Maybe you’ve heard people talking dirt and wanted to know more? Here’s the lowdown. There’s no dirty secret! Soil and soilless media are the planting medium that you use when you plant your seeds, bedding plants, or transplants. Each has its own benefits and drawbacks and once you understand the differences, you will know which ones to use when.

Soil

Soil is composed of sand, silt and clay, plus organic matter. A soil’s composition – the ratio of these components, affects how available nutrients are to plants.  Sandy or soil without any organic matter will not hold nutrients well.  Clay soil can hold lots of nutrients, but nutrients don’t travel well through it because it has small air spaces and water doesn’t move through it easily.  An ideal garden soil is a good balance of sand, silt, and clay, along with a good dose of organic matter.  This is referred to as loam.

Soilless Planting Media

Soilless media are usually made of sphagnum moss, peat moss, or coconut fibers. These can hold large amounts of water.  Some commercial products include different add-ins, like vermiculite, perlite, or hydrogels, to make the mix lighter or hold more water. Soilless media have no nutritional value and must be amended or receive a regular rate of fertilizer to grow plants.  Many soilless mixes have slow release fertilizers added to them.

Usage

Soilless media are used in greenhouses and for hanging baskets, pots, and planters. Soil is used in the garden and flowerbeds.  Choose the one you use based on the following characteristics.

Sterility

Greenhouse growers prefer soilless media because it can be sterilized.  This is very important in a greenhouse environment where a pathogen, like mold or disease, can spread quickly in the hot and humid environment.  Most of the time you won’t bother sterilizing your soil for planters and pots, but it is a good practice for starting seedlings which are very susceptible to fungal diseases like damping off. If you use garden or bagged soil in your planters and pots you will likely have fungal problems due to the presence of bacteria. 

Weight

Soilless medium is lighter than garden soil.  When you are working with trays of seedlings, or moving your full pots, you will appreciate this. In a garden, a soil’s weight isn’t as important, unless you have raised vegetable planters that can collapse.  

Blank Slate

Soilless media has no inherent nutritional content. For greenhouse growers who like to carefully control their plants’ nutrition, this is an advantage. For your home greenhouse or garden pots, you may wish to purchase mixes with an included slow release fertilizer to help maintain a constant supply of fertilizer.  Develop a fertilizing schedule, especially for any plants in soilless mix without added fertilizer.

Water Retention and Aeration

Soilless mixes hold a lot of water, but they also dry out faster than garden soil because they are fluffier and have more air incorporated into them.  Check and water your pots and baskets daily, if needed, even twice a day, during hot weather.  Some mixes include special media that hold water and release it slowly.  Straight garden soil does not work well in pots due its finer structure which results in compaction and the soil can then become waterlogged.

Disadvantages of Soilless Media

If you are growing plants in a greenhouse, pots, or hanging baskets, you should absolutely use a soilless media. However, some people add these to their garden.  There are likely arguments to do this, so if you wish to, you can.  The reason I recommend not using it is because you will have better results if you add compost for organic matter.  It has soil microbes and nutrients.  Peat is dry – if you add it to your flower beds and garden you must add water to it beforehand.  Mix water into the peat before planting – massage in with your hands or stir with a shovel. If you don’t, peat actually repels water. 

Testing Your Soil

Nutritional Deficiencies

You can buy inexpensive soil test kits to understand your soil conditions. If you have plants exhibiting signs of nutritional deficiencies, it is a good idea to test your soil to find out how to fix the problem. Adding fertilizer when it is a pH issue will not help.  See Understanding Plant Fertilizer for more information on plant nutrients.

pH

Ph is important in both soil and soilless media. It is an important factor that affects nutrient availability is a soil’s pH.  In general, the lower its pH, or more acidic it is, the more readily the soil will release the nutrients, however too acidic and it will become toxic to the plant.  Too alkaline of soil will not release the nutrients, even when present in sufficient quantities.  Different nutrients and plants respond differently to soil pH, but the ideal soil pH for most plants is between 5.5 and 7.5, with 7 being neutral pH. pH is important in soilless mixes as well. Canadian Sphagnum Peat is acidic and can lower soil’s pH when used in larger amounts.  Greenhouse growers regularly monitor their soil’s pH.

Salinity

Salinity is the amount of salt in soil and soilless media.  Salt destroys soil’s composition and it affects how well a plant can absorb water and nutrients.  You want to avoid too much salt.  Most fertilizers are salts. If you overfertilize or use home remedies like Epsom salts, you run a risk of damaging your soil and harming your plants.  Follow instructions on fertilizers regarding mixing and applying.  Greenhouses monitor the salinity, or conductivity, of their planting medium on a regular basis due to its importance to their crops’ health.  

Your water can also affect a soil’s salinity.  Some well water is quite salty and will damage soil. There is a small amount of salt in softened water and it shouldn’t hurt your soil. If concerned, you can install a bypass valve for your outside taps.   

Soil and Soilless Media Amendments

It doesn’t matter how healthy of soil or soilless media you have, eventually your plants will use up the nutrients if you don’t amend it.  Soil and soilless media amendments are products you add to your soil to improve or alter it.  Topdress (apply a two to three inch layer) with compost, manure, bagged mixes, or by adding fertilizers to the soil. Amend your soil pH with lime, wood ash, or sulfur, but follow instructions and do not use too much.  Saline soil can be helped somewhat by flushing with water.  Consult an agrologist if you require additional help with soil problems.

Compost

Compost is made from decomposing plant matter. You can make it yourself or buy bags and bulk.

The best choice for improving your soil is to add compost.  Compost adds organic matter which helps retain moisture, loosen or keep soil loose, improves soil microbes, and provides access to nutrients. While it is low in actual nutritional value for your plants, it is making it much easier for your plants to access the nutrients that are there.  As mentioned in the Understanding Plant Fertilizer post, not all fertilizer deficiencies are because the nutrient isn’t available, it may be that it can’t access it due to the soil’s pH, salinity, lack of organic matter, or leaching due to too much rainfall or over watering.

Is it Bad to Add Compost to your Garden?

While I don’t think there are many downfalls to using compost, you still must keep in mind your plant’s needs.  If your plant likes well drained, sandy soil, then adding compost won’t be beneficial. Many rock alpine garden plants prefer poor soil, for example.

Compost can be added to your soil throughout the season, but it shouldn’t be added to perennial beds after midsummer because you want your plants to not spend their energy on new growth that may not have time to harden off before winter.

Make Your Own Compost

There are a whole host of websites and videos on how to start your own compost pile or bin if you are interested in creating your own compost.  It is a great use of all the plant matter that you will produce in your yard.  Composters can be anything from small piles to large, multi-bin systems, depending on the space you have available.  Many communities have green bins as well that you can use. Composting keeps recyclable organic waste out of our landfills and is a good practice.

Manure

Manure is produced by decomposing animal waste i.e. poop. It adds organic matter and is higher in nitrogen than compost. You can purchase many different types of manure, like sheep, cow, chicken, etc.  If you get it from a farmer, make sure it is well-rotted manure.  New manure is too rich in nitrogen and will burn the plants – they cannot handle that high of concentration. Also, pay attention to the pile the manure is sourced from.  You can very easily introduce noxious weeds into your garden from a manure pile.  If possible, view the pile to check for weeds before adding it to your garden.

Using manure can also introduce pathogens to your garden, like E coli. It is important to make sure it is well-rotted – it should not smell like anything but rich dirt. It takes at least six months to compost to that stage. And it really isn’t recommended to use manure in your vegetable garden. Not only for pathogens, but also because it is so high in nitrogen. Compost is a better option.

Bagged and Purchased Media

Triple Mix

This is a common purchase to amend gardens.  It contains equal parts of compost, topsoil, and peat.  It isn’t as rich as straight compost and the peat adds extra organic matter.  There are varying qualities of mixes, so it may take some experimenting before you find one that works for your garden. It is a good choice if you don’t want to add a lot of nutrients, like later in the summer when you want your plants to start preparing for winter and not put their energy into new growth.

Topsoil

This is good black dirt or should be.  Quality can vary with the brand and you can order it in bulk from garden centers or local gravel yards.  Often, the soil is screened from a reclamation site and lacks organic matter.  If you find the soil is very fine and packs easily, you will need to add organic matter such as compost.

Potting Mix vs. Potting Soil

Though they sound similar, there is a difference. As already explained, potting mixes are soilless media with added products like vermiculite and perlite, some have bark, slow-release fertilizers, rapid boost fertilizers, water retaining pellets, etc.

Whereas, potting soil contains soil.  It should have been sterilized by the manufacturer to reduce soil born disease.  The mix works for houseplants and garden pots and planters.  It is heavier than potting mix because of the presence of soil.  There is better water retention, but it may become too packed and waterlogged.  It essentially reduces some of the advantages of the soilless mix, sterility, aeration, weight, primarily.  It does help with water retention and provides organic matter and nutrient retention.

Should You Use Soil or Soilless Media?

The application determines which planting media you will choose.  The advantages of soilless media make them ideal for greenhouse plants, starting seedlings in the house, pots and planters.  But they require steady watering and fertilizing to keep your plants healthy.  

Soil is ideal in the garden. It provides a fertilizer bank by retaining nutrients and moisture, weight and soil microbes aren’t as big of a concern, and aeration can be improved through the addition of compost. You can use it in planters and pots, but there can be problems with compaction and waterlogged roots.  Many people use potting soil, especially for houseplants and raised beds, because it offers a lighter mix but water and nutrient retention.

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