If you’ve ever wanted to pull your hair out standing in front of fertilizer on a store shelf, I get it because understanding fertilizer numbers and labels can be completely confusing, so let’s unscramble them!
Join me as I unravel what the fertilizer numbers mean, how they apply to flower and vegetable gardening, and how to decide which fertilizer is best for you.
Elements of Fertilizer & How Soil Affects Fertilizer Uptake
Plant metabolism is a very complex process requiring multiple elements and conditions for optimal plant functioning in growth, flower production, and the ability to replicate future generations.
Plants need roughly 60 elements to grow and 17 of them are considered essential. When these processes go out of sync, there is trouble for the plant, either in how it grows in its ability to reproduce.
All fertilizer is composed of 3 basic things: Macronutrients, Micronutrients, and Carrier Substances.
Nutrient availability to plants also depends on soil structure (or texture), soil pH, and the quantity of organic matter in the soil.
One of the keys to a great soil is how many air spaces it has. Air spaces allow room to hold water and fertilizer.
Sandy soils tend to lose nutrient or fertilizer content easily as water flows through it rather effortlessly.
Soils with high organic content, like loamy soils, hold onto their nutrients well. Organic matter, like composted bark behaves like a sponge, collecting and holding onto fertilizer and water yet draining it as needed.
Soil with a higher clay content also holds onto their nutrients well, but they don’t tend to drain well.
Soil pH is how acidic or alkaline a soil is. pH has an effect on how the soil reacts to a fertilizer chemically and how that makes nutrients more or less available to a plant. Soils that are more neutral, between 5.5 – 7.0 work the best for most plants.
What is a Fertilizer Macronutrient?
When looking at the word ‘macronutrient’, think of the word ‘macro’ or ‘big’. Fertilizer macronutrients are components that plants use the most.
Macronutrients are divided into primary and secondary elements.
Primary Macronutrients
The three primary plant nutrients are:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
These nutrients are represented in big letters on a fertilizer bag and in this order: N-P-K.
Plants use large quantities of these nutrients to support their growth. These nutrients contribute heavily to the role of plant enzymes, biochemical processes, and the health of plant cells.
Secondary Macronutrients
Secondary macronutrients are essential nutrients like the three above, but are needed in smaller quantities.
Secondary macronutrients in fertilizer are:
- Calcium (Ca)
- Magnesium (Mg)
- Sulphur (S)
What is a Fertilizer Micronutrient?
Plant micronutrients are components that are needed by plants in very small amounts but are still vital to plant development and growth. When a soil nutrients are tested, these micronutrients are found in very trace amounts.
Micronutrients aren’t usually a problem in home gardening, they are more of an interest in larger horticultural food production projects. Some micronutrients are:
- Boron (B)
- Chlorine (Cl)
- Copper (Cu)
- Iron (Fe)
- Nickel (Ni).
The Roles of the Three Big Fertilizer Numbers N-P-K: ‘Up, Down, and All Around’
The three big numbers that you always see on a fertilizer label stand for the percentage of macronutrients that are in the bag or bottle of fertilizer.
The first number is Nitrogen (N), the second number is Phosphorus (P), and the third number is Potassium (K). Let’s talk about those now:
N- Nitrogen’s Role in Fertilizer
When it comes to Nitrogen, think ‘UP’. Nitrogen helps plant growth UP above the soil.
Plant growth above the soil is leafy green growth and stem growth. Nitrogen promotes healthy green growth as it influences chlorophyll development.
Healthy leaves look dark green in colour. Pale or yellow leaves generally means the plant needs more nitrogen, unless the leaf colour is naturally chartreuse.
Nitrogen is considered to be a ‘mobile’ nutrient, meaning that it moves with water.
Give plants nitrogen when you want to green up plants after transplanting or after perennials have emerged in spring.
Most perennials only need one or two applications per season and most annuals need consistent weekly fertilizer. If you are looking for a fertilizer for deep green grass, you would look for one that has a high first number, like 25-0-4.
P – Phosphorous’ Role in Fertilizer
When thinking about Phosphorous, think ‘DOWN’. Phosphorus contributes to plant growth BELOW the soil.
Phosphorus plays an important role in healthy root development and bud and flower development.
A formulation for root development just after planting would be a 10-52-10. A formula for regularly feeding annuals all summer would be 15-30-15.
K – Potassium’s Role in Fertilizer
Potassium’s role is ‘ALL AROUND’. Potassium contributes to the overall health of a plant.
Potassium protects against infections, disease, helps in the ability to tolerate drought and cold, it improves the strength of cell wall structures and maintenance.
Potassium improves the development of flowers, fruits, and vegetables and enriches flavour. Fruit and vegetable fertilizers have lower macronutrient numbers than flower fertilizers. They look like 7-4-5 or 2-7-4.
Fertilizer Math: Why the Numbers Don’t Add Up
Each number on the fertilizer stands for a percentage of the fertilizer by weight regardless if the fertilizer is dry in a bag or liquid in a bottle.
So you have a fertilizer that is 15-30-15, the components of the fertilizer are 15% Nitrogen + 30% Phosphorous + 15% Potassium = 60%.
What happened to the rest?
The other 40% consists of micronutrients and the carrier substances. You won’t find a fertilizer entirely made up of N-P-K, as plants still need micronutrients, and the fertilizer elements need something to bind to, called carrier substances ,that allow them to dissolve in water or dissipate in soil.
How to Decide What Fertilizer is Best for You
What fertilizer is best for you depends on what plants you are growing. Annual flowers, perennials, and vegetables all have different nutritional needs.
Below is a brief guide to common fertilizers which work well for each. In all cases, read and follow package directions carefully.
Fertilizing Annual Flowers
Annuals have a high demand for regular weekly or biweekly fertilizer applications because they bloom all summer and have high metabolisms.
- After transplanting, you may apply a root boosting formula of 10-52-20 for a few weeks as they establish their roots.
- For growing all summer, a balanced fertilizer with a formulation like of 20-20-2, 15-30-15, and my favourite fertilizer, Nature’s Source, with a little larger boost of Nitrogen ratio 10-4-3 works well.
- I prefer to fertilize annuals weekly, unless it’s been a very cloudy, cold week.
Fertilizing Perennials
Perennials have a low fertilizer requirements. As a matter of fact, too much fertilizer can cause them to either not bloom or stop blooming. Be conservative in applying fertilizer to perennials.
- A slow release fertilizer like 10-10-10 can be broadcast over a perennial bed every year or so.
- You could give perennials a shot of a liquid fertilizer of 20-20-20 as they emerge in the spring.
Fertilizing Vegetables
The vegetable fertilizing category is quite a broad topic, enough for a blog on its own. I encourage you to research the vegetables individually. A few guidelines for vegetable garden needs are:
- Corn has a high requirement for Nitrogen
- Tomatoes are heavy feeders and should be fertilized weekly with a fertilizer of your choice, natural or organic.
- A granular, slow release fertilizer like 4-4-4 or 10-10-10 applied in the spring as you prepare your garden covers most general vegetable needs.
Summing Up Fertilizer
A well fed plant is a happy plant because it’s nutritional needs have been met. Plants that have enough macronutrients and micronutrients perform their metabolic tasks and function well.
A basic understanding of what the macronutrients on a fertilizer label stand for could be the difference between a head full of hair or not, lol. Enjoy your garden!
More to Explore
- The Ultimate Vegetable Companion Planting Guide from A-Z [+Chart]
- The Best Vegetable Companion Planting Chart for Alberta and the Canadian Prairies
- Complete Guide to Creating Show-Stopping Annual Flower Containers
- Eight Tips for Transplanting Plants in Your Garden
- When and How to Plant a Tree
©Sharon Wallish Murphy, Gardening with Sharon
