Green Man Publishing

How to make potting soil and sowing mixes

Beginning gardeners often make the perfectly reasonable mistake of using garden soil for starting seeds indoors. However this doesn’t usually work very well because indoor growing conditions are quite different from those found outside. In the confined environment of an indoor container, garden soil drains poorly, has a tendency to crust and can get compacted from repeated wetting and drying. Also it doesn’t hold much water (so dries out quickly), shrinks as it dries out and often contains weed seeds, pests and disease.

For growing in containers it is best to use a sowing mix that is specially formulated to prevent the above problems. These have a number of special attributes:

•      Low density.

•      Well drained and well aerated.

•      Hold a lot of water.

•      Have a fairly neutral pH.

•      Don’t crust easily.

•      Are firm enough to provide a strong anchorage for roots.

•      Have enough structural stability to resist compaction, even though it has a lot of pore space.

Buy or make?

There are lots of commercially available seed starting mixes and if you are only going to start small quantities of seedlings (or don’t have much room to store materials) you might want to simply buy a bag. Just make sure it is formulated for seed starting and ideally is 100% organic. It is much cheaper to make your own however and this is the way to go if you wish to grow a lot of plants.

Making your own sowing mixes

Sowing mixes are composed of several different ingredients, each of which fulfils a specific function in the mix:

•      Most mixes contain a large proportion of organic matter (compost, peat moss, leaf mold) to provide good water retention and aeration.

•      The mix should contain a coarse material (sand, vermiculite, perlite) to provide good drainage, as organic material alone tends to waterlog easily.

•      The mix must also contain a source of nutrients (compost, garden soil, or a fertilizer mix). It doesn’t need to have a very high nutrient content, because we want the seedlings to send out vigorous roots to search for nutrients. If the mix is too rich the plants get lazy and don’t do this. Pricking out and potting mixes must contain more nutrients, as the plants are bigger and have greater needs.

•      The pH of the ingredients should be a fairly neutral 6.0 – 7.0, similar to that of a good garden soil.

•      The mix should be free of weed seeds. It’s much better if it doesn’t grow more weed seedlings than crop seedlings.

•      The ingredients should be free of disease and insect pests.

•      The mix should also be low in salts which could burn delicate seedling roots.

Sowing mix recipes

Making mixes is a little like cooking, some people prefer to stick rigidly to the recipe, other people prefer to improvise with what’s available. I’m giving these to give you an idea of what is involved, but I rarely stick to any one formula.

Soil based mixes:
1 part garden soil (or 2 parts)  
1 part sharp sand  
1 part compost or leaf mold or peat moss    

2 parts soil  
1 part leaf mold  
1 part compost    

5 parts compost  
4 parts soil  
1 part sand  
1 part leaf mold or peat moss    

2 parts soil  
1 part leaf mold  
1 part compost        

Soil-less mixes Some gardeners prefer these because they are lighter and less likely to contain damping off fungi and weed seeds.  
1 part vermiculite  
1 part milled Sphagnum moss  
1 part compost / leaf mold    

2 parts turf loam  
1 part peat  
1 part coarse sand  

Screening

All of the ingredients for sowing mixes are screened to take out sticks, stones and other debris, and to reduce them to a uniform texture. The ingredients for soil block or plug tray mixes should be sifted through a ¼˝ mesh. Those for flat, cell pack, pricking out and potting mixes can be coarser, so sift them through a ½˝ mesh.

Mixing

Small quantities of mix are usually made up in a wheelbarrow or large plastic tub. Larger quantities are most easily mixed on a concrete floor. Just spread the materials out, one on top of the other, in a big pile.

Then turn the pile three times to thoroughly mix all the ingredients together. Do this by simply moving the pile a couple of feet, move it back again and then move it a third time to where you will actually store it. If you have two wheelbarrows, you can mix it by simply moving it from one to the other until it is thoroughly mixed

Moisture level

Make sure the materials for the mix are at their proper moisture level (not too wet or dry). The moisture level of flat and plug tray mixes can be tested by squeezing a handful into a ball. It should hold together well, but crumble when lightly squeezed.

Storage

Sowing mixes should be made as needed, as they dry out and deteriorate if stored for more than a few weeks. Fertilizers in the mix may start to break down and release ammonium, which can damage seeds and seedlings. It may also get infected by pests and various pathogens. Ideally you will store all of your ingredients in enclosed bins with the appropriate moisture content (and labels).

Common ingredients

Compost

Compost is the natural medium for seed germination, so well rotted compost is the basic ingredient in many mixes. It’s already pasteurized (if made properly), easily available, rich in nutrients, free of weed seeds, has a neutral pH, aids in water retention and drainage and can help suppress damping off. It is also cheaper than other ingredients, or free if you make your own. It isn’t widely used in commercial sowing mixes because it is too variable and unpredictable and not sterile enough.

Finely sifted compost can be used as a substitute for peat in sowing mixes, though it contains a lot more nutrients. It’s a good idea to sift and bag some compost in fall (write the date on your calendar), so it is ready for use in late winter, without having to mess around in the cold. If you aren’t that organized, then at least keep the pile covered (it should be anyway) to prevent leaching and make sure it stays dry enough to use.

Many gardeners make a special compost pile for sowing mixes, adding extra nutrients and turning it several times, to make sure it is thoroughly heated and broken down. Ideally this pile should be left under cover to age for 1 – 2 years before use.

Seed sowing mixes may contain up to 1/3 compost, while potting mixes may be as much as half compost (it is a good source of nutrients.

Soil

For many years, the trend in conventional growing has been toward soilless mixes. A major reason for this is concern about soil borne plant diseases and the excessive density of mixes where soil is a dominant ingredient. Soil is also heavy which adds to shipping costs.

Soil is still used in many mixes, as it supplies nutrients and provides support. Use the best soil in your garden; a good medium loam (not too much clay or sand) with lots of organic matter and not too many weed seeds is the best. Some gardeners sterilize or pasteurize their soil, but it isn’t really necessary if you use good growing techniques (I have never worried about the potential for infecting the seedlings with soil borne diseases).

Sharp sand

Sand makes a mix more porous and improves drainage, but doesn’t affect ph or contain nutrients. It is cheaper than any other drainage promoting material. It makes a mix quite heavy, which can be a drawback in some situations (such as for shipping plants). There is quite a lot of variation in sand, some types being much more suitable than others. The best types are quite coarse, with some large ˝ diameter grains (such as number 2 builders sand). Beach sand is too salty, river sand is often too smooth and fine, as it masons sand. These fine sands tend to fill in the pore spaces so don’t work so well.

Composted manure

This is a good source of organic matter and nutrients and can be used as a substitute for compost if that is unavailable. Make sure it has been composted properly, otherwise it may be full of weed seeds and other problems.

Worm castings

If you have worm bins (you should), these are a wonderful source of nutrients for the various mixes. See Composting for more on this.

Peat

Sphagnum peat moss is widely used in sowing mixes, particularly the soilless types. It has several properties that make it useful for sowing mixes. Most importantly it holds a lot of water, so increases the water retentive qualities of a mix. It has a coarse structure that helps to give good aeration. It is also light, fairly sterile, holds on to nutrients and is free of weed seeds, insect pests and toxic contaminants. It is also relatively inexpensive.

Peat is rather acidic (pH 3.5 – 5.0), so ground limestone is often added to raise the pH up to 5.0 or 6.0. Use approximately 2 tablespoons of lime per shovel of peat (4 ounces of lime will raise 8 gallons of peat 1 pH point).

I have already discussed the ecological implications of using peat moss (see Fertilizers) and you may want to avoid it altogether.

Live sphagnum peat may contain the spores of a disease causing fungus (Sporotrichum Schenkii), but this isn’t an issue with peat moss.

Coir

This is made from the fibers that surround coconuts and was once widely used in horticulture. It is becoming popular again in Europe because it is a good peat substitute. It is quite similar to peat moss, but less acidic (pH 5.5 – 6.8), holds more water and is more durable. It works well, but is more expensive than peat moss because it must be transported from the tropics, which seems like a waste of energy to me. If you don’t want to use peat there are other suitable alternatives (such as leaf mold).

Leaf mold

Leaf mold is a very useful ingredient for soil mixes. It is similar to peat moss in that provides a mix with good aeration and helps it to hold water. Unlike peat it contains some nutrients (especially Ca, Mg and P), is a renewable resource, easily available and usually free.

You may be able to obtain already decomposed leaf mold from deciduous woods. Be cautious when using it though, as it’s often very acidic and can sometimes actually inhibit seedling growth. Before using it in quantity you might want to experiment to see if it has any detrimental effect. You will probably have to add lime to it to raise its pH.

It is much better to make your own leaf mold, using the leaves of deciduous trees, especially oaks. Don’t make it from evergreens (either broadleaves or conifers) as they contain toxins that can inhibit plant growth. See Composting for more on making leaf mold.

Turf loam

Turf loam is made from decomposed turf and was commonly used for starting seeds before the widespread use of peat. It is a good source of organic matter and is also high in plant nutrients. It is made by leaving grass sod to decompose for 6 – 12 months. It is then shredded and sifted before use (see Composting for more on this).

Vermiculite

Vermiculite is hydrated laminar magnesium-aluminum-iron silicate and when heated to almost 2000°F, it puffs up like popcorn to 20 times its original volume. It holds up to 20 times its own weight in water and is added to sowing mixes to improve water retention and aeration. It contains some calcium, magnesium and potassium, has a neutral pH and its C.E.C. is sufficiently high at 100-150 meq to hold some nutrients. The coarser granular vermiculite is preferred for sowing mixes, because it doesn’t compact as much as the finer types.

There are drawbacks to vermiculite; it is quite expensive, it must be mined, is energy intensive to produce (hence not very sustainable) and sometimes contains asbestos. Because of this it is very important to avoid breathing the dust, by wetting it immediately. In keeping with my vow to not buy things I don’t need, I don’t use vermiculite (or perlite).

Perlite

Perlite is made by heating volcanic silica up to 1400°F, until internal moisture causes it to puff up like popcorn. It is often added to mixes because it is light, sterile and improves the drainage and aeration of a mix.

Perlite doesn’t contain any nutrients (but may contain toxic boron, aluminum and sodium), its dust is harmful to the lungs, its production is quite energy intensive and doesn’t really do anything that sand doesn’t.

Perlite is a volcanic rock that is heated and expanded to become a lightweight white material. It is sterile and has a neutral pH. When added to a soil mix, perlite can increase air space and improve water drainage. It is a hard material that does not break apart easily. Perlite pieces create tiny air tunnels that allow water and air to flow freely to the roots. Perlite will hold from three to four times its weight in water, yet will not become soggy. It is much lighter than, and can be used instead of sand

Lime This is used to neutralize acid ingredients such as peat moss. Dolomitic lime is preferred as it contains magnesium as well as calcium. See pH adjustment for more on this.

Fertilizer

A small quantity of powdered fertilizer is often added to mixes to increase their nutrient content (mix with a little sand and incorporate thoroughly). Use equal parts of blood meal, greensand and rock phosphate and about half as much kelp powder.

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