2 Aspects For The Best Potting Soils and DIY Recipe


A perfect potting soil is a balanced mixture of different mediums able to provide gases, water and nutrients without any excess – Photo by Lisa Fotios from Pexels

You might want to start growing your indoor garden to start harvest the most aromatic leaves you ever tasted before. However, you need the basic: the soil. Indeed, there are countless types of soils out there and you might wonder what will be the right choice or even if you can create the perfect soil yourself. Based on my experience, asking other gardeners and on extensive research here you can find all the information you need.

Hence, what are the two aspects that you should look for the best potting soil to grow indoors herb? The perfect soil 1) should have the right porosity to allow the flow of water and gas and also retain water and release it when needed 2) have the right type of nutrients and capability to retain them. An ideal potting soil is given by a balanced mix of peat moss, perlite and compost. In addition, limestone and fertilizer are also required.

If you want to know more about what makes a potting soil ideal for your herbs, how to create the perfect soil yourself and much more keep reading. 

First Aspect – Soil Structure: Herbs Need Gas And Water

A good potting soil needs to have first and foremost optimal physical properties. The presence of enough space to allow the exchange of gases between the roots and the outside world is paramount. Indeed, roots breath and, similar to us, they need air all the time if you do not want them to wilt and die. A compact soil in which even the water cannot penetrate (think about all the puddles that you might see in some parks after a rainy day) is not the right choice for your indoor herbs.

Moreover, the soil needs to provide adequate drainage. This means that the herb’s roots should never be flooded (other root rot) but neither left to dry out (the majority of edible herbs do require moist soil). Hence, the soil should be able to retain water and release to the plant. This ideal behaviour would make your life way easier increasing the chance of success in growing herbs indoors.

The above physical aspects are regulated by the structure of the medium adopted. You need to know that, although a large variety of medium exists in the market (perlite, vermiculite, compost, sand, etc….), only three of them are defined soils by gardeners and these are:

  1. Sand: it is made of particles from 0.2 to 2 mm in diameter;
  2. Silt: it is made of particles from 0.002 up to 0.2mm in diameter;
  3. Clay:  it is made of particles below 0.002mm in diameter.

Needless to say that a potting soil requires also water, air and organic matter to allow your herbs, as well as any other plant, to grow.

The capacity to retain water and gases cannot be considered separately. Indeed, they are both strongly dependent on the soil structure, or, in other words, by its porosity and consistency. Porosity is a measure of the dimensions and distribution of the pores in the soil. Both extremes (very low and very high) are bad for the majority of herbs

From one side we have soil with large pores (high porosity, like sand). In this case, it is quite likely that many of those pores will partially overlap across the soil volume creating multiple channels of empty space where water and gases can flow. This type of soil dries very quickly and are unsuitable for herbs if you do not water them very frequently.

On the other extreme, there are those soils given by very small and rounded particles (clay for instance) with very small pores. This type of soil presents a smooth looking, heavy and compact texture. In this case, water and air from the outside will have a massive difficult to reach your herb roots leading to a quick death of your plant (if you accidentally plant them in such a soil) due to the lack of air/water circulation. Moreover, herbs roots do not have the physical strength to move it around to develop.

In addition to the capability to move around (porosity), the medium material might have a specific capability to retain a large amount of water (compared to its volume). An example is the quite famous peat moss. As I will detail later, when providing the best potting soil recipe for beginners, you want to have a fraction of your potting soil with such material especially when we talk about indoor herbs as, although they do not like “wet feet” (gardening way to say wet roots) they thrive with slightly moist soil.

Hence, like everything in life the best lies in the middle as the best potting mix will present medium with excellent water retention capability with porosity “enhancer” medium that presents big particles (perlite). Indeed, mixing material with different structure can give, in the right ratio, benefits both-side mitigating the negative ones. 

Moreover, an ideal potting soil should be able to provide stability to the plant’s roots. This is not a big issue for the majority of you as your herbs, potentially on the windowsill of your kitchen, will not be exposed to winds or harsh weather that can destabilize them.

The ideal potting soil should present a good water retention capability medium with a average/high porosity. In more qualitative terms this means that the best potting soil for herbs should feel light and airy.  

Another feature, water-related, is that your soil should not shrink/ expand significantly when it gets dry/wet. Indeed, if you might have experienced with some of your past “forgotten” potted plants that the soil, once dry, was not touching the wall of the container this was probably a bad soil. Indeed, this will cause further damage to the plant as unable to exchange gas through its roots as trapped by this very compact soil.

Second Aspect – Soil Nutrients: Herbs Food

The second fundamental aspect you need to consider in choosing or creating the best potting soil for your herbs, beyond a good physical structure, is its nutrient and chemical content. Indeed, an ideal potting soil should be able to supply the nutrients needed by your herbs, ideally for a long time. That’s why it is common practice to add fertilizer or organic-rich material (one of the best is worm cast) to the potting mix. How do you know if a potting soil has the right amount of nutrients? Well, you can check this article.

The amount of organic material cannot be too high as this will damage your herbs (too much nutrient is so bad that can “burn” the plant roots). Moreover, such medium although excellent nutritional wise, might not have the ideal physical structure (porosity) affecting the soil. In addition, you will see when I will introduce you the easy beginner recipes, that multiple nutritional sources (like compost and fertilizer) might be needed in order to complete each other.

The physical structure discussed before also have a side effect on the capability to retain nutrients. Indeed, a soil with high porosity, such as sand, will let a large amount of water passing through it very quickly. Hence, those nutrients that bound with water (and there are many) can be easily washed away in such types of soil making them bad candidates for herbs.

Expert gardeners call them “hungry” soil referring to the amount of “plant food” (fertilizer) that need to periodically receive in order to allow herbs as well any plant to grow. It is not impossible to grow in these soils, but it is quite hard and costly.

At the other extreme, we have those soils that due to their water retention ability to stay rich in nutrients for a long time. One example is clay. But, as discussed before, this might affect the breathing capability of your soil. Moreover, clay is known among gardeners to have a further capability to retain important nutrients for herbs like Magnesium, Sodium, Potassium and Calcium. Without getting into the scientific reason behind it, this is because clay acts as a magnet (very simplified here, please expert gardener/soil scientist do not fustigate me!) can attract those nutrients.

A potting soil mix needs to be able to deliver water, gases and nutrients to the roots of your herbs. That’s why its composition matter for the success of your indoor garden

Is that enough? No of course. The pH plays is another key protagonist in the chemical balance in the soil. 

The soil capability to retain nutrients is also strongly affected by its pH. A soil with a low pH (<5.5, acidic) make some elements that are normally locked and inaccessible to the plant, like Aluminium, Magnesium and Iron, readily available. This is bad as your herb will likely die sooner than later. Many expert gardeners recommend a pH between 5.5 and 7.5. How do you know if this is the case of your soil? In this article, you can find info regarding soil testing while in this one an overview of the best soil testers in the market.

As an extra (but essential) addition is that a good combination of physical and chemical properties, and hence a soil rich in nitrogen and carbon source (the plant respiration) will create the best environment for the development of healthy microbes that will boost the full development of your herbs by producing further secondary nutrients and plant hormones .

Prepare the Best (Non) Soil: Do It Yourself

If you are reading this section then you are willing to learn the secret to create the best for your herbs and, on top of that, you will have the advantage to learn tons more on how the soil works by creating, like a baker, your mix

One of the easiest soil recipes, especially for beginners, also used (with some minor differences) in  potting soil commercially available is a balance of three mediums 1) able to provide organic matter (compost) 2) to retain the corresponding nutrients and water (peat moss) and 3) provide a good porosity (perlite). 

The ideal potting soil mix should be light with up to 30% of its volume of air. The following potting mix was obtained based on my knowledge, asking around to more experienced gardener, listening to podcasts and watching video from the very pro. It is an easy recipe for beginners and provides an excellent potting soil with great physical and chemical characteristics suitable for herbs as well as a great variety of other plants. Here is the recipe (expressed in volume, you decide if 1 volume is 1 litre, 100 gallons or whatever is suitable for you):

1 unit peat moss: peat moss is the result of thousands of years of decaying vegetable material underwater. Peat moss, once extracted (even down to up to 10m), is dried out, compressed to be then placed in large bags that you can find in your local gardener shop and it is also part of main potting soil mix commercially available.

Peat moss is a (historical) institution among gardeners. This is because it can retain a significant amount of water and release it slowly when the roots demand it. This is a great feature as it makes the soil very easy to manage, important for hobbyist and beginner gardeners in particular.  Indeed, it is way harder for your herb to suffer from overwatering or underwatering in a soil mix using peat moss as this medium acts as a buffer providing a stable amount of water to your herbs.

For these above reasons, it is not a surprise that is widely used/recommended by many gardeners, from beginners to experts, from small indoor to large outdoor applications. Moreover, peat moss is free from diseases and pests and has an interesting microbe content that helps to limit the spread of them.

A box full of peat moss – a soft, light and airy medium that resemble soil but it is not – Photo from Dug Buckers in Flickr

1 unit volume of organic material: the most commonly used is compost. This is the result of decomposing, in a specific way, organic material such as dead plants, animal manure and sewage sludge. Independently from where it comes from all compost presents the characteristics to hold water and high nutrient content (such as Nitrogen, Phosphorus and Potassium as well other “secondary” nutrients).

Moreover, compost releases such nutrients very slowly released avoiding the spike caused by the most common indoor herbs fertilizers (in liquid form) that can literally burn the roots of your herbs. 

In addition, such nutrients are tightly part of the compost making them difficult to be washed away, but at the same time, they are easily accessible by the herb roots. Compost also hosts a plethora of microorganisms that boost herb growth by producing, as bioproduct, other nutrients and herb hormones

As all the above advantages were not enough, compost is not only the major contributor to the nutrient content in the soil but also has an important structural function. Indeed, it creates clumps of soil (due to its stickiness once wet) that increases porosity further promoting the health of your herbs

0.75 volume of perlite: if you bought a bag of potting soil, it is very likely that you say perlite in it. Perlite appears white in colour under the form of large grains (around double the size of rice grains). This medium is classified as glass (more precisely is a volcanic rock) and has the capability to retain air (indeed is quite light in weight and can float when adding water). However, the main function is structural. Indeed, due to its large size, gives to the soil a high porosity, needed to allow nutrients and gases to circulate

Close look-up of perlite, its coarse structure is ideal to provide aeration in the soil

Pulverized limestone: this strong alkaline material (pH around 10) originated from coral and mollusc sediments (so very high in carbon content) and it is added to the soil recipe with the only purpose to counteract the high acidity of peat moss (that has pH around 3, the same of orange juice). As very alkaline you need only a small amount (around ⅛ of a cup for every 3 gallons of peat moss).

Fertilizer: Although the presence of nutrient-rich compost you might want to add fertilizer to your soil. Indeed compost represents only a third of the whole soil volume and does not have all the right amount of nutrients your herbs needs. As you are producing soil for all your pots (and probably more than 1 or 2) I will invest the money in a decent soil fertilizer whose nutrients are slowly released into the soil, definitely beneficial for the plant.

Pellets of fertilizer, this is a typical form in which slow-release fertilizers are sold and the best for your potting soil when creating the mix

Procedure: I usually use a large bucket in which I add first the peat moss. Tip: the peat moss needs to be abundantly watered (until soaked) before being mixed with the others “ingredients”. Indeed, the very first-time peat moss is kind of “water repellent”. Once soaked (to know the right level of watering many suggest to squeeze the peat moss and see if it release a few droplets of water, if so it is ready) you take out the peat moss and place in a second bucket where you add the other mediums and mix them vigorously to let the air get in either with a wooden long spoon (those very long ones you find in the supermarket for soups) or with bare hands (in this case better wear some gloves). I usually add fertilizer at the end (the specific amount depends on the type of fertilizer you are using).

If you want an extra mile I will recommend performing the tests described in these articles (Test Your Soil Like a Pro and Find Out the pH of Your Soil Easily) on your potting soil after 2-4 weeks it has been used for your herbs. Indeed, making the test straight away will not take into account the contribution of the fertilizer that takes time/watering to be absorbed by the soil (slow-release, you remember).

A good rule of thumb in designing your potting soil is to avoid any medium in the potting to be more than ⅓ of the total volume. In this way, the downside associated with any medium does not become dominant in the final soil mix and it is possible to be corrected by the other components.

Potting Soil Mix Variation

The soil mix provided above is a simple recipe ideal for beginners and approved by a large variety of experts in the field. However, gardening is an always dynamic and evolving area with new discoveries to help your herbs and (and why not) the planet to be a better place. Indeed, based on recent studies, you might consider the following variations.

Peat Moss: An Uncertain Future

Peat moss is at the centre of an intense discussion in the gardener world. Indeed, although widely used in the majority of potting soil available in the market and a great addition for a large variety of DIY potting soil mix it presents several environmental downsides. Indeed do you know where peat moss comes from? 

Peat moss is the result of thousands of years-long decomposition process of a species of moss (called Sphagnum) that develops in waterlogged areas. Hence, such material is produced in geological time-scale, at a pace way slower compared to the one at which it is extracted (some estimates 500 times slower). This means that its supply will definitely end up at some point and it is so necessary to look for alternatives.

Moreover, peat moss is one of the most effective natural carbon dioxide collectors on the planet and for such reasons should not be mined. Also, peat moss, when undergrounds, acts as a water filter by purifying the water that passes through it and preventing dangerous floods. Finally, it is an important element for some specific flora and fauna habitats in the UK as well in the USA where it is mined the most.

If you live in the UK peat moss commercially available might experience a decline in the near future. Indeed the government has expressed a few times the intention in reducing dramatically the extraction of peat moss. At the moment, the majority of UK peat moss is imported from Northern Ireland (and in Canada for the USA supply), where, unfortunately, it is still intensively mined 

Moreover, a drawback of peat moss is its inability to retain water once it has been completely dried becoming very difficult to use. Finally, its low pH, although can be corrected, still requires extra care with the addition of limestone.

Hence, although peat moss is a great medium you might want to opt for a peat-free potting mix (you will hear this term a lot these days, especially from potting soil vendors). For this purpose, you can replace peat moss with one of its major substitutes: coconut coir.

This material is obtained as a bioproduct of coconut harvesting/cleaning process that otherwise would go to waste, so even better for the environment. Coconut coir is the “fleshy/fibre” part of the coconut that surrounds its “wooden” shell (that you never see, as, at the supermarket, you buy already cleaned wooden shell).

One of the most common forms in which coconut coir is sold: dried and compressed brick. In this case, you need to add water to it – Photo from Jungle Garden in Flickr

The coconut coir presents a good structure with plenty of pores (positive aspect) but, unfortunately, this decays way faster than peat moss obliging you to prepare new soil at a higher frequency. Coconut coir is pH neutral (positive aspect), but very poor in nutrients compared to peat moss. Hence, you might need extra fertilizer to compensate. Moreover, its cost is way higher, probably due to the drying process involved in its production.

There are also other less known alternatives (such as paper, sewage water, biochar, etc…) although coconut coir is one of the best at the moment. It is important to remember that up to now there is no perfect substitute of peat moss due to its unmatched ability to retain and release water according to the plant needs as well as for his great structure (high porosity).

That’s why, up to now, still many gardeners (not all though) still recommended peat moss.

Can I Use Garden Soil?

You might have in front of you a nice and prosperous rich garden. Hence, why not take some soil from there, put it in a pot so to save money and time. After all, that soil is already proving to be fit for purpose right?

Not so fast! Indeed, experienced gardeners always agree in avoiding as much as possible to use garden soil as potting mix (or part of it). Indeed, garden soil might have harmful bacteria that might affect your herbs.

Moreover, if you just grasp with your hands, you can immediately notice that garden soil is heavier and more compact. This is not a problem outdoors as a plethora of insects and animals (earthworms) are constantly moving the soil creating those empty channels that roots uses to breath. However, in a “more sterile” environment as a pot, such insects are not there (you can in theory, but are difficult to keep under control as detailed in this article) causing breathing problems to your herb that will probably suffer.

Avoiding garden soil for your potted plants is also justified by safety reasons. Indeed, garden soil might contain harmful to human bacterias (animals such as cats and rodents might urinate or defecate over there, good for plants, less for you) that might spread very easily in the close environment of a kitchen where, after harvesting a couple of basil leaves, you are preparing the food for your dinner (perhaps forgetting to wash your hands as your basil leaves and soil are supped to be clean).

A garden might look like a quick, and free source of a large amount of potting soil but in reality, is a BAD idea – Photo from tejvanphotos in Flickr

Another aspect in favor of a proper potting soil is its ability to be tailored for specific plant needs as you are deciding what goes in there and in which amount. On the other hand, you do not have much choice with garden soil. Of course, this comes to a cost as the potting soil does not come for free as the garden soil. However, considering that a few gallons good quality potting soil, like the Foxfarm this one on Amazon, comes for a dozen dollars, it looks to be a reasonable expense for a long-lasting soil that will fill many small-medium size pots. 

Are All Potting Soils Sold Equal?

Like everything in a marketplace, you can more expensive high-quality product and very cheap ones that are on the essential/minimum needed side. The same applies to the soil. To be honest, I am quite a frugal person, that try to minimize any waste of money (I still using clothes bought 7 years ago to go to the gym!). However, in this case, I would not recommend a cheap soil, indeed the difference in the results can be dramatic. Cheap soil has hurt gardener (literally and metaphorically) for many of the following reasons:

Pests: A cheap potting soil might have not undergone a proper sterilization process. Many gardeners have found unwanted surprises such as Symphylans in their potting bags. These are a few millimetres long white insects (a pest) similar to a centipede that lives in the soil. In addition, it is not nice (neither safety if you have children around) to have them wandering around your kitchen! Others have found fungus gnat in their potting soil. These resemble fruit flies, but differently, from them, they might affect the roots of your herbs if in a large number. A few people also found living grasshoppers in their potting soil bags!

Symphylans, also known as garden centipede, is a pest that might infest cheap potting soil bag

Heavy metal: and no, we are not talking about music bands here. Several cases, especially in the USA, have been recorded of heavy metal coming from the compost contained in the soil. This is possible as compost is also obtained from sewage water and, a not proper treatment, might leave some of this dangerous material in the compost. These metals might pose a serious health problem as once absorbed by your edible indoor herbs, they can end up in your body once you eat them (basil for instance).

Trash: yes, you read well. In some cheap potting soil bags, gardeners found pieces of trash. This is a dangerous hazard as you are going to touch the soil with your hands when preparing for potting and few people get injured with slivers and pieces of wood. Moreover, others found stones in their bags too.

Plants killer: this might be the worst problem of all. Indeed, some cheap soils have also proven to rapidly kill those herbs unlikely enough to be planted there. Indeed, an excess of nitrogen (the big deal among gardeners, this article for more information on macronutrients) can cause a rapid growth over a short period of time followed by a sure death.  

Hence, my main takeaway is of not budgeting in choosing a potting soil, for one simple reason. The difference in price between a good potting soil and a cheap one, in absolute, is not massive (we are talking about 5-10 dollars max difference for the same 50 litres bags for instance) and it is well worth considering that a bag is sufficient for a large number of pots, more than you might need if you are just starting as a gardener.

How long potting soil last? It is suggested to replace potting soil after around 3 years as it starts losing its quality and structure;

Can potting soil be recycled? Yes, it is possible, through a rejuvenating procedure to establish the initial property of the soil although material such as compost, fertilizer and others might be needed to reintegrate the chemical and physical features that the soil lost;

Can I use potting soil in an outside garden? Yes, although it is not recommended as the potting mix will dry way quicker outside.

Further Readings

Best Potting Soil

Adding sand to your mix?

The coffee ground in your potting mix?

21 Tips to grow massive basil

Test the soil: full beginner guide

Best soil testers for beginners

A simple video on producing a potting soil mix using coconut coir

Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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