Soil Conditioner vs Compost – All The Differences

You are in the gardening shop just heard the word “compost” and “soil improver” without knowing what they really are. So, is compost better than soil conditioner? Which one should you use?

Compost is decomposed organic matter to improve the nutritional content of new or existing soil while soil conditioner is a medium (solid or liquid) given by a variety of components to improve the physical and chemical properties of the existing soil. A soil improver might also contain compost.

However, knowing the difference is not enough to understand when and where you need to use one or the other (or if you even need to use them).

Soil Improver and Compost

Let’s discuss first what soil improver and compost are.

Compost

As I discussed in one of my previous articles here, compost is a medium obtained by decomposing, in a controlled manner, organic matter. The compost you buy in the supermarket is often the product of waste material from different types of industrial processes. This is a great plus considering that allows converting waste into useful material. 

The material used to produce compost varies greatly. The commercially available compost can be produced from animal manure (from farming), almost any green material (vegetable scraps), and even water sludge. Different origin affects quality differently. I do prefer compost coming from organic sources such as vegetable scraps. 

Another alternative is to join thousands of people in producing your own compost at home, a green way to reduce dramatically waste that would go otherwise to landfills. In this case, you just need to avoid a few organic materials building your compost such as animal-based products (cheese, milk, blood, etc…)

Compost, once ready looks like soil: a dark brown in color, light and airy at the touch. However, it is not soil, far from it as discussed here. It is also typically slightly humid thanks its ability to retain moisture.

Soil improver

This is generally something more. Indeed, a soil improver (also called soil conditioner) is obtained by mixing different (even as little as two) mediums such as:

1. Compost: for its ability to retain water and nutrients;
2. Fertilizer: for its nutrient content;
3. Perlite: increase aeration and drainage;
4. Peat Moss: for its ability of water retention ;
5. Pine Bark: to retain soil moisture;
6. pH correctors: to correct the pH of the original soil is too far from neutral;
7. Wetting agents: for its ability to attract water.

Soil conditioner does not necessarily need to be a solid material. A few gardeners actually use liquid soil conditioners with just two ingredients as shown in the video at the end of this post in case you are interested (for outdoor applications).

As a soil conditioner has the objective to improve the quality of existing soil, there is no single recipe that works for all. Each manufacturer will find its way or a combination of mediums.

Soil Improver vs Compost: What They Do

Now you know what soil improver and compost are. However, I did not tell you why and when you should use them?

Compost 

This is an essential “ingredient” in the majority of potting soil that can be found commercially or that you can create yourself (as illustrated step by step in this article). This is because of its nutrient content and capability. It acts like a sponge that retains water and releases it when needed by the roots. Moreover, compost has the quality to limit the spread of disease, promote the growth of good bacteria, and release slowly to the plant (positive aspect) the nutrients present in the soil. 

Compost can be used when creating a potting mix for indoor herbs or, for outdoor applications, placed on top of the existing soil. In this case, you might also need to turn it over to guarantee a uniform mix of existing soil and compost. 

You can also add an inch layer of compost once every year to your potted herbs. However, I do prefer creating or buying new potting soil to replace the current one every 2 years or so. This indeed guarantees that also the physical structure is kept at its best. Moreover, especially if you are a beginner, it might be more useful to build your own soil (here is a recipe) and transplant your own plant.

A goldmine of valuable experience that requires very little time and also allows you to check the health state of your herbs by inspecting their roots. Moreover, you can also learn by getting your hand dirty the difference between exhausted soil and a fresh one when touching them.

Soil Improver

This, on the other hand, is added only to existing soil. It is not designed to be used as a “soil” where to grow your plant. Hence, I would not fill pots with soil improver and place my herbs there. It does not have, alone, the physical and chemical properties of good soil.

Talking with gardeners, you can notice that soil conditioners can be used for both outdoor and indoor applications. Nevertheless, it looks way more common for outdoor ones. This makes sense. Indeed, if you have an outside outdoor garden or a large land area dedicated to crop production, your soil might be exhausted after a few years. Hence, to compensate for the loss of its ideal physical (and eventually chemical) properties, you can use a soil improver. Indeed, given its size, you do not have the options to replace tons of soil with a new one.

However, this is not a problem for indoor gardens. Indeed, even with a dozen pots, those might count for only for a few kgs of soil. So, it is more effective to replace the current soil buying a high-quality potting soil bag (previous article) or create a new one (here for the recipe). I would consider soil improver for your potted herbs only if you have a seriously large number (50+) and of a large dimension.

One common application of soil improver is to correct the physical properties of degraded clay soil. This soil, although more abundant in nutrients than others, can suffer from compaction and insufficient aeration. Indeed, clay particles are so small that they can create a dense gas/water “proof” barrier between plant roots and the outside. The addition of a soil improver can break clumps and make the soil lighter, allowing roots to access gases and water from the outside.

Soil improver can be used in more mixed soil as well. In this case, the improvement will both increase the nutrients level and enhance the soil structure by replacing components that might be exhausted after a long time (like compost).

Compost Or Soil Improver For Your Potting Mix? 

Compost is also called “black gold” among gardeners. Hence, as you might easily guess, this medium is a critical component in creating potting soil, including for indoor application as discussed in detail in this article. Its ability to retain nutrients and water and its high nitrogen and carbon content are vital aspects that will benefit the majority of herbs. 

Compost can also be placed on top of existing soil in your pot. However, I recommend not to spread it too close to the base of the plant. If not, you might risk burning it as too much nitrogen might have a negative effect as too much food might have on us.

Soil conditioner, on the other hand, is something that I would not use in a potting mix given the simplicity in creating new soil. This is also confirmed by the large number of instructions you can find around by typing soil conditioner. You will notice that the majority are bed-garden applications or for large fields.

Compost And Soil Improver Near You? 

You now decided that you want to buy either compost or soil improver for your indoor plants. Well, the best choice is to go to your local garden shop or even in a plant nursery. There you can have the opportunity to have a great chat with the vendor that quite often is an experienced gardener himself. However, given time and travel constraints, you might not be able to. 

In such a case, you can use one of the large variety of online retailers. I found myself pretty OK with Amazon (and heard good things about Walmart for our USA gardener friends). They both present a large variety of products with different size (from a few liters to bags of 1000liters!) for all prices with good compost products (as the Charlie one on Amazon or this other one in Walmart) and soil improver (Amazon and Walmart).

Related Questions

Is compost all equal? No, the origin of the compost (from worm cast to sewage sludge) affects its final nutrient content. Typically the worm cast-based compost is regarded as higher in quality the gardener world.

Is it possible to grow plant in soil conditioner? Generally not. Soil conditioner usually does not have at the same time, all the minimum fundamental physical and chemical properties to sustain in the long term a herb.

Further Reading