Horticulture Magazine

Potting Soil

potting soil in large grey planters

What Is Potting Soil?

Potting soil is the general name given to growing media that is used in pots and containers. Some people will simply refer to potting soil as compost. But this can get a little confusing, since it is not the same as an organic kitchen waste and garden waste compost you would make at home. The difference is that while a potting soil contains composted materials, it has been especially formulated to provide optimal conditions for plant growth. Often, potting soil will contain a blend of different ingredients.

Potting soil can also be a term that is used to refer to a specific type of growing media which is specifically formulated – not just to provide good condition for a wide range of plants – but tailored specifically to cater to the needs of those plants which are in pots or containers.

There are a number of different potting soil types available commercially, including:

  • Peat-based options.
  • Peat-free alternatives for the more environmentally minded.
  • Loam-Based or Soil based potting soils.
  • Organic potting mixes for those who really want to do the right thing for people and planet.
  • Ericaceous mixes specifically for acid-loving plants.
  • Mixes specifically for starting seeds.

Some commercial potting soils are fairly simple in their composition, while others are more complex and contain other non-organic ingredients. It is important to understand that there is a lot of variation – not only in constituent ingredients but also in quality – when it comes to potting soil or potting mix that is commercially available.

The term potting soil was first used in print in an 1861 issue of the ‘American Agriculturist’. But this term is also rather misleading, because the potting media is not actually soil at all. Most potting soils are actually soil free, and those that are loam or soil based only contain a proportion of soil, which is sterilized and only shares some, not all, characteristics with the soil in your garden.

Whether you are potting on indoors sown seeds, or creating a container garden – choosing the right potting soil is key. But it is important to note that environmentally speaking, it is often best to make your own potting soil to fill the pots and containers in your garden.

What Are Its Benefits?

The benefits of using a potting mix (either commercial or home-made) for your pots and containers is that, if you make the right choices, you can provide optimal conditions for your plants.

A good potting soil or potting mix can:

  • Maintain good structure and oxygenation.
  • Manage water availability – with the right balance of moisture retention and drainage for the specific plants you are growing.
  • Provide nutrients for plants in the right quantities over the specific period of time.

Different potting mixes will deliver nutrients to your plants for different lengths of time. Some will be depleted of nutrients much more quickly than others. It is important to understand that when growing plants in containers, you will usually have to provide further nutrients to your plants with liquid feeds and other supplements over time. And in certain cases, the potting soil should be replaced entirely with new, nutrient rich material after a certain length of time.

How To Use Potting Soil

The first thing to remember is that choosing potting soil or container growing media should be a careful and thoughtful process.

Different potting soils have different strengths and weaknesses, and can cater to different needs of different plants. Choosing one should always begin with an understanding of the needs and preferences of the specific plants that you will be growing in terms of moisture/ water, soil structure, fertility and pH level.

As an organic, sustainable gardener, you should also think about the environmental impacts of the choices you make. This might include avoiding the use of peat based composts, perhaps also avoiding coir or other materials that have had to travel a long way (resulting in carbon emissions). You might also consider creating your own potting mixes from locally available ingredients, to avoid purchasing materials for your garden in plastic packaging.

Once you have chosen a potting soil, you will, of course, use it to fill plant pots and containers prior to planting (or sowing) into them. Unlike an all purpose medium, or all purpose compost, potting soil is specifically used to grow plants in pots or other containers, and not typically used to amend or mulch the soil in a garden.

What Plants Is Potting Soil Good For?

The plants a particular potting soil is good for will depend on what its ingredients are and its exact composition. A general purpose potting soil that you purchase (or make yourself) will usually be suitable for a wide range of houseplants, vegetables, herbs, flowers and other plants grown in containers.

Some, however, will not be as good for seeds or seedlings. And as mentioned above, some are specifically formulated to meet the needs of acid loving plants. This is why it is always so important to read the ingredients and information on a pack when you are purchasing a growing medium. And you must have at least a basic understanding of plant needs if you plan on formulating your own.

What Are Its Disadvantages?

The pros and cons of a potting soil depend very much on its ingredients and composition. As mentioned above, potting soils can vary significantly both in their make-up and their quality.

Typically, if you are choosing a commercial brand, you will get what you pay for. Cheaper potting soil does not always represent good value for money, and if you choose a cheap and inferior type then plant health may suffer and you just won’t get the results you hope for.

Making your own potting soil without carefully considering the needs of the plants you wish to grow can also lead to disappointments if you do not get it right. But one thing to say for making your own is that you won’t have wasted as much money if things to do not go according to plan. And, of course, will also have avoided the environmental harm that many commercial brands can bring.

All but organic and sustainable potting soils will cause some damage to our environment. So you need to ask yourself whether the true cost of these materials is worthwhile.

Frequently Asked Questions

What Is The Difference Between Compost And Potting Soil?

While the word ‘compost’ is sometimes used to refer to potting soils, they are really somewhat different things.

A compost is made up of decomposed or partially decomposed organic material. While a potting soil contains a proportion of decomposed materials, but usually has other materials mixed in. Many potting soils, for example, will contain mineral constituents like sand, for example. And may contain perlite, vermiculite, and perhaps additional synthetic or organic fertilizers.

A home made compost will be decomposed materials – usually a mix of carbon rich brown materials and nitrogen rich green materials. It’s nutrient profile and characteristics can vary quite a lot depending on exactly what went into it and how exactly it was made.

A potting mix, unlike a compost, is exclusively used in pots and containers. It is less beneficial when added to garden beds or borders. Compost can be used as an ingredient in filling pots and containers, but should usually only be used in conjunction with other materials.

Compost can be an ingredient, for example, in homemade soil-based potting media, or homemade soil-less growing media. Typically, however, it should be mixed with other ingredients in order to create a more stable medium, with good water and aeration properties.

For example, many plants can be happy in a general purpose mix of 1/3 compost, 1/3 leaf mould, and 1/3 loam/ loamy topsoil from a garden. You can also mix a homemade compost with other ingredients like composted bark,  coir etc. to make a soilless mix.

Can You Mix Compost With Potting Soil?

While you can make your own potting soil or potting mix from scratch, you need to be careful about mixing your own homemade compost with a commercial potting soil mix. Remember that a commercial potting soil has been specifically formulated to provide for the needs of your plants. And a good example will be well balanced, with good moisture retention/ drainage and aeration. Make any changes to this container growing medium and you may risk disrupting this balance.

That said, you can potentially eek out expensive potting soil by adding a small proportion of your own homemade compost to the mix – as long as the compost does not account for more than around a third of the overall volume of the mix.  This should help to avoid compaction or drainage issues while providing nutrients that plants need to grow strong. This could be one way to improve cheaper and less ideal potting soils that have been purchased.

How Should You Dispose Of Old Potting Soil?

One important thing to understand is that the nutrients in potting soil will be depleted over time, and must be replenished. It is also important to understand that diseases can built up if you use the same potting soil without thought year after year. That said, refreshing the potting soil in containers does not necessarily mean that you have to throw the old potting soil away.

If the potting soil harboured a plant that succumbed to disease, then you should dispose of it carefully, to make sure that the problem does not spread. Make sure that you do not allow the contaminated material anywhere near your garden growing areas or composting system.

However, where no problem has become evident, you can often recycle the potting soil. To do so:

  • Carefully remove all plant matter (including root systems) from the potting soil. Dead plants that have not been infected with pests or disease can be added to your composting system.
  • Remove debris from the medium by hand, or with a garden sieve.
  • Put the potting soil on a tray and heat it in your oven for half an hour – make sure you do not do this at too high a temperature. This should kill off any pathogens/ harmful organisms left within it.
  • Allow the soil to cool, and leave it in a sealed container, out of the elements for storage.

This process should make most potting soil safe for reuse. But it is important to remember that much of the nutritional content will have been depleted, and microbial life will have to be restored to allow these nutrients to be released and for plants to grow well.

To make sure that the potting soil is able to successfully support plant life in future, you will need to supplement and amend it. We need the soil to contain microbes, which will continue to break down organic matter and release nutrients. We need to add more organic matter – e.g. kelp meal, alfalfa meal, to improve the structure and fertility of the mix. And can also add other things, like molasses, for example, to boost microbial life.

Replenishing old commercial potting soil is not always easy for beginners. But as long as you understand about adding back organic matter and microbes, you can begin to learn how to build a healthy soil mix.

If you have made your own simple potting soil with homemade compost, leaf mold and loam/ loamy soil then you will find it somewhat easier to make new mixes, since you can add more of these ingredients from your garden over time, as well as using liquid organic plant feeds to give plants a boost.

If you have used an organic potting mix, you can also spread spent potting soil that has not been contaminated with diseases as a mulch along with additional organic matter around mature trees and shrubs in your garden.

How Long Does Potting Soil Last In The Bag?

Potting soil will last almost indefinitely in an un-breached bag. However, as soon as a bag is opened to the elements, and gets wet, or is in contact with the soil, it will begin to leach nutrients and will become depleted over time. The organic material will continue to break down. The material will usually have depleted to the point where it is no longer ideal for plant growth in a matter of months.

How Can You Remove Algae From Potting Soil?

Algae on potting soil around seedlings or plants is often a sign that you are overwatering. Algae likes damp and humid to wet conditions. Avoid watering too much and make sure there are no drainage problems. In some cases, it may be better to water from below rather than from above. Make sure there is good air flow around your containers.

Algae may also develop more readily where light levels are lower. Make sure your plants are in a bright and sunny area, or consider using LED grow lights if growing indoors early in the year.

This problem is more common on peat-based media, since peat is very moisture retentive and provides the ideal conditions for algae to form. If you see this problem a lot, consider switching to peat-free media.

This problem is also more likely to occur if you use garden soil rather than a soil-less potting mix. Garden soil can harbour spores (as well as fungi which can cause problems like damping off). Consider sterilizing soil before use, or switching to a soil-less recipe.

Make sure containers are cleaned and sterilized before use to reduce the likelihood of this problem occurring.

Once you take care of the environmental conditions and make sure these are right, you can often simply scrape the algae off the surface of the soil. In certain cases, it might be a good idea to add an organic mulch or soil cover (like sand, pebbles or grit) in the tops of your containers. With mature plants, you can repot if there is a major problem with competition from algae. But algae is usually only a serious issue when it comes to competition for seedlings and very young plants.

What Is The Average pH Of Potting Soil?

A typical potting soil will be specially formulated to have a balanced pH of around 6-8. Though the pH of a potting soil will vary considerably depending on what exactly went into it. Peat is more acidic in nature than other organic materials.

Can I Use Garden Soil In My Pots?

Yes, in a pinch, you can use a healthy garden soil in pots or containers for mature plants. Though it is important to note that this will not necessarily be the best choice, and won’t provide optimum results.

Garden soil is best not used on its own, however, especially for seed starting. It can contain microorganisms that cause plant diseases, and may harbour pests. Another thing to note is that garden soil tends to be heavier and denser than potting soil. It is common, when using garden soil in pots and containers, to encounter issues with drainage and aeration.

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