What Do Fertilizer Numbers Mean?

Fertilizer Numbers

Fertilizer is a vital part of houseplant care for many plant varieties. Feeding plants helps them grow new foliage and possibly even bloom. Applying the proper fertilizer at the correct time can help your houseplants reach their full potential. This sounds easy enough, but how do you identify the appropriate fertilizer? And what do those fertilizer numbers mean? Here’s what you need to know about how to fertilize your houseplants.

Fertilizer Numbers

Why Do Houseplants Need Fertilizer?

Plants in nature have plenty of opportunities to receive natural fertilizer. As nearby vegetation and leaves, branches, and flowers from the plant itself die, that organic material breaks down. It takes some time, but as organic material biodegrades, it feeds the surrounding plants. Essentially, plants in nature are routinely top-dressed with compost, which supports new growth.

Houseplants are not so lucky. Even if a houseplant drops a leaf or two, it will likely be removed before it can break down, and a couple of leaves are not enough to support continued growth. This is why fertilizing is vital because feeding your plant provides the nutrients the plant would receive in nature but are not readily available in a home.

Know Your Plant  

Plants have different nutritional needs. Some houseplants like fiddle leaf fig and Christmas cactus are known as heavy feeders. These are plants that require frequent feedings. Although, for what it’s worth most plants should only be fed while actively growing. Even if a plant is a heavy feeder, it may only need fertilizer for a limited time.

Other plants do not require any fertilizer. A single application may help them look their best, but regular fertilizing is unnecessary. Succulents, like aloe, are houseplants that prefer minimal, if any, feedings.

Know what your plant needs and fertilize accordingly. The type of fertilizer used also matters.

NPK Fertilizer

Fertilizer mixes available for purchase are made from three main components: nitrogen, phosphorus, and potassium (sometimes potassium is referred to as potash). The letters NPK represent each element’s symbol on the periodic table. 


Nitrogen (periodic symbol: N) is naturally found in chlorophyll and helps support foliage growth.  Fertilizers high in nitrogen are excellent for leaf growth. Use fertilizers with increased nitrogen for leafy plants that do not bloom. Plants that receive a lot of nitrogen will set stunning greenery, but the plant will not have any energy left for flowers.


Phosphorus (periodic symbol: P) helps plants grow a robust root system. This element also aids in flowers and fruit, although fruit is more relevant for garden and landscape plants. Fertilizers high in phosphorus are ideal for houseplants that set flowers.


Potassium (periodic symbol: K) helps plants conduct photosynthesis, which is the process of turning sunlight into energy. That energy is necessary for the plant to grow. Potassium helps plants grow big and strong.

Breaking Down Fertilizer Numbers

Fertilizer numbers always appear in the order of nitrogen, phosphorus, and potassium. The numbers represent the percentage by weight of each element in the fertilizer mix. When a fertilizer has numbers like 15-30-15, that mix has 15% nitrogen, 30% phosphorus, and 15% potassium. Those fertilizer numbers total 60%; in this example, the remaining 40% of the fertilizer comprises secondary elements and carrier products.

Carrier products are elements in the mix that allow the fertilizer to function and work as needed. Including carrier elements enables the fertilizer to be shelf-stable, have the proper texture or granular size, and have functionality like a slow release. 

Organic Fertilizer Numbers

Organic products often have low fertilizer numbers. For example, you may see a product labeled as 2-3-2. Fertilizer numbers are based on how much of the product is available right away. Low numbers are not a bad thing — it just means that the fertilizer needs time to interact with soil microbes to deliver the full benefits. Organic fertilizer is effective and often has a long-lasting impact.

Homemade Fertilizer

Store-bought fertilizer is a quick and easy way to support your houseplants. An NPK plant food is beneficial if you want to provide a particular element. For instance, if you want to encourage foliage or give your plant a balanced diet. Following fertilizer numbers makes this easy. There are plenty of homemade houseplant fertilizers, like coffee grounds and banana peels. DIY houseplant fertilizers may provide limited benefits. Banana peels are a great source of phosphorus and potassium and are great for flowering plants. Banana peels do not contain nitrogen, so they are not as helpful for leafy plants. 

This isn’t to say you should avoid DIY fertilizers — they’re great and beneficial. Know what you are using, and make sure your plant gets the nutritional care it needs.

Making Sense of NPK Fertilizer Numbers

Fertilizer ratios help you give your plants the specific care they need. The right fertilizer makes all the difference if you want a lush and leafy prayer plant or a flowering anthurium. When in doubt, select a balanced fertilizer with numbers like 10-10-10 because that will provide complete nutrition. Your plants will benefit from each element, and you’ll have thriving houseplants to show for it.