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 of 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) is called '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 at all.
But when the mineral salts in your fertiliser drop out of suspension they can form crystals or sediment at the bottom of the bottle.
Give it a shake and you may hear the crystals or your fertiliser may change from clear to cloudy.
Think of it like trying to dissolve sugar in a cold cup of water vs a hot cup of tea. And it turns out heat 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.
The salt out point is the temperature at which the nutrients start to fall out of the liquid solution and form crystals.
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 happen.
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.
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 degrees 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. But if you've already had salt out occur, there are a few ways to fix it (coming up next).
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. 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.