How to Fertilise Plants Grown in Semi-Hydro (LECA & PON)

One of the most common questions when switching to semi-hydro is what to do about fertiliser. It's a fair question, because the answer genuinely is different from soil — and getting it slightly wrong is one of the few ways a semi-hydro setup can actually cause problems, rather than solve them.

Why Semi-Hydro Fertilising Is Different From Soil

Soil naturally holds onto some nutrients and buffers the roots against sudden changes in concentration. LECA and PON don't do this in the same way — they're inert, meaning they don't contribute nutrients themselves, and they don't buffer the strength of whatever's in the water as readily as soil does. In practice, this means your plant gets exactly what's in the reservoir, with much less natural cushioning if the strength is off, which is part of why semi-hydro plants typically need a different fertilising approach altogether.

What Kind of Fertiliser to Use

A fertiliser specifically formulated for hydroponic or semi-hydro use is generally the better choice, since these are designed to dissolve fully and provide a balanced nutrient mix suited to roots sitting in a reservoir, rather than in soil. Standard soil fertilisers can sometimes be used at a much more dilute strength, but they're not designed with semi-hydro in mind, so results can be less consistent. Whichever you choose, always follow the dilution guidance on the product itself rather than guessing — semi-hydro setups are generally far less forgiving of an overly strong mix than soil is. Be aware that some brands of PON come ready fertilised, so reading instructions is a must.

How Often to Fertilise

Most semi-hydro growers fertilise every time they refill the reservoir, using a diluted feed in the water rather than fertilising occasionally at full strength. This little-and-often approach tends to suit the constant water availability of a semi-hydro setup better than the feed-then-rest cycle common in soil growing.

[Photo idea: someone diluting fertiliser into a watering can or jug, alt text e.g. "Diluting liquid fertiliser for semi-hydroponic plant feeding"]

Why We Feed at a Lower Strength

Plenty of experienced semi-hydro growers, ourselves included, feed at well below the strength stated on the bottle — often around a third — rather than following the full recommended dose every time. It's a similar idea to the difference between steady training and a crash diet: one builds something that lasts, the other just pushes for quick results that don't always hold up well. A lower, steadier feed tends to support more even, sustainable growth rather than forcing a sudden growth spurt that the plant hasn't really built the root strength to support.

We feed our own plants this way, and it's one of those small habits that took a bit of trial and error to land on, but it's made a noticeable difference to how settled our plants stay over time. If you're naturally inclined to feed a little less rather than a little more, you're in good company.

Signs You're Over or Under-Fertilising

A few visual clues can tell you which way you might be drifting:

• A white or crusty build-up on the LECA surface or pot sides usually points to over-fertilising (a mineral salt build-up) — worth diluting your feed further or flushing the pot through
• Pale, yellowing leaves or notably slow new growth can suggest under-fertilising, especially in a plant that's been in the same semi-hydro setup for a while without any feed
• Brown, crispy leaf tips can also be a sign of fertiliser strength being too high, particularly in more sensitive plants

Flushing Your Pot to Prevent Build-Up

Every so often, it's worth flushing a semi-hydro pot through with plain water — filling and draining the reservoir a couple of times without fertiliser, to clear out any accumulated mineral build-up before it becomes a problem. This is a useful habit to build in every few weeks, rather than something to only do once an issue's already visible.

The Bottom Line

Fertilising semi-hydro isn't complicated once you know the difference from soil: a little and often, at a lower strength than the bottle suggests if you want steadier rather than forced growth, and keeping an eye out for the visual signs that tell you which way to adjust. Get this right, and it's genuinely one of the lower-maintenance parts of growing in LECA or PON, rather than something to dread.

Stocking up on growing media or fertiliser? Browse our semi-hydro supplies and accessories, or explore our full range of semi-hydroponic plant pots, handmade in Cornwall.