- The three numbers on fertilizer are the N-P-K ratio, which stands for nitrogen, phosphorus, and potassium.
- These three nutrients have separate functions and are essential to a plant’s growth and health.
- Fertilizers can be organic or synthetic, and come in a dry or liquid form.
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As most plant parents know, feeding your plants is an essential part of their care routine. Plant fertilizers are meant to increase a plant’s fertility and encourage growth. Typically they’re either organic (plant- or animal-based) or inorganic (man-made chemicals), and some are water-soluble (dissolvable in water) while others are dry (granular).
Choosing the right fertilizer can seem confusing, especially when it comes to deciphering the numbers on the label and knowing which type of fertilizer will work best with different types of plants. Erin Marino, editorial lead and plant expert at The Sill, shares her expertise on plant fertilizer so you can feel confident and knowledgeable the next time your plants require feeding.
What does each number mean?
Plant fertilizers depict three numbers on the label indicating the ratio of each nutrient, or N-P-K ratio. The first number is for nitrogen (N), which increases leaf production. The second is for phosphorus (P), which promotes fruiting and flowering, and the third is for potassium (K), which promotes hardiness and is essential to the metabolic process. These three elements are critical in supporting any plant’s health.
“You can have an all-balanced fertilizer, for example you’d see 10-10-10 for houseplants, or choose 15-30-15 depending on what you are trying to facilitate for your plant,” says Marino.
Each number represents the weight percentage of each nutrient in the package; for example, if a plant fertilizer contains a 10-15-10 ratio, this means it has 10 percent nitrogen, 15 percent phosphorus, and 10 percent potassium. The remaining 65 percent is a combination of other micronutrients such as iron,
, boron, and copper.
A higher concentration of any of the three macronutrients will impact a plant’s need correlated to the macronutrient’s function. In other words, high nitrogen will cause greater leafy growth, high phosphorus will cause more fruiting or blooming, and high potassium will cause greater hardiness.
“Nitrogen, phosphorus, and potassium are the primary macronutrients your plant needs,” says Marino. “Fertilizers with higher ratios are more concentrated than those with lower ratios, so if you see high numbers, you’ll want to dilute with more water before fertilizing your plants.”
Which fertilizer numbers are right for your plants?
While balanced, all-purpose fertilizers with a 10-10-10 ratio or 5-5-5 ratio are best for most houseplants, narrowing down the needs of the plant will allow you to choose a fertilizer with the number that best fits those needs.
“If you’re brand new to using fertilizer, it’s best to search for one that fits your specific needs,” says Marino. “The ratio is determined by the purpose, so searching with the purpose first will help you find the right ratio.”
If the plant is green and leafy, higher nitrogen is best. If your plant is a flowering or fruiting plant, higher phosphorus is best. If you are growing vegetables, higher potassium is best.
If you don’t already know that flowering plants like anthurium and bromeliads will benefit from a fertilizer with higher phosphorus, or leafy plants like fiddle leaf figs and monsteras will benefit from a fertilizer with higher nitrogen, consulting your local nursery or a simple internet search on your plant’s fertilizer needs can shed some light.
“All three nutrients are essential, but some plants can benefit from more of one than another,” says Marino. “For example, the popular houseplant pothos needs all three nutrients but will benefit more from a higher nitrogen ratio, since it’s a fast-growing trailing plant that produces many leaves than it would from a fertilizer with a higher phosphorus ratio since it doesn’t bear fruit or flowers.”
Once you’ve understood the meaning behind the numbers on plant fertilizer, the next step is to differentiate between organic versus inorganic, and dry (granular) versus wet (liquid).
Wet vs. dry
While Marino says that choosing between wet and dry is more of a preference, there are pros and cons to both.
“Granular fertilizer tends to be used outside more and is generally cheaper, so it can be used for large crops, and has a better shelf life,” says Marino. “For indoor plants or small container plants outside, I recommend using a liquid fertilizer which allows you to dilute the fertilizer with water (over-fertilizing can kill a plant) and can get down to where the roots are.”
Most water-soluble plant fertilizers have instructions on the label on how to dilute the fertilizer, but as a rule of thumb half to quarter strength dilution is recommended, which is about half a teaspoon per gallon of water.
Organic vs. inorganic
Choosing between organic and inorganic, or synthetic, fertilizers can also be preferential. But it can also depend on your plants’ nutrient and growth needs since organic fertilizers work in a slow and natural way, while synthetic fertilizers work rapidly.
“Synthetic fertilizers tend to be more concentrated, so if you purchase in liquid form and dilute it, it can be more cost-effective. You can use less and get the same amount of nutrients,” says Marino. “Organic fertilizers are made from natural ingredients so they tend to be less concentrated and take longer to break down once applied. This helps avoid over-fertilizing and provides steady nutrients over a longer period of time.”
When to fertilize
The great thing about most fertilizers — whether they are organic, synthetic, wet, or dry — is they have instructions on the back label on how and how often to fertilize. For houseplants, the rule of thumb is to fertilize every four to six weeks and about every two to three weeks during their growing seasons in the spring and summer.
For gardens with edible crops, granular or dry fertilizer should be mixed in with the soil before planting in the spring. For gardens with perennial flowers, fertilize before the spring growing season and do not fertilize during a frost.
Over-fertilizing plants is usually worse than choosing the wrong numbers since over-fertilization can cause the plants to burn and leave them weak. If your plants’ requirements are different than what is indicated on the fertilizer numbers, plants will usually show signs.
For example, choosing a fertilizer with high phosphorus even though the plant doesn’t flower will not produce green leafy growth, thus proving the necessity for a high nitrogen fertilizer which does increase leafy growth. According to Marino, knowing your plant is the best way to determine which fertilizer numbers need to be prioritized.
Choosing the right fertilizer no longer has to be an intimidating process with the right knowledge at hand. Understanding the N-P-K ratio, the plant’s needs, and the pros and cons of different types of fertilizers can empower you to make the right decision and help feed your plants to keep them healthy throughout any season.