What's the difference between single part, 2 part and 3 part fertilisers?
If you come from the world of houseplants, you may only ever have used single part fertilisers. But if your background is in hydroponics, 2 part or even 3 part fertilisers might be more familiar.
That's not to say those world's don't cross-over. There's a reason it's taken as a good sign if the maker of a single-part houseplant fertiliser has a hydroponics background.
And because houseplant hobbyists are embracing the world of hydroponics and semi-hydroponics like never before, such as growing in substrates like pon, perlite and leca, what you feed matters much more when you grow in sterile semi-hydroponic substrates.
What is a 1 part or single part fertiliser?
Single or 1 part fertilisers are essentially what they sound like, but do come with a warning. They are often marketed as 'everything plants need' or 'all-in-one'. But even if they make the claim to be complete, always check the label. 'Most of...' or 'Almost everything' would be more accurate for many single-part fertilisers. If you're unsure what to look for on the label, keep reading below about essential vs beneficial nutrients.
Most single part fertilisers are not truly complete even though they claim to be. That's most, not all. There are a small number that are, but since most are not, that's one main reason why 2 and 3 part fertilisers exist.
Two popular single part houseplant fertiliser brands that are truly complete are Growth Technology's Focus range of fertilisers, such as Foliage Focus, and the other well-known one is Dyna-Gro, recently rebranded Superthrive, such as their Foliage Pro formula.
To include all the essential nutrients plants need, traditionally you had to opt for a 2 part fertiliser, also called and A and B fertiliser. The main reason is because some nutrients don't play nicely when mixed together. Some may bind and cause nutrient lockout, becoming unavailable to the plant.
Even today, most single part fertilisers can't offer the full list of essential nutrients (most, not all), but technology like chelation is one way some single part fertilisers can include more or all of the essentials, all-in-one, without lockout. Chelation keeps nutrients from reacting, both solutions and in soil, so what's in the bottle is actually accessible by the plant.
Both single, 2 part and 3 part fertilisers include NPK. That stands for Nitrogen, Phosphorus and Potassium. Some single part fertilisers also include a handful of other essentials nutrients. But only a few contain ALL the essential nutrients.
What is a 2 part fertiliser?
A 2 part fertiliser is still designed to be used together, but you buy it in 2 bottles. Typically called an A and B formula. You use both BUT you can't mix them together without diluting them in water first, one-by-one. If you add the two parts directly together in their concentrated forms, the minerals will react and some will become unavailable to the plant. A and B fertilisers aren't as common in the houseplant world as they are for hydroponics.
What's the difference between the A and B fertilisers?
The main difference is in what macro-nutrients are in the 2 parts. The A part is normally the one with nitrogen in it and the B part normally contains the phosphorus and potassium. Both parts will have a mix of other essential micro-nutrients also.
Just like single part fertilisers, some brands of A and B fertilisers include other beneficial nutrients also. Just because a fertiliser is in 2 parts doesn't mean it will have all the essentials, so always check the guaranteed analysis on the label. Speaking of, do check there's actually a guaranteed analysis on the label! Some fertilisers make grand claims about being complete with no testing and no proof. Some also get a nutrient analysis done once but not for each batch, so can be different inside the bottle than what the label claims.
How much 2 part fertiliser to use
Unlike a 3 part fertiliser, most 2 part fertilisers are already balanced to make them easier to use, usually requiring the same amount of each part, each time you water, the same as you would with a single-part fertiliser.
For example, 2mls of A and 2mls of B for a total of 4mls per 1 litre water. Just make sure to add the A to the water, stir, then add the B separately. Don't mix them together first then add to water.
What is a 3 part fertiliser?
Plant fertilisers in 3 parts take a different approach. Normally split into a Grow, Micro and Bloom formula. Normally Grow (sometimes called Foliage), will be nitrogen-based, whereas the Bloom (also called Flower or Fruit), formula will be potassium-based. Micro is the rest of the micro-nutrients.
Even in single-part and 2-part fertilisers, you'll often fine there are Grow and Bloom formulas. Typically a Grow / Foliage fertiliser favours nitrogen, and a Bloom / Flower / Fruit will favour potassium and phosphorus. If you don't know what type you've got, have a look at the NPK ratio on the guaranteed analysis on the label.
With a 3 part fertiliser, after testing what your plant needs, and taking into account what growth stage the plant is in (eg: vegetative or reproductive), you then mix as needed in the right ratio and test again before use.
Because 3 part fertilisers don't have fixed ratios like single and most 2 part fertilisers do, 3 part fertilisers don't tend to be recommended for beginners without the correct equipment. Normally the manufacturer will provide guidelines for the different ratios to use.
Can an A and B or 2 part fertiliser be used for houseplants grown in soil?
The answer is no and maybe, it depends. Although A and B fertilisers are formulated for hydroponics, that's not to say you can't give them to houseplants growing in soil, however do check with the manufacturer or your supplier first, as the amount to give will often be different.
Typically you'll use a fair bit less of a hydroponic fertiliser when used for soil-grown plants to avoid fertiliser burn or nutrients reaching toxic levels. Some may also list on the label what the rate is for hydroponics and semi-hydroponics, versus soil and potting mix.
Can a single-part fertiliser be used for hydroponics and semi-hydroponics?
Again, the answer is no and maybe, it depends. Most 1-part fertilisers made for growing plants in soil rely on the soil microbiome to help supply some of the essential nutrients, so most are not complete.
However, if you grow in water or sterile hydroponic substrates like leca and perlite, the fertiliser needs to do the heavy lifting so you don't end up with nutrient deficiencies and stunted growth.
There are some single-part fertilisers you can use in hydro and semi-hydro: One is Growth Technology (GT) and another is Superthrive Dyna-Gro. If you check the label, you'll see both give you the different rates for hydroponics and semi-hydroponics vs growing in soil or potting mix.
Why do indoor plants need a different fertiliser than outdoor plants?
When it comes time to decide what the best fertiliser is for your houseplants, knowing what's in the bottle matters. That's because houseplants don't benefit from the complex soil microbiome that plants grown outdoors do.
Outdoors, healthy soil includes a community of microorganisms, full of fungi, bacteria, beneficial insects and more that create an ecosystem that (among other jobs), breaks down soil and other organic inputs into essential nutrients for your plants.
But it's a different story when we grow plants indoors, typically in a chunkier, low-soil or soil-free potting mix, or sometimes in sterile substrates like leca. They need a helping hand.
As indoor plants typically have little to no soil microbiome helping them, that means houseplants rely on us to provide those nutrients from their fertiliser instead. That's one reason finding a complete fertiliser matters more for indoor plants, and even more so if you shift to sterile semi-hydro substrates like leca.
What are the essential nutrients for houseplants?
The complete list of essential nutrients for houseplants is:
Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, boron, zinc, copper and molybdenum
That's 12 (so far). Add oxygen, hydrogen and carbon from air and water and now we're up to 15.
Then there are a handful of beneficial nutrients which you won't always find in your fertiliser. That's because they are not essential for all plants, and some may even be harmful. Some are considered essential depending on factors like the plant, what you grow them in and what water source you use.
The beneficial nutrients are chlorine, nickel, cobalt and silicon. That potentially takes us up to 19 nutrients, but because not all of them are essential for all plants, it's more likely to see the 12 essentials listed on a fertiliser that is truly complete. Houseplant fertilisers also intended for vegetables and other crops may go up to 16. Here's why...
Silica
Considering all the hype around silica and the multiple studies proving a long list of benefits for all plants, you'd think Silica would be essential, but it's not. One reason is because silica is generally abundant outdoors, but our indoor plants miss out due to their substrate. That's one reason you may have heard silica is essential for indoor plants.
To remain available to plants to uptake, silica doesn't play nicely when mixed with the other nutrients in fertiliser, so although you won't find silica IN your fertiliser, you can find it readily available by itself. GT Silica and Superthrive Dyna-Gro ProTeKt are two examples.
Chlorine
Chlorine (which plants absorb as chloride) is essential, but because it's usually found in tap water, rainwater and soil (especially if you live near salt water like us here in New Zealand), you may not see chlorine listed on your fertiliser label.
Some houseplants are sensitive to chlorine, but even for those that tolerate chlorine well, chlorine can get to toxic levels even in small amounts. You may want to look for a fertiliser that is chlorine-free, or check the guaranteed analysis to see how much is in the bottle.
One fertiliser I've safely used on my indoor plants that does includes chlorine lists it at 0.006%, however if I use a fertiliser with chlorine in it, I also use a couple of drops of instant chlorine remover in my tap water so I'm not doubling up. You could also use distilled water which is chlorine-free. Distilled water is not the same as filtered water. Keep in mind some household water filters like Brita jugs may reduce but don't remove chlorine.
Nickel
A quick detour to your vege patch: If you're growing crops that are nitrogen-fixers like spinach, then nickel is essential in micro amounts. Not essential for indoor plants, but some indoor plant fertilisers do include nickel, making them handy for both indoor and outdoor use.
Keep keep in mind it doesn't take much to get into toxic territory as we're talking TINY amounts, so if a fertiliser includes nickel check the guaranteed analysis to make sure you're not overdoing it for your houseplants (one I've used on mine that does include nickel is listed at 0.0001%).
Back to the vege patch again: Cobalt is not considered essential for houseplants and other non-legumes, but if you're growing legumes like peas and beans, cobalt is essential, again in very small amounts. That's why you won't find cobalt on your typical houseplant fertiliser label, but one I've used on my houseplants that does include cobalt has it listed way down at 0.0005%.
In case you find something amiss with a fertiliser that says it include cobalt and nickel, but doesn't list it on the label, that can be because the levels are so low they aren't required to be listed in the analysis (labeling laws also change country-to-country and sometimes even state-to-state). Check the manufacturer's website or ask your supplier as they will normally have a more detailed breakdown they can share.