The sneaky plant killer: Ultimate Guide to Fertiliser Burn that could save your indoor plants
Yes, there can be too much of a good thing when it comes to fertiliser and our indoor plants. It's rare for too much fertilizer to burn plants right away. Solved with an extra flush through of plain water. The bad news is fertiliser burn is normally a sneaky plant killer that creeps up slowly over time. The slow-burner type (which is more common for houseplants), is also called salt stress. The immediate (and less common), type of fertiliser burn is called salt shock. Either way, you can help your plants survive it.
Knowing the signs to watch out for and how to avoid it may just save your plants. Because yes, fertiliser burn is not just a matter of looks. It absolutely can be life or death for your houseplants. Here's how to save your plants...
What causes fertiliser burn?
First things first, it's not what it sounds like. It's not the fertiliser literally burning the plant. It's not even a burn. What we call fertiliser burn is most often caused by the plant's response to too much salt in the soil around the roots. The two main sources of salts in the soil are your water source and (you guessed it), fertiliser.
When the salt levels in the soil get too high, water stops moving from the soil into your plant through the roots like it normally would. Sometimes it even goes the other way and high levels of salts in the soil can pull water OUT of the plant! If the soil's dry, the salts in the soil can absorb what little water there is, leaving nothing for the roots. And yes, even if the soil is wet, the roots can still react to too much salt by starving your plant of water.
What are the signs of fertiliser burn to watch out for?
Primary signs of fertiliser burn
Early signs - usually seen before you see any physical 'burn' above ground - is a white salty 'crust' forming on the surface of the substrate. Also make sure to check the drainage holes underneath for a white build-up there too. Since we rarely look underneath our plants, it's a good reason to always lift up and check under the pot as a habit every time you water.
Never water 'through' that salt crust. Whether you top-water, bottom-water, wick-water or use a self-watering pot, always remove that salty build-up if you see it. Watering will temporarily dissolve those excess salts back into the soil and surround the roots in salt again. Personally, I prefer to repot if there's so much salt that it's leaving that white crust on the soil or around the drainage holes, but at the very least, scrape off the top layer of the soil before watering again.
If not caught in time, above the surface is usually next. Excess salt can turn leaves brown, usually from the leaf tips and margins inwards. That happens because your plant's reacting to the lack of water from the roots, by drawing moisture into itself from its foliage instead. Although not strictly speaking a burn, that's why foliage can look scorched or burned.
If you still don't fix the cause, leaf tips and margins can progress to entire leaves turning brown and dying. But what's more of a concern is what's going on below the surface.
What you can't usually see is the root damage. First root growth slows then stops, then roots turn brown and die. As root hairs or the entire roots die, next up can be root rot. And if you've ever had to deal with the dreaded root rot, you know how fast that can kill a plant (here's everything you need to know about root rot and how to save your plant).
Secondary signs of fertiliser burn
Although yellow leaves isn't typically associated with fertiliser burn, a lack of water can also result in a lack of nutrients in general, and a nutrient deficiency CAN cause yellow leaves.
High salt levels in the soil can also cause nutrient deficiencies another way, through soil pH. Soil pH has a Goldilocks Zone: not too acidic, not too alkaline, just right. High salt levels in the soil can mess with the soil pH making some nutrients too available or unavailable to the plant. Too available and some nutrients can turn toxic, causing nutrient burn.
For most plants the goldilocks zone is pH 6 to 7.5. Too acidic and nutrients like manganese can become toxic while calcium, phosphorus and magnesium become less available. Too alkaline and the macro-nutrient phosphorus along with the majority of micronutrients become less available.
Is there such a thing as a low-salt fertiliser?
Once you know that it's actually the salts in fertilisers that can be to blame for fertiliser burn, it would be natural to assume fertiliser companies would make a no-salt fertiliser. But strictly speaking, whether in dry or liquid form, all fertilisers ARE salts.
But there is such a thing as a reduced-salt fertiliser. Some fertilisers have a higher salt index than others due to the ingredients they use. So in a way, yes, shifting to a fertiliser that avoids high-salt index ingredients CAN go a long way to help reduce the risk of fertiliser burn.
When you think of reduced-salt, you might think of table salt. But fertilisers don't typically contain sodium (some do though, check those ingredients!). However, one of the highest salt-index ingredients you will find in most fertilisers are chlorides.
There are a handful of sodium-free and chloride-free fertilisers available in the market these days as plant hobbyists get more clued up about fertiliser burn. One is Growth Technology, known in the industry just as GT. GT is available here in New Zealand, or for my overseas readers, you can now get GT here on Amazon.
Beware under-watering!
Over-watering can drive fear into the heart of houseplant lovers. So much so, that some of us end up under-watering. The problem is under-watering increases the risk of fertiliser burn. Something I didn't find out until recently.
And no, I don't just mean some plant parents (like me), who don't water often enough, leaving plants too dry too long. Guilty as charged. Under-watering also includes watering too lightly.
Over-watering is NOT possible from giving plants plenty of water when they're thirsty. You absolutely should thoroughly wet the substrate top to bottom so no roots miss out. That does not cause over-watering, or the dreaded root rot that comes with it.
It's a water-logged substrate that leaves root sitting in water without enough air, and when you water again TOO SOON that you need to be worried about. Use the right substrate for your plant and water when it needs it and you're good to go. Really, if you think about it, the name 'over-watering' is pretty misleading.
Giving your plants too little water by not fully saturating the substrate can not only cause some roots to miss out and die (which also causes root rot by the way). Watering too little can also put plants into salt stress. The salts in the soil can absorb what little water there is, stealing water from your plants. Next thing you're dealing with fertiliser burn.
DIY may be a disaster
Another way to invite fertiliser burn in to your home is by using DIY fertilisers. Worm tea, banana skins and the like. As well as some of those also inviting Fungus Gnats to move in (here's how to get rid of fungus gnats once and for all), it's hard work getting the levels of nutrients our plants need perfect.
When there's too much of one nutrient and not enough of another, you can end up giving plants too much of a good thing (yes, even if the source of the nutrients is organic). The excess can't be absorbed and builds up in the soil, causing salt levels to rise, pH to change, and increasing the risk of fertiliser burn :(
These days I leave getting it exactly right to the experts and I choose a complete and balanced fertiliser for my indoor jungle. I admit it doesn't have the same feel-good factor as brewing up some banana tea like I used to, but I prefer the less risky method of giving plants what they need without the risk of excess or deficiencies (or fertiliser burn!).
Avoid mix-feeding
If one is good, two must be better, right? I've been asked a lot over the years if it's okay to mix two fertilisers together. Especially one that's organic-based and one that's synthetic. Best of both worlds, right?
Actually, no. I'd stick to one or the other. By all means alternate between them, just avoid combining them. One reason is nitrogen. The first and highest ingredient in most fertilisers.
A lesser-known cause of fertiliser burn is simply too much nitrogen, which can mess up a process called cellular respiration, essentially the key part of how plants make the energy they need to stay alive. The result? Fertiliser burn, including root rot and leaf death (especially of the lower leaves).
Knowing WHAT you mix is key. If you've got something like GT Rootzone or Japanese cult-favourite HB-101, those are not fertilisers and CAN be mix-fed with fertiliser. If in doubt, ask the store you bought them from if what you have can or can't be mixed with fertiliser.
Temperature matters
Just like drier soils have a higher potential for fertiliser burn, so do cold, drier soils. Heating the colder rooms of the house, or using a heat mat in winter, can keep soil temperatures warm and reduce the risk of fertiliser burn. Warm, moist soil reduces the risk.
Even using lukewarm-temperature water when you water your plants can help reduce the risk, as the salts become more soluble in warmer water and excess salts can be flushed out more easily along with the excess water.
Bottom watering is risky
Before you panic and never bottom-water again, I tend to bottom water most of my 100+ plants these days. But the reason top-watering is safer when it comes to fertiliser burn is top-watering - as long as you water heavily enough so water pours out the drainage holes - means the water that drains out takes some of the excess salts sitting in the soil out with it.
That's why most fertiliser companies (even those who make sodium-free and chloride-free fertilisers), will tell you to do an occasional 'flush water' with plain water. Although I bottom water the majority of my plants, and I use a reduced-salt fertiliser every time I water, I still do a top water every two or three months.
Avoid 'sharing' or re-using water between plants
Another way to increase the risk of fertiliser burn is sharing water. Whether you top water or bottom water, re-using water that has been used for another plant first, means the water can become higher and higher in salts. Not from the fresh fertiliser used, but from the natural accumulation of salts in the soil that water's just passed through. Sharing water also increases the risk of root rot as the spores that cause root rot are water-borne.
Fertiliser burn may usually be a sneaky, slow killer but hopefully now you know the signs and how to reduce the risks you can save your indoor plants. If you want to keep learning, here's more related guides from the blog...
Keep learning...
Find out the common causes of yellow leaves and how to fix it fast (before it's too late) >