Becky Weeks - Feb 27, 2021
Selecting the best soil for your seeds
It's that first warm day coming out of a particularly cold stretch that gives you hope that spring is on the horizon. You trudge out to the mailbox and there they are-- the seeds you've been waiting for. Excitement rushes over you and soon you're off to the garden center to pick up some soil for starting your seeds. Arriving at the store, you're instantly confronted with dozens of different types of bagged soils. There's potting soil, potting mix, seed starting mix, garden soil, and container mix. This one is organic, but that one has moisture control. What on earth is "Coir"? It's overwhelming and often easy to settle for whichever bag has the nicest looking picture on it, but there are measurable differences among various bagged soils and arming yourself with a little information beforehand can set you up for success when it comes time to start your seeds. This article, therefore, is intended to give you some basic information about the properties of soil, some common ingredients, and how they interact to determine the quality of the soil mixture.
Soil factors that impact seedling growth
Before we discuss the common components in soil mix, it's important to understand some basic qualities of soil materials and their impact on plant health.
Water retention is important when formulating growing media because plants grown in containers have less root space than they would if they were grown in an open field. The optimal growing mix will stay moist between waterings, while not becoming saturated or water-logged. Plants that are grown in water-logged soils have stunted root development and are prone to disease.
Porosity describes the total amount of pore space present in a given media. It is usually expressed as a percentage of total volume or weight and is important because it determines the amount of water and air that can be accessed by the seedling's roots. Plant roots require oxygen for respiration and give off carbon dioxide, which needs to be able to dissipate, since high levels of CO2 in the root zone can be detrimental to plant growth (He et al., 2019). Porosity also affects the ability of soil to shed excess water. Waterlogged soil is an invitation for fungal growth, which can be deadly for young seedlings.
Nutrient retention within a given media is usually measured as it's cation-exchange capacity (CEC). In simplified terms, CEC quantifies how many binding sites are available in the soil to hold onto positively-charged nutrients. The higher the CEC of a soil, the more nutrients it can store. When soil CEC is low, nutrients will need to be supplied continuously to compensate for nutrient loss through leaching.
Shrinkage describes the fluctuation of soil volume over time. If you have ever started seeds in peat-containing media, you probably have witnessed shrinkage as the soil pulling away from the edges of the container during drying, causing water to pour down the sides. Usually, these types of soils regain their volume once rehydrated. Other, more permanent causes of shrinkage include decomposition, which is more common in poor quality mixes that contain wood substrates. Decomposition is usually exacerbated when nitrogen is applied. Finally, shrinkage can occur when a medium has too much variation in soil particle size. Over time, the finer particles will settle into the larger pores, reducing air space and inhibiting water drainage.
Sterility, when it comes to growing mix, means that the soil is free of pathogens that could harm fragile seedlings. Absolute sterility is not necessary, since most soil components (such as peat, bark, and vermiculite, for example, are not harvested from environments where soil pathogens are prevalent). However, if the soil contains components that could have been in contact with common soil pathogens (such as compost or manure, they should be pasteurized before mixing. See section below for more information about using compost in soil mix.
Soil pH is important because most seedlings perform best when grown in medium that is between 5.2 and 6.2 (Bilderback, 1982). When the pH of a soil is above 7 or less than 5, the availability of some nutrients becomes limited (NRCCA) . Dolomite lime (CaCO3 * MgCO3) is most often used to control the pH of soil mixes, but you will occasionally see Calcite (CaCO3) and Gypsum (CaSO4) used as well.
Bulk density is the weight of a mix in relation to its volume. The trend in the bagged soil industry has been to reduce bulk density to save on shipping costs. This is not necessarily in conflict with the interests of the gardener unless fillers, like polystyrene (Styrofoam) or woodchips, are used. Also beware of mixes that contain large chunks of material, which are used by manufacturers to reduce density. These materials mix with finer particles, resulting in a shrinkage-prone mix that has reduced aeration and poor drainage.
Common bagged soil components
Now that you know some of the things to look for in a good growing mix, let's examine some of the most common components of bagged soil.
Sphagnum and peat moss
Nearly every bag of soil you encounter will have sphagnum moss or peat moss in it. Sphagnum moss is a genus of water-loving plants that grow in bogs, swamps, and other wetlands. It is favored for potting mix primarily because of its excellent water-retention properties, holding 500% or more of its weight in water (Mastalerz, 1977). This is all thanks to sphagnum's porous hyaline cells that comprise up to 80% of its stem and leaf weight (Turetsky et al., 2016). It also has excellent porosity which allows more air space to be present in the soil, thereby increasing the amount of gas that can be exchanged by the plants' roots. Finally, a naturally-occurring phenolic polymer called "sphagnic acid" protects the plant's cell walls from degradation (Freeman et al., 2001), making it an excellent shrink-resistant ingredient for growing mixes.
Sphagnum moss vs Peat Moss
As you read labels, you may find that some soils contain sphagnum moss while others contain peat moss. So what is the difference? They are in fact different parts of the same plant. Sphagnum is the growing portion of the plant that lives on the surface of the swamp or bog, while peat is the dead portion of the plant that has sunken below the surface. Whereas the aerial portions of the moss can regenerate after harvest, mining for peat involves digging out the bog, thereby destroying the entire plant and the ecosystem entangled within it. Therefore, many environmentally-conscious gardeners prefer to use sphagnum moss over peat, since sphagnum is a renewable resource and its harvest is less destructive to the wetland ecosystem.
Less common, but increasing in popularity is coconut coir. Coir is the reddish-brown fiber found on the husk of a coconut--you may also recognize it as that coarse, hairy mat that is often used to line hanging baskets. It is a natural byproduct of the coconut industry and is favored over peat by some environmentally-conscious gardeners. Proponents of coir also point out that it also has a more favorable pH, compared to peat, and is easier to wet. Coconut coir is still relatively uncommon, but you will sometimes find it in higher-end and/or organic soil mixes.
Bark is another common ingredient in most bagged soils. Bark has the benefits of being inexpensive (it is a byproduct of lumber production) and low-density. It can also be hammer-milled and screened to create a uniform particle size. Depending on the species of tree from which it was derived and the particle size, bark can make excellent growing media. For example, pine bark is preferable to hardwood bark because it degrades more slowly, reducing the amount of nitrogen that is depleted from the soil through decomposition. Hardwood barks are not suitable for growing mix unless they are first composted to reduce the carbon to nitrogen ratio. A bark particle size of 0-1/4" is optimal, in terms of water retention and porosity (Bilderback, 1982).
These days it is becoming increasingly popular to include wood chips in potting mixes. Unless composted for long periods of time (several years or more) wood makes a very poor container medium and I would recommend avoiding any growing mix that contains it. Wood consists of mostly cellulose, has poor moisture-retaining abilities, and it is just unpleasant to work with when used in potting mixes. The sharp pieces stab your hands (sometimes under a fingernail-ouch!) and they always seem to awkwardly spear themselves into the containers making it difficult to spread out the soil. Wood-containing mixes may be cheaper, but the savings aren't worth the trouble, in my opinion.
Photo by University of New Hampshire Extension
Also known as aluminum-iron-magnesium silicate, vermiculite is a naturally-occurring mineral, with deposits found in the United States and other parts of the world. The vermiculite used in potting mix has been superheated to reduce its density and expand it's cation-exchange capacity. It also contains significant amounts of potassium and magnesium, which are needed for plant growth.
Perlite is a volcanic rock that, like vermiculite, has been expanded by heating. Perlite offers little in terms of water and nutrient retention, however its highly porous nature makes it great for improving the aeration of potting mixes. Nowadays, some inexpensive growing mixes will substitute polystyrene (Styrofoam) instead of perlite. Styrofoam is likely used to reduce the density of the soil making it less expensive to ship. Compared to perlite, Styrofoam has 42% less pore space and retains 90% less water (Matkin, undated). As far as soil components go, Styrofoam is in my opinion, worth its weight.
Both sphagnum moss and pine bark have slightly acidic pH values, therefore many bagged soil manufacturers will add lime to neutralize the soil. The most commonly used lime for bagged soil mix is dolomite, which is just lime (calcium carbonate CaCO3) mined from a deposit that contained significant amounts of magnesium. On rare occasion, you may also see gypsum (CaSO4) used as a liming agent. Note that mixes that contain uncomposted hardwood bark should not be limed as the pH of the medium increases naturally with decomposition (Bilderback, 1982).
Most growing mixes contain significant amounts of sphagnum, peat, and/or bark. As these materials dry out, they become increasingly hydrophobic, such that it can become very difficult to rehydrate them. To compensate for this, some manufacturers will add surfactants to improve water penetration. Surfactants are detergent-like substances that reduce the surface-tension of the media, thereby encouraging water to run through the media rather than around it.
Many high-end mixes, and especially organic ones, will contain worm castings. Earthworm casting are made when the worm, well, does its business. Castings contain significant amounts of ammonia, urea, and body tissues that are rapidly mineralized (Curry et al., 1992). Earthworm castings have been shown to improve germination rate and enhance shoot and root growth in developing seedlings (Hidalgo et al., 2011). It is believed that this is due to the increased nutrient availability in soil that has passed through the earthworm gut. For example, field studies have reported that earthworm castings contain 2-3 times more available potassium when compared to the surrounding soil (Basker et al., 1993).
Each manufacturer has their own proprietary blend of ingredients. It is not uncommon, for example, to see silica, moisture-retaining gels, fertilizers, growth enhancers, or a host of other marketable amendments advertised on the bag. If you are just using the soil to start your seeds, it probably isn't worth splurging on these mixes. After all, the seed contains everything it needs to get the seedling out of the soil (with the exception of water, of course). Fertilizing at this early stage likely is of more benefit to pathogens than it is to the seedling. However, if you are going to be direct-seeding into a larger container that will be moved outside, you may want to consider a soil that has more nutrients to support the plant later in life.
Frequently asked questions:
Here are some of the most common questions we're asked when it comes to selecting or making growing mix for seeds.
Q: Can I make my own seed starting mix?
A:Absolutely! However, formulating a seed starting mix that everyone likes is like trying to find universally-loved potato salad recipe--it doesn't exist. If you are interested in making your own mix, I would recommend first determining what ingredients you want to include, those you want to avoid, and then researching to find recipe that has the right ingredients in proportions that will support plant health.
Q: Can I use garden soil for starting my seeds?
A:I wouldn't recommend it. While garden soil can grow perfectly healthy plants in its natural environment, when placed in containers it exhibits something called the container soil effect. Consequently, a well-draining, highly-aerated garden soil will become dense and saturated when placed in a container. The type of the container doesn't matter, because the container effect is caused by a change in the length of the column affected by gravitational pull (Mastalerz, 1977).
Q: Can I add compost to my mix?
A:If you are considering adding compost to your mix, I'll ask just one question: Are you the type who carefully controls the inputs, temperature, and age of your compost pile? Or are you the type that occasionally trudges out to sling some banana peels on top of the partially decomposed heap (finger pointing back at myself here)? Properly produced compost can be a wonderful addition to soil starting mix, but poorly produced compost can be an invitation for fungal problems. If you want to use your compost, but aren't sure it was produced with scientific precision, try mixing a pint of it into the bottom of the hole when you transplant. That way your plants can still harness all of the wonderful benefits of compost, but only after they've become strong enough to resist common pathogens.
Q: Is organic potting mix better than conventional?
A:Whether or not you purchase organic seed starting mix is a matter of personal preference. Organic mix can be a bit more expensive, but many gardeners consider it essential. On our farm, we use a soil mix that contains ingredients for which an organic certification shouldn't matter (sphagnum, bark, perlite, lime). We have, however, used Fertilome's Organic Potting Mix with great success.
Q: Do I need to pre-wet the mix?
A:Most potting mixes come premoistened to make them easier to spread out in the pots. However, we recommend re-wetting the media after you have filled your pots or trays and then letting it drain thoroughly before planting. We often repeat this step just to be sure it is moistened throughout prior to sowing.
Q: Will adding fertilizer make my seeds grow faster?
A:Initially, probably not. We do not recommend adding fertilizer until the seedlings have at least one set of true leaves. Over-fertilizing your seed starting mix will only cause problems. Once the plants are up and going, it should be fine to fertilize, however, only supply it as liquid form and only at one-third to one-half the recommended rate.
Picking out the best bagged soil
Okay, you've done your homework and you're ready to purchase some soil. When you get there, check out the various bags to determine their ingredients. If those look good, feel the bag to try to get an idea of what the media inside is like. Here are a few hints to help you find the best soil by feel.
- Feel the outside of the bag to see if it has any large chunks of material in it. If you feel any pointy objects, it likely contains woodchips or worse yet, ground up pallets. Move on to the next brand.
- Make sure the bag is not saturated with water. If your garden center keeps soil outside, you may have to dig down a layer to find a dry bag.
- Pick up the dry bag of soil to gauge its density. If it feels overly light compared to other brands, the soil is either too dry or contains a lot of fillers.
- Press on the outside of the bag. If it is brick-hard, it will be difficult to wet (and also work with). If it feels soft and moves a bit when you pick up the bag, you have a winner.
In summary: choosing the best soil for your seeds
There are a number of factors to consider when choosing the best soil for starting your seedlings. Moisture-retention, porosity, nutrient-retention, and pH are all important variables that affect plant health. Therefore, choosing a mix that has the right soil components will go a long ways in getting your seedlings off to a great start. Finally, once at the garden center, taking a bit of time to pick up the bag and feel its contents will provide some helpful clues about the quality of the soil inside.
Becky Weeks, Ph.D.
Geneticist, Gardener, Founder of Thresh Seed Co.
Becky is a geneticist with a passion for gardening. Prior to starting Thresh Seed Co., she spent fifteen years researching plant genetics and development, later applying those principles to aid in the breeding of commercial corn and soybean varieties. She lives on a farm in Iowa with her husband and two children, where she enjoys growing just about anything and experimenting with the breeding of new vegetable varieties.