The Best Organic Indoor Plant Fertilizers For Healthy Houseplants

A guide to some of the best organic indoor plant fertilizers, including the benefits of using organic fertilizers over inorganic ones & tips on how to get the best from your indoor plant fertilizer.

A range of indoor house plants in pots. Image copyright Annie Spratt via
Image: Annie Spratt

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As an organic gardener, I don’t use herbicides or artificial fertilizers on my allotment. Instead, I try to feed the soil with natural, organic matter, such as manure, compost, leaf mould or comfrey tea.

However, for indoor plants, it’s easy to forget that they’re strictly limited by the amount of soil & nutrients in the pot – they don’t have the ability to spread roots to surrounding soil or benefit from nutrients naturally found in rainwater.

So houseplants often have to rely on supplemental feeding to help them stay healthy during their growing season. This is where natural & organic indoor plant fertilizers can come in handy if you want to avoid synthetic ones.

Inorganic Vs Organic Indoor Plant Fertilizers

There are two main types of plant fertilizers: inorganic (man-made) and organic (derived from plant or animal). Inorganic fertilizers are synthetic, artificially processed forms of plant nutrients, whilst organic fertilizers are made from materials derived from living things or inorganic minerals.

Inorganic plant fertilizers

Inorganic fertilizers are processed from minerals, gasses and inorganic waste material and are often called artificial fertilizers because they go through a manufacturing process when they’re made.

Inorganic fertilizers often only contain the key basic nutrients plants need; namely nitrogen, phosphorous & potassium (often called NPK fertilizers). Concentrated & generally cheaper than natural/organic versions, artificial fertilizers contain nutrients in a form that is readily available to plants. However, because they are so soluble, these nutrients can be quickly lost into the soil.

This loss of nutrients also has environmental impacts – from making soil more acidic to negatively affecting air and water quality, as well as aquatic life. If you have a plant that’s struggling and needs a quick boost from a certain nutrient, an inorganic fertilizer may help it survive, but overall they contribute very little to the fertility of the soil & may even decrease it.

Organic plant fertilizers

Organic plant fertilizers, on the other hand, are derived from natural plant and animal sources, such as microbes and organic waste or materials. Organic fertilizers contain nutrients that are minimally processed & remain in their natural form. Common examples include seaweed or bone meal.

Organic fertilizers generally contain nutrients at lower levels that inorganic ones (because they’re less concentrated), plus they work less quickly because organic matter has to decompose before it’s in a form plants can absorb. However, this slower provision of nutrients ensures a steady release of nutrition into the soil, containing your indoor plants.

Organic fertilizers also improve water movement into the soil and, over time, add structure to the soil, feeding beneficial soil microbes and boosting soil fertility.

Importance of NPK Fertilizer Ratio

Whatever indoor plant fertilizer you opt for, it’s essential it has a balanced distribution of three essential nutrients in order to flourish. These nutrients are phosphorus, nitrogen and potassium. Here’s what each one does:

  • Nitrogen (N) promotes green leafy growth
  • Phosphorus (P) helps develop healthy roots and shoots
  • Potassium (K) encourages development of flowers & fruits.

Different plants need different proportions of these nutrients in order to flourish and this is known as the ‘NPK ratio’. I didn’t know what an earth this formula was going on about when I first started gardening, but it really just means you need to give plants the right balance of nitrogen, phosphorus and potassium for their growing needs.

For example, tomatoes benefit from a potassium boost to help them fruit, so you might see a tomato fertilizer with an NPK ratio of 4-3-8. This means it contains 4% nitrogen, 3% phosphorus and 8% potassium. Leafy greens, on the other hand, love nitrogen rich soil to encourage lush leaf growth.

NPK ratio for indoor plants

Always check your individual plant’s specific needs when adding an indoor plant fertilizer, but as a rule of thumb indoor plants usually require higher levels of nitrogen and potassium than phosphorus. So an NPK ratio of 4-1-3 or 6-1-4 might be about right for many indoor plants. However, keep in mind there’s never a one size fits all rule, as indoor succulents, for example, tend to thrive on a more balanced ratio.

Often, a commercially bought indoor houseplant fertilizer will do the NPK ratio work for you and say something like ‘suitable for all houseplants’ on the label. Saying that, it can help to know your indoor plants, so you can fertilise them in line with their specific needs.

Types Of Indoor Plant Fertilizer

Fertilizers are available in a number of different forms – those most suited to indoor plants are generally in the form of liquids, granules or spikes.

Liquid fertilizers are added to the water when watering plants, granules are mixed into the soil, where they release nutrients for root uptake & fertilizer spikes are inserted gently into the soil, where they release nutrients more gradually.

Benefits of Organic Indoor Plant Fertilizers

Using an organic fertilizer is a gentle & safe way to feed your indoor plants, helping to:

  • increase the soil’s organic matter
  • improve the soil’s structure
  • avoid the risk of plant burning (because the fertilizer is less concentrated)
  • boost the soil’s nutrient & water-holding capacity.

Organic fertilizers are also renewable (inorganic fertilizers aren’t), making them more sustainable than artificial ones.

7 Natural & Organic Indoor Plant Fertilizers

Here are 5 organic & natural indoor plant fertilizers, including liquids, granule and capsule options.

1 – Neptune’s Harvest Organic Fish & Seaweed Fertilizer

Fertilizer Type: Liquid
Suitable For: Indoor & outdoor plants, potted houseplants

If you don’t mind the odd whiff of fish, Neptune’s Fish & Seaweed fertilizer is a popular choice with organic gardeners.

Suitable for indoor plants, Neptune is made from a blend of fish hydrolysate and seaweed, both of which are known to help build natural sugars in plants. Gentle & effective, it’s designed to help plants grow stronger, produce more flowers & last longer. It’s easy to apply by hand or sprayer & if you use it solely as a feed for indoor plants, it should make 64 gallons.

To use as an organic fertilizer specifically for indoor plants, add 1 tablespoon per gallon of water and water houseplants lightly, once a week. The NPK ratio is listed as 2-3-1 & if you’re worried where the ingredients are harvested from – Neptune’s website says their fertilizer is made with freshly caught fish & seaweed from the North Atlantic.

2 – Joyful Dirt Organic Indoor Plant Fertilizer

Fertilizer Type: Granules
Suitable For: Indoor houseplants

If you’d prefer an organic plant fertilizer specifically formulated for houseplants, Joyful Dirt is a popular choice. Containing mycorrhizae, kelp & bone meal to help houseplants develop strong, healthy root systems, it’s listed as suitable for organic use by the Organic Materials Review Institute (OMRI).

There’s two ways to use it – for slower release of nutrients, simply shake some organic based fertilizer onto the soil on either side of your houseplant & water as usual.

Alternatively, mix 1tbsp with half a gallon of water. You’ll likely find this method is better suited to feeding multiple plants at once, as it’s recommended you use the mixture immediately.

3 – MARPHYL Organic Liquid Fertilizer

Fertilizer Type: Liquid (vegan friendly)
Suitable For: Indoor & outdoor plants

MARPHYL Organic Liquid Fertilizer is derived from wild marine phytoplankton – nutrient rich microalgae found in the ocean. Phytoplankton are exceptionally nutritionally dense, due to their natural abundance of vitamins & minerals.

Naturally high in nitrogen, phosphorus & potassium, as well as trace amounts of calcium & magnesium, MARPHYLL organic plant fertilizer naturally boosts plant health by replenishing the soil as your plants grow. Suitable as an organic soil enhancer/fertilizer for indoor plants, you can use it as a water soluble soil drench, foliar spray or even with a hydroponics setup.

To use, stir in 1 part concentrated liquid to 20 parts water. Recommended use is fortnightly.

Marphyl soil enhancer is listed by OMRI as an organic product. It’s also non GMO & vegan friendly, making it a great option if you’d prefer to steer clear of fertilizers containing animal or fish products.

4 – Epsoma Organic Cactus Fertilizer

Fertilizer Type: Liquid
Suitable For: Indoor cacti & succulents

If you’re nurturing a host of cacti & succulent plants in your home, this organic liquid feed from Epsoma is specially formulated for their growing needs. Succulents are pretty tough plants, used to growing in arid areas low in nutrients. However, this doesn’t mean regular fertilizing can’t still do wonders for their growth.

Cacti & succulents do well at storing water & nutrients, so when feeding them, a lower, balanced dose of nutrients is generally advised. This Epsoma Organic fertilizer for cacti & succulents has a 1-2-2 NPK ratio & is easy to use – simply add half a cap of plant food to a quart of water & use around your plants every 2 to 4 weeks.

Derived from natural ingredients including, hydrolyzed poultry manure, soy & fish protein, bone meal & kelp, Epsoma organic fertilizers also include options specifically for other indoor plants, as well as orchids.

5 – Blackstrap Molasses

Did you know you can use blackstrap molasses as a fertiliser to boost plants? I often add a tablespoon of molasses to my watering can to give plants a boost, especially hungry plants like tomatoes. However, you can just as easily use it on indoor plants.

Naturally high in nutrients including calcium, magnesium, iron, and potassium, as well as sulfur & other micronutrients, blackstrap molasses provides plants with a boost of energy that’s easy to absorb. The natural sugars also encourage the growth of beneficial microorganisms in the soil. It’s also thought molasses may deter pests like aphids, as well as some bad fungi.

If you plan to use molasses, make sure you use unsulphured blackstrap molasses. It’s more nutrient dense. Also opt for organic if you can get it – it’s generally not that expensive. To use blackstrap molasses as an organic fertilizer, add about a quarter of a teaspoon of molasses to 1 litre of water. Then use on your hoiseplants as a drench or foliar spray.

6 – Household Waste Fertilizers

If you’d prefer not to use a commercial indoor plant fertilizer at all, there are also a number of household waste items you can use.

Crushed egg shells help lower acidity & add calcium, used coffee grounds add nitrogen & banana peel boosts potassium. Not only are these options completely natural, they also won’t cost you a penny.

You can also use green tea for a boost of nitrogen. Organic products are obviously ideal, of course, as they’re far less likely to contain chemical residues. Here’s a guide on how to use these items as indoor plant fertilizers.

7 – Homemade Compost Tea Fertilizer

I regularly use comfrey or nettle tea on my plot, but they’re super smelly, so not many of us are likely to want to bring a stink like that into the house! So an alternative is to make an organic compost tea fertiliser, which, made correctly, shouldn’t smell bad and can be used on houseplants, so long as you dilute it.

Compost tea is a liquid, fast release, fertilizer made by extracting beneficial microbes from compost by steeping (or brewing) it in water. Whilst it’s almost impossible to gauge a true NPK ratio for homemade compost tea, many gardeners swear by it for giving plants a nutritional boost.

Here’s how to make it:

  1. Fill a bucket with tap water and leave to rest overnight to allow any chlorine to evaporate out (chlorine can negatively affect the tea). Alternatively, use rainwater, as this won’t need to rest.
  2. Using a ratio of 1 part compost to 5 parts water, add organic compost to the water and stir really well for about 2 minutes. Compost tea needs aeration, so this step is important. After that, continue to give the compost tea a good stir, once, or twice, a day, for about 5 – 7 days. Then it’s ready to use.
    Keep in mind, compost tea should not smell bad – just have a natural ‘earthy’ type smell. If it stinks in a bad way, bin it, as it’s not been aerated sufficiently.
  3. Dilute the tea to around 1 part tea and 10 parts water. This will dilute the nutrients to a level where they can be safely absorbed by your houseplants. Once diluted, you can apply your compost tea fertilizer directly to the soil of your indoor plants, or as a foliar spray (strain it first if using as a foliar).

Once it’s ready to use, I find it’s safe to apply compost tea to indoor plants every week, or so, during the growing season. As a liquid, fast release fertiliser, plants will take up the nutrients immediately. You can stick to once a fortnight, though, if you prefer. Or simply keep an eye on how your plants are looking.

Make sure you use your compost tea within a day or two, or the beneficial microbes will start to die off.

Keep in mind as well, the strength of the tea will depend on how long it’s brewed for, as well as the quality of compost. So you may wish to play around with dilution rates, to find a ratio that best suits your plants. As an organic indoor plant fertilizer though, it’s very effective as a cheap alternative to some of the more expensive organic choices on the market.

When To Fertilise Indoor Plants

It’s best to fertilise indoor plants when they’re actively growing, as this is when they’ll benefit most from a boost in nutrients. This is generally early spring to late summer. Frequency depends on the type of fertilizer & indoor plant, but typically varies from around every 2 weeks to every 3-4 months.

However, still always fertilise houseplants with caution & follow the instructions on commercially sold fertilisers – organic or otherwise. Used incorrectly, indoor plant fertilizers can still harm your plants.

How To Use Indoor Plant Fertilizers

It’s always best to use a fertilizer in line with the instructions if it’s shop bought, but here are a few tips on how to apply an indoor plant fertilizer to help you get best results:

  • Don’t overfeed plants – this can cause over-enthusiastic growth that may attract aphids. No one wants these pesky things in their house (unless you’re a ladybird)
  • Don’t use fertilizers at a higher concentration than recommended – it could burn or damage your plant
  • Give houseplants a drink before applying liquid feeds to avoid burning their roots.

Organic Indoor Plant Fertilizers – Conclusion

For me, organic & natural indoor plant fertilizers are definitely the eco friendly choice over inorganic, artificially produced ones. There’s a lot of evidence they help build better soil structure too.

Always follow the instructions on a commercially bought fertilizer & try to get to know your houseplants, so you get a feel for what works for them. No one will ever know your houseplants like you do!

Online reviews can also help you gauge how well a fertilizer performs on different types of indoor plants.

Or why not go completely natural and make your own organic indoor plant fertilizer with compost, coffee grounds, banana skins or crushed eggs shells. I find egg shells around a plant can also help deter the pesky aphid..

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