What Fertilizer Ratio Should You Use When?

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As a soil scientist, I am passionate about the idea of delivering specific nutrients at certain times. Knowing what fertilizer ratio you should use when in the garden is important for ensuring maximum fruit and vegetable harvests. This is even more true when it comes to people living in cold climates or gardening in Canada.

Choosing a fertilizer based on the plant’s stage of growth is a great way to save money & have better results. Certain nutrients are known to stimulate different types of growth. Tailoring your fertilizer based on the plant’s health is also a great way to help a plant through a bug attack. You can even use specific nutrient ratios to ensure better results when transplanting or trimming.

How To Read Fertilizer Labels

We have all seen the three letters N-P-K, which are usually translated into a number. For example, 5-7-4, which is the percentage of the product that has each value. This means the higher the values the less filler there is in the product. An example of a product with a higher percentage of the nutrient value is miracle gro all-purpose. This product rings in at 24-8-16, this means in every 10 lbs of products we will find, 2.4 lbs of Nitrogen, 0.8 lbs of Phosphorus and 1.6 lbs of Potassium. The remainder of the product is filler to help aid in solvency, slow-release, and stabilization in the solution.

What Fertilizer Ratio Is Best For Flowering Plants?

The best fertilizer ratio for flowering plants is 10-5-15 or 8-12-6. The biggest difference between each option is the phosphorus and potassium. If you are using any form of mycorrhizae addition for the soil you will want to aim for the lower levels of phosphorus (10-5-15). If you are not utilizing microbe inoculants with the plants then you can use the fertilizer with higher levels of phosphate.

Microbial inoculants form a symbiosis with the plant to help obtain more nutrients. In particular mycorrhizal, garden products are meant for solubilizing phosphorus. This symbiosis takes energy on the part of the plants. This means if the plant can access bioavailable phosphate it will not expend energy on forming a symbiosis with the fungi. The inoculant won’t go to waste but it is likely to sit and dormant until all the excess phosphate has been absorbed.

When Should I Use NPK 20-20-20 Fertilizer?

A 20-20-20 fertilizer or a 10-10-10 fertilizer is considered a balanced fertilizer. But the reality is that this balanced fertilizer does not always serve your garden. The nitrogen level in these is likely appropriate in most cases but the phosphate (especially in the 20-20-20 fertilizer) is higher than a plant’s season needs.

I mentioned above that phosphate can ultimately affect our soil microbiota and plant root symbiosis. The other reality with another application using a balanced fertilizer also means pollution. A balanced fertilizer can result in pollution of waterways it becomes a water-soluble compound.

In a balanced fertilizer, such as the 20-20-20 fertilizer, we find that the nitrogen is either washed away (leached) or gases off as NH4 (volatilized). However, with potassium (K) and phosphorus (P), we see something drastically different. Both these nutrients are NOT garden soil mobile. Meaning year after year they can build up.

So, you’re probably wondering. Ashley…

Is fertilizer build-up a good thing?

No, garden soil nutrients need to be in a range. There is a minimum and a maximum level to the ideal garden soil nutrient level. If we are too low (below the optimal range), the plant is unable to grab the nutrient because it is “locked up” in our garden soil cation exchange capacity. If the soil nutrient level is too high (above the optimal range), the nutrients begin to compete for “soil real estate”.

When Should I Use NPK 20-20-20 Fertilizer?
When Should I Use NPK 20-20-20 Fertilizer?

What Is The Best Ratio For Lawn Fertilizer

For lawn fertilizer, you want high nitrogen, low phosphorus & potassium. This means a 24-0-1 or for a poor rooting lawn 26-0-5. The goal here is always going to be greenery. Our root development should not be the focus when looking at an established lawn.

For a new lawn or a recent turf instalment, however, you will want higher levels of potassium. Aim for something with 2% – 5% potassium & lower nitrogen. Something like a 9-0-2 would work great for a new lawn application.

What Are The Best Number For Fertilizer?

This is a question that could not be answered, even if we wrote a novel. I provided a chart below of the best number for fertilizer depending on your situation and needs as a gardener. Always feel free to reach out if you have any confusion. With the best number for fertilizer chart, you can go plus or minus 5 on each value and still be in an ideal range.

Plant TypeSoil TextureRecommend FertilizerApproximate N-P-K Numbers
Leafy GreensClay – Clay LoamAll-Purpose20-5-10
Fruiting Vegetable GardenClay – Clay LoamBloom Formula20-5-10
Rooting Vegetable GardenClay – Clay LoamBloom Formula20-10-10
Onions, Leeks, Scallions, BrassicaClay – Clay LoamBloom Formula10-5-10-6
LawnClay – Clay LoamLawn Formula24-0-2
Leafy GreensSand – Sandy LoamAll-Purpose20-10-15
Fruiting Vegetable GardenSand – Sandy LoamBloom Formula20-5-15
Rooting Vegetable GardenSand – Sandy LoamBloom Formula10-5-15
Onions, Leeks, Scallions, BrassicaSand – Sandy LoamBloom Formula20-5-15-6
LawnSand – Sandy LoamAll-Purpose32-0-5
HouseplantsPotting SoilAll-Purpose + lime24-10-10
Leafy GreensPotting SoilBloom Formula + lime20-10-15
Fruiting Vegetable GardenPotting SoilBloom Formula + lime10-15-10
Rooting Vegetable GardenPotting SoilBloom Formula + lime10-15-10
Onions, Leeks, Scallions, BrassicaPotting SoilBloom Formula + lime10-15-10-6
What Are The Best Number For Fertilizer?

The Best NPK Ratio For Vegetable Garden

The best NPK values for your garden are going to depend on what you are growing and your soil texture. Check out the chart above to reference the suggested ranges according to your soil texture. For a more generic view of NPK values relative to vegetables, garden-type let’s explore.

NPK Ratio For Leafy Greens

The best NPK ratio for leafy greens is always going to be something with higher nitrogen. This value can range from 15 to 32 and look like a 15-5-5 or a 32-5-10. The more sand your soil contains the higher you want the nitrogen and potassium (within reason). With clay soils, we can generally get away with lower nitrogen values. The exception to this rule is aged clay soil, this can be identified by looking at the soil colour. Red clay soil = aged soil, to learn more check out the article on soil colour.

If you are growing leafy greens in containers or potting soil you can use a 20-5-10. The reason for this is the potting soil tends to allow some nutrient loss through the water. Similar to that of sandy soil but to a lower extent. To limit fertilizer losses through potting soil always “pre-water” and then apply your fertilizer water.

The goal here is always going to be more upper biomass. To achieve vegetative growth we always want to use higher levels of nitrogen. But keep in mind higher nitrogen also means quicker growth. This faster growth can also result in leggy or soft tissue. This tissue is much more susceptible to things such as aphids, thrips, etc. To help manage this lower your nitrogen values.NPK Ratio For Fruiting Garden Plants

NPK Ratio For Root Vegetables

NPK ratios for root vegetables should be lower in nitrogen overall with a higher focus on potassium. This is because our goal is to achieve a bigger root and less upper foliage.

NPK Ratio For Fruiting & Flowering Garden Plants

NPK ratios for fruiting & flowering will be an all-purpose during the vegetative stages of growth. The vegetative stage of fruiting and flowering garden plants lasts till approximately mid-late June in Canada & cold climates. Once the flowering stage begins we can add higher levels of phosphate and potassium again within reason.

NPK Ratio For Tomatoes & Peppers

The most common fertilizer question I get for tomatoes and peppers is: “what about calcium and magnesium”. I want you to focus on using the fruiting and flowering NPK ration regime with your tomatoes and peppers, once they are transplanted outside. Prior to the planting outdoors I want you to fertilizer with a calcium magnesium fertilizer. This should be provided to tomatoes and peppers when they are seedlings. The exception to this rule would be potting soil growers. You will want to either mix in lime or use a cal-mag all season.

NPK Ratio For Alliums Garlic, Leeks & Onions

The fourth number refers to Sulphur. N-P-K-S, this is VITAL to alliums and brassica species. This is what gives them their smell and flavour. If you have alkaline soil then feel free to add elemental to the Sulphur as a whole in hopes of lowering the pH of the soil. If your soil pH is acidic or neutral then add the Sulphur containing fertilizer only to the needed application areas. I like to do this when transplanting for the most part.

What Does Nitrogen Fertilizer Do

Nitrogen (N) is the fuel for vegetative growth above ground. Nitrogen is the key ingredient behind chlorophyll which we all know is essential to photosynthesis. Nitrogen in excess can result in rapid growth and thin leaves. These thin leaves can encourage pests and diseases to take root. That means more is not always a good thing. 

Nitrogen is applied as a fertilizer regularly throughout the growing season because nitrogen is easily lost from the soil. The main mechanisms by that nitrogen leave the soils are leaching and volatilization. These are both fancy words for saying through gravity and into the air respectively.

What Does Phosphorus Fertilizer Do

Phosphorus (P) is essential for roots and flowers. Due to this, it is important to increase the level of phosphorus when expecting flower development. This is not as important in the seed starting stages. Keep in mind this increase should be within reason and relative to your soil type. Excess phosphorus is not good for the soil’s microbiota or CEC.

What Does Potassium Fertilizer Do

Potassium (K) activates enzymes. These enzymes help the plant handle stress such as transplant shock, changes in temperature and soil moisture. This is the ideal supplement to give after transplanting outdoors. I also like to fertilize my seedlings with a bit of extra potassium if I know they are dramatic when transplanted (melons, vines & squash).

What Does Calcium Fertilizer Do

Calcium (Ca) is important in transporting information around plants. A great example of this is the Tel Aviv plant study where they measured increases in calcium in an injured area of the plant. Beyond that, it is important to the plant’s cell structure.

What Does Magnesium Fertilizer Do

Magnesium (Mg) is used in enzyme reactions within the plant which are essential to chlorophyll. Without Magnesium, photosynthesis can not occur.

What Does Sulphur Fertilizer Do

Sulphur (S) is the foundation of the plant’s immune system. It also gives brassica and onions their flavour and smell. If your garlic, onions or leeks tend to come across as flavourless give this a shot.

What Fertilizer Ratio Do You Use During the Seedling Stage?

When the plant is a seedling the main focus is on roots and shoots. Therefore you want to go with a fertilizer with higher nitrogen and phosphorus. Click here to check out my entire blog post on fertilizing seedlings.

This is a list of the fertilizers to use for seedlings:

Organic Fertilizer Options For Seedlings

BudJuice – Nitro

Gaia Green Blood Meal

Inorganic Fertilizer Options For Seedlings

Miracle-Gro All Purpose

What Fertilizer Ratio Do You Use After Transplanting?

If we have hardened off our plants properly then our main focus will be roots and treating stress. This means our main nutrients will be phosphorus, potassium and if possible sulphur.

This is a list of fertilizers to use after transplanting:

Organic Fertilizer Options For Seedlings

BudJuice 0-0-15

Inorganic Fertilizer Options For Seedlings

Promix Fertilizer

What Fertilizer Ratio Do You Use For Flowering Plants?

If you are trying to encourage a plant to flower or already have flowers emerging then you will want to supply the plant with higher levels of phosphorus. If you are having an issue with flowering it most likely is due to overapplying nitrogen to the soil.

This is a list of fertilizers to use with flowering plants:

Organic Fertilizer Options For Seedlings

BudJuice 2-14-0

Gaia Rock Phosphate

Inorganic Fertilizer Options For Seedlings

Miracle-Gro Bloom Formula

What Fertilizer Ratio Do You Use For Stressed Plants?

A stressed plant is any plant that has gone through a heatwave, a cold snap, transplanting, or from pest/disease pressure. This means higher levels of potassium and sulphur when possible.

This is a list of fertilizers to use on stressed plants:

Should you be using liquid or dry fertilizers? 

The quick answer to this is liquid, however dry has its time and place. For more info on the differences between the two check out my blog post here.

There you have an in-depth guide from a soil scientist going over exactly what fertilizer to use when. You are now prepared to give your plant the nutrient it needs when it needs them. Let me know in the comments below which fertilizers you enjoy and why. For more info on gardens, and science checks me out on YouTube.

What Fertilizer Ratio Should You Use When?
What Fertilizer Ratio Should You Use When?