Soil secrets: how your potting mix affects how much you water
Potting mix (a general term to describe the soil that your plant grows in) is one of the most overlooked parts of houseplant care. It's important to understand and get right, because not only does it provide essential nutrients to your plants, it influences how often you'll need to water them too.
What's actually in your potting mix?
Most houseplant mixes available are a blend of materials (known as ‘substrates’), each one playing a different role. Additives like perlite and horticultural grit improve drainage but don’t hold water , whereas coir and sphagnum moss do the opposite – they retain moisture while staying open enough for roots to breathe. The combination of materials that make up a potting mix determines whether it dries out in a couple of days, or holds moisture for a week. This is why it's essential to buy (or make your own) a potting mix that is best suited to the plant growing in it. 
Why do different houseplants need different mixes?
Your houseplants may all live under the same roof, but it’s likely they come from vastly different environments in the wild. For example, a cactus and a fern have almost opposite requirements. Cacti and succulents need a gritty, free-draining mix that's allowed to dry out completely between waterings, mimicking the arid, rocky or sandy soil they typically grow in. Ferns, on the other hand, often need the opposite: a moisture-retentive, humus-rich mix that's much less prone to drying out.
And then there are plants like orchids, growing in an airy bark-based media that drains fast and dries quickly between watering, which is best suited to their epiphytic nature.
If you opt for the wrong potting mix, you'll be fighting an uphill battle no matter how carefully you water.
How do different substrates affect watering?
Most potting mixes start with a nutrient-rich base (usually compost) which feeds the plant but has little bearing on how the mix holds or drains water. It's the additives blended into that base that alter the texture and structure of the mix, and in turn, how often you'll need to water.
- Perlite is lightweight and volcanic, holding no water but it does. iImprove drainage and aeration.
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Vermiculite is a natural, layered mineral rock that is puffed by heat, making it hold both moisture and air, which is suited to plants that need consistent moisture.
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Horticultural grit improves drainage and prevents compaction. It also adds weight and stability to the pot.
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Coir (made from coconut husk fibre) retains moisture while staying open in structure. It’s widely used in peat-free mixes.
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Sphagnum moss has a very high water-holding capacity, often used for orchids and carnivorous plants.
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Orchid bark is fast-draining with good airflow, suited to roots that need air between waterings.
The more inorganic material in the mix, the faster it drains. The more organic material, the longer it holds onto moisture.
Why does peat-free mix behave differently?
Most houseplant mixes are peat-free now, and they come with one quirk worth knowing. They can look dry on the surface while still holding plenty of moisture underneath, which catches a lot of people out and leads to overwatering.
If a peat-free mix is allowed to dry out too much, it can also turn hydrophobic, meaning water poured on top runs straight down the side of the pot rather than soaking in. If that happens, a gentle pour, given time to absorb and repeated once or twice, will rehydrate it far better than one heavy go.
Does the quality of the mix matter?
It does. Better-quality potting mix holds moisture evenly throughout the pot. Cheaper mixes can develop dry pockets or drain unevenly, which means parts of the root system can be dry while others are sitting wet, even though you've watered the whole pot.
So how do you know when to water?
Equipped with knowing what your potting mix is, you can make an educated guess on how frequently you'll be picking up the watering can. Even so, it's always best to check before pouring away - simply checking the surface layer isn't sufficient. Push a finger a couple of centimetres into the mix, and you'll get a far better sense of what's going on beneath. Lifting the pot helps too, as a dry pot feels noticeably lighter than a freshly watered one. Once you've got a feel for it, watering stops being a guess and starts being a routine you can trust.
Getting the mix right makes everything else easier, but it's only half the job. The right can lets you put that understanding into practice, whether that's a gentle soaking after a fresh repot or a precise stream for a quick top-up. Take a look at our indoor watering cans.