Take a close look. Can you see a layer of sediment at the bottom in your bottle of liquid fertiliser? When you shake it, does it go cloudy when it used to be clear? Can you see the little lumps or crystals in the bottom? Give your bottle of liquid fertiliser a gentle shake. Do you hear something hard hitting the sides?
What you're seeing (or hearing) could be 'salt out'
The nutrients in your fertiliser are mineral salts. Those mineral salts normally stay dissolved and suspended, mixed into the liquid. You normally can't see them.
If your fertiliser is fully or partially organic, it can be natural and normal to get some sediment on the bottom which can cause it to become cloudy when shaked.
But if your fertiliser is fully synthetic, like most houseplant fertilisers are, then they are typically clear with no sediment in the bottom, and certainly no crystals.
But when the mineral salts in your fertiliser drop out of suspension they can form crystals at the bottom of the bottle. Give the bottle a gentle shake and you may hear the crystals rolling around or hitting the sides.
Think of it like trying to dissolve sugar in a cold cup of water vs a hot cup of tea. And it turns out temperature is the major reason behind why it happens.
Does salt out mean a fertiliser's 'gone bad'?
Although normal and not usually anything to worry about if it's a small amount, it can cause a slight imbalance in the ratio and amount of nutrients available to your plants.
But that doesn't mean you have to throw out the fertiliser. It hasn't gone bad. It's just separated. Use the solutions coming up below to get those salts back into suspension.
How to avoid crystals forming in your fertiliser
The best way to avoid salt out is to watch the lower end of the temperature range your fertiliser is exposed to. Salt out can happen when fertiliser is stored in cold temperatures.
Owning my plant store going on 5 years now means some things repeat at the same time every year. It's usually late winter and spring that I get more concerned questions from customers worried about whether their fertiliser's gone bad and what the crystals are, after temperatures got too low over winter.
The salt out point is the temperature at which the nutrients start to fall out of the liquid solution and form crystals.
Crystals can also form as a reaction to contaminants. so take care not to get other things in your fertiliser bottle. Replace the cap quickly after use, store with the cap on and closed, and avoid using one pipette across multiple products.
The mineral salts in your fertiliser can react to the minerals in your tap water, or to contaminants like silica or other fertilisers and nutrients getting mixed in with your fertiliser.
What temperature does salt out happen in fertilisers?
The amount of nitrogen and potassium in the main factor that decides the salt out point. In general, higher nitrogen (the N in NPK), or higher potassium (K), fertilisers have a higher salt out point.
The higher the salt out point, the warmer the temperature that salt out can start to occur.
Because different fertilisers have different N and K levels, and because the type of N and K used also changes the salt out point, there is isn't one temperature that salt out occurs in all fertilisers.
But as a general guide, expect it to be somewhere between below 0 up to 15 degrees Celsius or around high 20's to low 60's in Fahrenheit. Aim to store your fertiliser above the higher end of that range to avoid salt out.
Store your fertiliser indoors in winter, in a heated room, or the warmest room in the house. In a typical New Zealand winter, even when stored inside a heated home, it's hard to avoid overnight temperatures dropping below the higher end of that range.
The good news is there are a few ways to fix it (coming up next), if you've already had salt out occur.
Are some fertilisers more prone to salt out than others?
Yes, some are more prone to salt out, but it may not be the ones you think. That's because one way to avoid salt out is to add water and dilute it further.
You do that every time you fertilise, sure, but one things that matters when it comes to changing the salt out point, is how much water is in the bottle when the fertiliser is made.
As a general rule, the better quality fertilisers also tend to be the most concentrated. That's good for us. It means we're paying for the nutrients not the water.
But that also means they can be so concentrated that they're just on the edge of what the solution can hold without some mineral salts falling out of suspension.
I's not always the case, but that's why you may notice salt out happening more with better quality, more concentrated liquid fertilisers.
What to do when you already have crystals or sediment at the bottom of your fertiliser
One solution, if it's just a little sediment starting to build up, is a quick shake before every use.
If you skip fertilising your plants over winter, give the bottle a shake every couple of weeks anyway (this is why I DO fertilise my houseplants over winter).
Another solution is putting the bottle in a warm water bath to dissolve those salt crystals and sediment, so it goes back into suspension (then give it a good shake after a warm bath).
Another is to wait till the end, mix any leftover sediment or crystals with an equal amount of hot water to dissolve the crystals, then triple-dilute with water.
So if you'd normally use 1ml per 1 litre water, once dissolved, use 1ml per 3 litres water (as the salts will be more concentrated).
The best tip to avoid salt out in the first place?
Only get what you need for the growing season ahead.
It's very tempting to buy fertiliser in bulk as there are some significant savings from most brands when you get bigger sizes.
BUT you're usually better to get a fresh bottle of fertiliser at the start of the growing season (around late-winter to early-spring).
And get yourself a size you know you'll use it up by the time winter comes around, or within a year of opening.
Keep learning
Find out > Should you fertilise your houseplants over winter?
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident writer for NZ Gardener magazine on the topic of indoor plants. You'll see her houseplant care articles in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.