birds and poultry to grind their food (since they don't have molars).  My feathered friends get theirs from the eave gutters where the grit from the roofing shingles tend to collect.  It is sold in feed stores in three grades. Chick grit (or chicken starter grit) is the fine (smallest) grade.  The bags are labeled "starter" or "fine", hence the name Chick grit. This is the size used as a seed topping.  The next larger size is labeled "grower" or "special pullet".  The largest size is about the size of a pea and carries the label "large" or "coarse" depending upon the supplier.  Cost in the Chicago area is about $4.75 for 50 lb. bag.  Chick grit is sometimes added to the germination mix to improve drainage, especially for alpines, but is usually used as a seed topping.

Other forms of chick starter grit are manufactured from oyster shell or calcium carbonate and can be used in the same manner, with some reservation as to longevity and the amount of long-term calcium release when the product is used for creating alpine soils. These are also offered in screened sizes and labeled in the same manner as granite grit. The "starter size" or "chick grit" of both of the above products is slightly smaller and lighter than granite grit, and therefore suitable for topping smaller seeds. Similarly, other products marketed under the names Terra-Green or Turface are inert sterile calcined mineral absorbent (high temperature processed ground up clay) which may be used as a seed topping or in mixes requiring "sharp" drainage.  Certain grades of silica sand, bank sand, and kitty-litter are also used in these applications, but I would experiment with them very sparingly.  A fine grade of aquarium gravel would be suitable too, but expensive. In some areas, granite grit may not be available due to freight costs, and you may find grit made from "quartzite". This substitute has proven itself the equivalent of granite grit in my own operation.

Adjusting your mix:

The germination mix I use is Sunshine #5 which is called a "plug mix".  It contains sufficient wetting agent for excellent bottom absorption when watering.  Regional distribution across North America may not be even.  In every area, there will be at least one good brand which offers at least one formulation that will be satisfactory.  Finding the best starter mix for you involves searching and testing.  Many suppliers will release one lb. samples for this purpose.

Because these mixes are relatively expensive ($15 to $23 for 4 cu. ft. compressed bale) and because Canadian sphagnum peat moss is not a renewable resource, I prefer to cut the prepared mix with cheaper sterile additives which improve the mix to my needs while lowering the average cost, a little. I add four parts plug mix to two parts perlite and perhaps one half part vermiculite. Sometimes, the seeds have special requirements, and I will make up a special mix just for them.  For example, when sowing Lisianthus, the seed company recommends three parts mix plus seven parts perlite. If I know in advance that a particular seedling is sensitive to damping-off (see the FAQ on seedling diseases), I will combine four parts mix with two or three parts perlite, and dispense with the vermiculite. Potted plants in the sun need more moisture retention, so I will increase the percentage of vermiculite, etc.  Adjusting your basic mix is a matter of "feel" based on your experience.  In some applications, I will add one or two parts of grit to the soil mix for plants requiring "sharp" drainage.

Seed sowing technique:

The fewer the number of seeds you plan to start, the greater the number of seed starting systems available for purchase.  As you increase your seed starting plans over the years, more and more of those seed starting systems become inadequate.  The pots and trays that I use are suitable to the scale at which I operate, that is, the sowing of 300 to 400 packets of seed per year. There are recreational gardeners sowing anywhere from 1 to 2000 packets of seed per year.  At some point, you will discover the need to sow seeds in communal rather than individual pots, especially where considerable numbers of seed types need conditioning before germination. Therefore, the method I describe is peculiar to the scale of my hobby, and may not be suitable for many gardeners.  

My communal pots are 3.5 inch square pots which are four inches deep.  The pot is filled with mix to a 3 inch depth, and tamped down lightly.  Nine to thirty seeds are sown on top of the mix, and covered with 1/16th inch to 1/4 inch of grit, depending upon the diameter of the seed.  Larger seeds get the heaviest topping. Seeds requiring light for germination are sown on top of 1/8 inch of grit, and watered into the topping and covered thinly with a very light dusting of oyster shell.  The "starter" or chick grit size of oyster shell is basically powder.  It has the advantage over silica sand in that it never cakes up. The pots are placed in a tepid water bath until the topping is fully moistened. Since these pots are disposable types, they are labeled on the sides with a pressure sensitive label showing plant identification, germination temperature, and growing-on temperature.  The seeds are sown from the palm of my left hand.  That is, the seed stock is placed there, and with the blade of a tiny pen knife, I pickup a one to four seeds at a time so that they can be distributed in the nine imaginary sowing squares in each pot. To control the distribution of very fine seeds, it is important to me to pick up exactly the number of seeds to be dropped into each sowing area.  I never broadcast sow.

After germination, the pots are moved to the proper location for growing-on temperature.  As soon as the seedlings have their first or second set of true leaves, depending upon vigor, they are transplanted to individual 2 1/4 inch seedling pots.  When the leaves of each plant begin to interfere with plants in adjacent pots, the seedlings are transplanted to 3 1/2 inch, then 5 inch, 6 inch, or 8 inch pots as necessary to maintain constant vegetative growth until it is time for setting out. Additional grit is applied as topping with each transplant to prevent the growth of algae and mosses, and to protect the crowns of the seedlings from excessive moisture. In climates where algal growth on the seed topping is excessive, you can use a product called Agribrom misted onto the topping at the rate of 15ppm. The additional medium added during transplanting to accomodate larger pots is the same germination mix through transfer to 3 1/2 inch pots. For transfer to larger size pots the additional medium added during transplanting is potting mix.  My potting mix is composed of 1/3rd germination mix plus 1/3rd compost plus 1/3rd garden soil. There are many variations of this practice for seed conditioning and special requirements.

After the first transplant, the seedlings need nutrients and trace elements.  The trays are watered from a solution which I make up in 1/2 gallon plastic bottles containing one half tablespoon of household vinegar and 1/8th to 1/4 teaspoon of general purpose fertilizer.  The purpose of the vinegar is to make a slight adjustment in Ph since my water supply tests out at 7.6 on average. Again, there are many variations on this practice based on the Ph preference of each type of plant. Seedling trays are moved to protected positions out of doors, under the carport, in the greenhouse, on the patio, etc. as soon as possible.


The following is excerpted from the practice of a popular nursery owner in Southern Illinois:

"I use Scott's Pro Metro Mix.  I sift it for seed sowing to remove the large bark pieces through a 1/2 inch screen.  Nothing added, unless I am doing alpines, in which case, I add 1/2 part grit.  My transplant mix is one part Metro mix, one part peat, 1/2 part vermiculite, 1/2 part perlite, and one gallon of silica sand for each  3 cu. ft. bag of mix.  For alpines, I add more perlite and some grit.

I use grit as a pot topping for seed growing.  One of the main reasons is to control the various fungal diseases grouped under the general heading of damp-off.  Seedlings come up through the grit easily, and since it dries quickly, and doesn't hold much moisture, the incidence of damp-off is much less than when using moisture retentive substances such as vermiculit or sphagnum.  It also helps to control the growth of lichen and moss under the lights.  I also use it on cuttings.  In addition, I use grit as a top dressing on those plants that dislike moisture around the crowns.  Most of those are rock garden plants.

If the seeds need light, just cover the post with a very light layer of grit, just until you can't see the soil.  For very fine seeds, such as begonia or campanula, I sow the seeds on top, then use a fine mist sprayer to wash the seeds down into the grit.  I also find it useful, when sowing like this, to sprinkle a very thin layer of white silica sand to help hold in moisture. Just as with other seed toppings, you can eventually tell by looking what pots need watering.  The grit does not stay completely dry, and as with soil, changes color when the pots begin to get dry.  I find that the watering is on a much more regular basis with grit as a topping.  Almost all of the pots dry out in the same intervals.  I end up watering indoor sown seedling pots about twice per month.  I also mist them once a day with a mist setting on the hose sprayer to help break up the seed coats which sometimes prevent the cotyledons from getting free."


The following is the sowing procedure described by a nursery owner in Michigan:

"We sow almost everything in Sunshine mix which we mix with perlite, one part sunshine to two parts perlite. We fill flats in advance and water before we sow the seeds. All seed is sown directly on the surface and covered with a layer of chick starter grit, this goes a long way to prevent damping-off  problems. Sow on the surface and cover to several times the thickness of the seed with fine thick grit; with very fine seed, pots should be gritted first and the seed should be sown on top of the grit and watered down into it.

Keep moist but do not over-water. Fungicides are no substitute for good culture and in general it is best to avoid them. We rarely have damping-off problems and do not routinely spray fungicides to prevent it. Chick grit and care in watering is the best prevention. Be sure to use a thick enough layer to inhibit moss growth.

Difficult species are almost always best sown in a minimally heated white, not clear, poly-covered greenhouse. Easy species can be done under clear poly or fluorescent lights. White poly-covered cold frames should work well on a small scale, or better still build a small Nearing Frame. Seeds that need multiple 40-70 cycles are best placed in the refrigerator vegetable drawer in a ziploc bag with sunshine mix. This works better than baggies and towel for most species. We routinely extend the cycle time on the species Norm Deno lists as multicycle germinators. This is very helpful.

Freezing is seldom necessary or helpful, it will crack seed coats but there are better ways to do it. Freezing in household freezers is much too abrupt and may rupture membranes killing the seed. If you must freeze, place seed in a coldframe outdoors in the fall. Seed can be stored in the freezer for extended periods but only if the moisture content is very low. In general this is not recommended. Remember for many species extended dry storage is actually beneficial with old seed germinating much better than fresh seed.

Seeds requiring GA3 can either be placed on coffee filters soaked in the appropriate concentration of GA3 (1000 ppm is a good place to start) and the seedlings pricked off as the radicle emerges, or sown in flats and misted with a one time application. You can also soak larger seeds until they swell and then sow out. Keep concentrations as low as possible to reduce stretch problems.

We fertilize young seedling with a soluble balanced fertilizer at every watering (250 ppm total nitrogen) which is higher than generally recommended but the plants thrive on it. No burn problems at all, with a bit more fertilizer most plants will be more likely to outcompete the mosses which generally dislike high fertility media. Water until runoff and let flats dry out between watering. Beware of high pH In the water. We killed a ton of stuff due to this. We now adjust our water pH to 5.6-6.0 by injecting battery acid directly into the irrigation water along with the fertilizer. Buy a portable pH pen – it’s well worth the investment, pH-induced iron chlorosis is an insidious problem. Plants weakened by it quickly fall victim to fungus gnats and aphids. Algae and fungus gnat problems are also less severe if flats are allowed to dry out a bit, it's a very fine line.

Properly dry flats will be flattened when you water but show little sign of wilting before the water hits them, and will recover quickly with no tip burn. Be patient, the only thing to do with some species is to sow and wait for a couple of years. Don't kill shade-lovers by placing in full sun. Be careful of high light levels, even sun-tolerant plants may be sensitive as seedlings and it's easy to crispy the little ones. Many seeds will germinate immediately with the majority coming up a year or more later. Often the early germinators turn out to be the weakest plants. We often weed out the early stragglers to wait for the main flush of germination. Don't throw away seed pots for at least four years on the long germinators. Before you trash a pot dig around and find a seed and cut it in half -- if it hasn't rotted then it will germinate eventually.

There are many formulas for success but the most important ingredient is patience. Don't let your seed pot dry to dust, and don't let them get soggy and moss covered -- if they do carefully cut off the moss layer leaving the seed behind and regrit and wait some more. It in doubt about what to do, think about what would happen in nature and try to emulate it. This is a great help in determining whether to start cold or warm."


As the previous writer said, "there are many formulas for success." I know of no existing literature describing the use of chick grit, and this article was written primarily to fill that void.

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