Quick Guide: Fertilizer Basics for Indoors and Out

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A selection of fertilizers – which to choose?

Why do we need to fertilize?

Plants growing wild in nature will receive nutrients from the breaking down of organic material in the areas in which they grow. These materials disintegrate over time releasing nutrition back into the soil. When it rains, those components soak into the water and become available for plants to uptake. There are also nutrients and minerals deeper in the soil that plants will access as the roots extend downwards.

Plants don’t have access to organic material as readily in our home gardens where they grow in fairly controlled environments such as raised beds, containers and pots. For instance, it’s unlikely that gardeners would leave leaf matter to build up over the years or that wildlife would wander through leaving ‘deposits’ of organics. It may happen a little, but not enough. Nutrients found in bagged potting soil can deplete over time as plants use them and gardens in newly built developments may have poor quality topsoil. This is why we might need help from commercially available fertilizers.

What do the numbers mean?

You’ve probably noticed that fertilizer packaging always advertises 3 numbers on the front of the label (for example 20-20-20). These numbers represent the concentration of three main macronutrients. Nitrogen – Phosphorus – Potassium (or N-P-K). These three nutrients are the most essential to plant growth and survival, but it’s important to note that they are not the ONLY important nutrients. There are 13 micronutrients that plants rely on as well (more on that below). So… what does each macronutrient do?


Nitrogen is responsible for helping plants to grow foliage. Nitrogen is also a component of chlorophyll so plants cannot photosynthesize and grow without it. Plants low in nitrogen can develop yellowing leaves because they cannot produce the green colouration through their chlorophyll. Nitrogen availability in the soil can fluctuate depending on moisture, ph of the soil, temperature, and composition of the soil. Plants will take up nitrogen through their roots however some plants can absorb it from the air (legumes like peas are described as ‘nitrogen fixers’) It’s important to note that overuse of nitrogen can result in pollution. It’s also important to note that nitrogen application during the wrong growth period can be detrimental to some plants i.e. If they need to focus on root growth prior to a dormancy period.


Phosphorous is a vital part of plants and their tissue. Phosphorous helps plants to perform processes such as creating sugars and starches, transferring energy, creating blooms, roots and so on. Plants take up phosphorous through their roots and can be aided by mycorrhizal fungi. Gardens low in phosphorous may produce small leaves as plants struggle to process water and transfer nutrients. Low P can also result in a change of leaf colour (tomatoes may see purple tinted leaves.) Plants suffering from a short supply of phosphorous may be more prone to disease, produce less fruit and slow growth. It is important to note that overuse of phosphorous can result in pollution if the soil already has adequate amounts or if plants are unable to access it.


Potassium is incredibly important for the functions of a plants stomata (cells which aid in the movement of water, oxygen and carbon dioxide through the leaf surface) Potassium helps root systems to mature, starch and protein productions, and fortify plants against disease. While generally soils aren’t deficient in potassium it can be unavailable for plant uptake as it can be bonded to other minerals. Plants without adequate potassium may wilt or have low heat tolerance. They can have stunted growth and small leaves.

The 13 Micronutrients

What type of fertilizer should we choose?

There are several types of fertilizers available including liquids, slow release pellets, organic fertilizers and synthetic fertilizers. How do you know which type is best? And why do they all have different amounts of N-P-K?

All fertilizers need water for the nutrients to be available for plant uptake. Liquid applications will soak into the soils but can need re-application more often. Slow release pellets will become available over time as water dissolves them. Organic fertilizers will often have more variation in their micronutrients and synthetic chemical fertilizers can be easier to use and store. Which one you chose will depend on how you like to garden and grow your plants.

The ratio of N-P-K will depend more on which type of plant you’re trying to grow. Some plants enjoy a specific ratio which is why you may see fertilizers that are made for a specific type of plant. Fertilizer for grass will have a high nitrogen number because growing that green grass foliage is what we want to see while fertilizers for flowering plants may have a higher phosphorous number. This is a simple way to choose a fertilizer however, the composition of your soil is really what we need to know. There is no point to putting more nitrogen on soil that already contains enough. We can’t know what our soil is lacking unless we test it but that’s a subject for another day and this is a quick guide! If you’re growing plants in raised beds or pots and feel the soil has been depleted then choosing a fertilizer based on which type of plant you’re growing is a good starting point. If you’re growing on a larger scale directly in the ground a soil test might be very helpful.

What’s the difference between 5-5-5 and 10-10-10? The 10-10-10 is more concentrated but otherwise they are the same!

When should we fertilize?

The cheeky answer to the question of ‘when to fertilize our plants’ is ‘when they need it.’ We know, that isn’t very helpful but hear us out – as long as the soil has adequate nutrient, the time of year doesn’t really change whether or not we should feed our plants. If nutrient is available, plants will use what they need when they need it. That said, plants will utilize more nutrients when they are actively growing and less when they are dormant or receiving less light. If your plants are in a growth period, they will be using more nutrients so new growth is a good sign to provide some food. Plants like annual hanging baskets will need fertilizer more often as they burn through it quickly and are in a confined space. Plants in the ground will need it less often or perhaps not at all.

How do we apply fertilizer?

Follow the application directions on the fertilizer package. It is advisable to reduce the strength of a fertilizer for plants that might be sensitive to processing salts (like calathea or prayer plants) and to err on the side of caution. It’s easy to increase fertilizer slowly over time but more difficult to rehab a plant that is suffering from fertilizer burn. Some plants are heavier feeders (Monstera) than others (Cactus.) Researching the particular plant you wish to feed is advised.

When should we stop fertilizing?

Plants that are about to go into dormancy should not be encouraged to grow foliage as it can cause them to expend energy when they should be going to sleep. This is important to consider for trees and shrubs as we head into winter. Plants receiving inadequate light (like houseplants in winter) should not be fertilized as they may not have the light resources to produce healthy foliage – the result can be weak or lanky growth. If soil is new out of the bag or otherwise in good health, avoid fertilizing as it can be unnecessary and provide no benefit to the plant.

Bare soil can lose nutrients to erosion

What can we do to improve soil health?

Aside from using fertilizer, we can also try to improve the soil by adding organic material. This is something we can do over the course of the year for our outside gardens – it’s not so easy to do indoors. Using mulches like bark, leaves or straw will help your soil obtain fresh organic material. These materials break down over time and enter the soil helping to improve drainage, mycelium activity and increase nutrition. We can also add compost to our planting beds. You can make your own compost with waste from your garden and kitchen or you can purchase a commercially available product like Mushroom compost.

What else can we do to ensure our plants get nutrients?

The healthiest soils on the planet won’t help a plant if it cannot uptake nutrition through the root systems. This could happen when there isn’t adequate water for transporting the nutrients or if the root system is damaged. Damage can occur to roots by inconsistent watering. Dry or rotten roots will not be able to function. You may see evidence of poor root health that looks an awful lot like nutrient deficiency and this is why -the nutrients may be there but the roots can’t get at them. Tomato plants with blossom end rot are most likely suffering from poor root health vs a lack of potassium. Plants with yellowing leaves are more likely to be suffering from poor root health vs a lack of nitrogen. Just something to keep in mind and if you’re seeing trouble, try checking the roots.


The subject of fertilizer is a complex one and breaking it down into one easy to digest post wasn’t easy! We hope this gives you the basics so a visit to our fertilizer section won’t be so intimidating but please always ask your questions in store or comment on this post. We’re happy to help and provide more information on this interesting aspect of growing!

https://extension.umn.edu/manage-soil-nutrients/quick-guide-fertilizing-plants https://www.bhg.com/gardening/yard/garden-care/why-you-should-fertilize-plants/

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Heather Eigler
Heather Eigler

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