Do You Need Fertilizer To Grow Plants (3 Things To Know)


Maybe you want to grow plants without synthetic fertilizers, or maybe you want to save a little money by going without.  Either way, it is natural to wonder if you really need fertilizer for your garden.

So, do you need fertilizer to grow plants?  You may not need fertilizer to grow plants if your soil has all of the necessary nutrients for growth.  Compost and aged manure will help to restore nutrients to soil while also adding organic material.  If your plants show signs of nutrient deficiencies, get a soil test to determine nutrient levels and soil pH before adding any fertilizers.

Of course, nutrient deficiencies are more likely if you plant the same thing in the same spot every year (crop rotation can help to avoid this problem).

In this article, we’ll talk about how to tell if your plants need fertilizer, what to use, and when to use it.  We’ll also look at what to do and what to avoid when fertilizing to keep your plants healthy.

Let’s get going.

Do You Need Fertilizer To Grow Plants?

You do not need fertilizer to grow plants – at least, not in all cases.  Sometimes, your garden soil will have all the nutrients that plants need to grow.

ammonium nitrate
You might be able to grow plants without fertilizer if your soil has the necessary nutrients. Get it tested before fertilizing!

In other cases, some nutrients in your soil will become depleted over time.  This will happen after years of planting in soil without replacing nutrients with compost, aged manure, or fertilizer.

Sometimes, your soil has nutrients, but plants grow poorly due to wet or dry soil.  In that case, fertilizer will not help.

Do Plants Grow Better With Fertilizer Or Without?

Plants will grow better with fertilizer – as long as you don’t overdo it!  If you use fertilizer when it is not needed, there is a chance that you will burn your plants (due to salts).

However, in the right situation, fertilizer provides extra nutrients that might be lacking in soil.  These extra nutrients offer important benefits for plants, including:

  • More extensive root system
  • Thicker stems
  • Larger leaves
  • Higher bud and flower formation
  • Increased fruit production
  • Better disease resistance

In light of these benefits, you shouldn’t be afraid of fertilizing your plants.  You just need to know when to do it (and how).

ripe tomatoes on vine
A proper dose of fertilizer can give plants thicker stems, larger leaves, and more fruit production.

How To Tell If Plants Need Fertilizer

There are two basic ways to tell if plants need fertilizer:

  • Do a soil test
  • Watch for symptoms of nutrient deficiency

The first option is better, since you can prevent slow growth and a poor harvest if you take care of the soil.

How To Do A Soil Test

There are two ways you can do a soil test:

  • DIY soil test
  • Lab soil test

We’ll look briefly at how to do each type of test.

DIY Soil Test

You can do a soil test yourself to see if you need to give your plants fertilizer.  The most basic DIY soil test kits will tell you the levels of nitrogen, phosphorus, and potassium in the soil.

soil test kit
A soil test will tell you the pH and nutrient levels in your garden.

You can find DIY soil test kits online or at garden centers.

One option is to buy a digital soil tester, which uses a metal probe to test nutrient levels in soil.

For example, this digital 3-way soil analyzer from A.M. Leonard measures temperatures, soil pH, and nutrients levels.

Digital soil testers are reusable many times, but they are not the only option.  You can also use a less tech-heavy method.

For example, here is an NPK soil test kit from Gardener’s Supply Company (they recommend using distilled water for testing, so you might want to buy some while you are at it).

You can learn more about NPK (nitrogen, phosphorus, and potassium) and what it means in my article here.

You can learn more about what a soil test tells you in my article here.

Maybe you don’t trust the results from a DIY soil test kit, or maybe you want more accurate results.  In that case, consider sending a soil sample to a lab for testing.

Lab Soil Test

Another option for soil testing is to send a sample to a lab.  They will run tests and tell you all about the soil, including:

  • Soil pH (acidic, neutral, or alkaline)
  • Nutrients (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, etc.)

The benefit of a lab soil test is that the results will probably be more accurate than what you would get with a DIY test kit.  The downside is that there is a fee, and you may have to wait a while to get your results.

Generally, you want to take a sample of soil from wherever you are going to plant your crops.  It is best to take a sample when the soil is dry.

soil
Take a dry soil sample to send to a lab for testing.

If you can do it in the spring, you might have enough time to fertilize (if necessary) before planting.

Most local agricultural extension offices (found at state universities) offer soil testing services for a fee.  Check out the USDA website to search for agricultural offices by state.

If you send information about what you are planting, the lab can give you detailed recommendations for fertilization.

You can learn more about how to do a soil test in my article here.

Symptoms Of Nutrient Deficiency In Plants

You might be able to tell that your plants need fertilizer just by looking at them.  Sometimes, nutrient deficiencies will show up early in the season so you can treat the problem.

Some of the most common symptoms of nutrient deficiencies in plants include:

  • Discolored Leaves (including light green, yellow, red, or purple leaves)
  • Interveinal chlorosis (yellow leaves between green veins)
interveinal chlorosis
This leaf is suffering from interveinal chlorosis, caused by a nutrient deficiency.

It can be difficult to determine which nutrient is deficient without a soil test.  However, you can narrow it down by looking at which leaves become discolored first:

  • Old leaves (bottom leaves) discolored first – deficiency of a mobile nutrient (such as nitrogen, phosphorus, potassium, and magnesium).
  • New leaves (top leaves) discolored first – deficiency of an immobile nutrient (such as calcium, sulfur, iron, and zinc).

Remember that mobile nutrients are ones that plants can move easily through their tissues.  Immobile nutrients are ones that plants cannot easily move through their tissues.

When a plant has a mobile nutrient deficiency (such as nitrogen), it moves the nutrient from the bottom leaves to the top leaves.  The growth of top leaves is prioritized, since they will get more sun than the lower leaves.

You can learn about why nutrient deficiencies occur in my article here.

You can find a table with symptoms of various nutrient deficiencies in this article from the University of Missouri.

When To Fertilize Plants

If you find that your soil has a nutrient deficiency, then you might need to add some fertilizer.  However, it is best to make sure that you are doing it at the right time and with the right frequency.

Otherwise, you will be wasting time, money, and nutrients for no reason.  You may also harm the environment, so be careful not to overdo it with fertilizer.

How Often Do You Fertilize Plants?

There are two important times to fertilize plants: at transplant and if you discover a nutrient deficiency during the growing season.  You may also want to switch fertilizer types during the season to encourage flowers and fruit.

The Michigan State University Extension suggests that perennials only need to be fertilized in spring and possibly after flowering.

You should fertilize plants for the first time when you transplant them (for tomatoes, peppers, and other plants that you start indoors).  Mix some fertilizer into the soil where the transplants will go, and water them in after you transplant them.

tomato plant 3-5-2021
If your soil needs fertilizer, add it when you transplant.

If you discover a nutrient deficiency during the season, find a fertilizer that will supplement that particular nutrient.  For example:

The best way to deliver nutrients to plants without burning them is to use a light side dressing.  Side dressing means putting fertilizer on the soil (or mixing it into the soil) near the plants.

The University of Missouri Extension has a table for suggested times for nitrogen side-dressings in this article.

Just don’t add nitrogen in high amounts too late in the season. According to the University of Minnesota Extension, adding nitrogen too late in the season will promote leafy green growth of plants, but may prevent flowers and fruit from forming.

To avoid this, use a low-nitrogen fertilizer for later in the season. You can learn about low-nitrogen fertilizers in my article here.

What Is The Best Time Of Day To Fertilize Plants?

The best time of day to fertilize plants is early in the day, when it is cool.  This is also a good time to water, so you can water in the fertilizer after applying it.

watering can
Fertilize plants early in the day, when it is cool, so that you can water in the fertilizer after application.

Avoid fertilizing plants when the soil is very dry, since this can cause fertilizer burn (more on this later).  Also, avoid fertilizing and watering late at night, since wet leaves might not have a chance to dry out overnight, increasing the chance of fungus on plants.

How Long Does Fertilizer Last In Soil?

Some fertilizers only last a couple of weeks in the soil.  Others can last for months.  It all depends on the type of fertilizer and the soil texture and composition.

According to the Oregon State University, some of the fastest release organic fertilizers last only 2 weeks.  These include:

  • Burned eggshells
  • Fish emulsion
  • Urea (found in manure)
eggshells
Some fertilizers, such as burned eggshells, only last 2 weeks in soil.

On the other hand, the University of Florida suggests that some slow-release fertilizers may last in soil for up 18 months (at which point they will have released all of their nutrients).

Organic fertilizers (such as compost and aged manure) can release nutrients into soil for months, making them a good natural slow-release fertilizer to pair with chemical fertilizers (if needed).

Can You Fertilize Plants Too Much? (Over Fertilizing Plants)

You can fertilize plants too much, and it can happen in several ways, including:

  • Adding too much fertilizer to soil at once (follow the package instructions to avoid this!)
  • Adding fertilizer when it is not needed (do a soil test to avoid this!)
  • Adding “hot” manure to plants (let the manure age before applying to avoid this!)
  • Fertilizing plants in dry soil (water plants before and after fertilizing to avoid this!)
manure
Adding manure that is too fresh to your garden can burn plants with salts.

When you fertilize plants too much, they will show symptoms of over fertilizing, including:

  • Wilted, dry (“crispy”), and/or discolored (yellow or brown) leaves
  • Slow growth
  • Root rot
fertilizer burn on leaf
If you over fertilize plants, you may see burned leaves with brown spots.

Fertilizer burn is more likely if you are using a fast-release fertilizer, since all of the nutrients will be available at once.

Fertilizing plants in dry soil is even worse.  The salts in fertilizer can prevent a plant from absorbing water or cause a loss of water due to osmosis (water moves from low salt concentration to high salt concentration).

Should I Water Plants Before Fertilizing?

It is a good idea to water plants before fertilizing to get the soil wet.  According to Clemson University, you should not fertilize plants in dry soil where water is unavailable, since plants need water to absorb nutrients.

water jugs
You can mix water-soluble fertilizer with water and then apply the solution.

Another option is to dilute a water-soluble fertilizer in water and then apply the water/fertilizer solution.  Follow the package instructions to find the proper ratio of water to fertilizer!

Should I Water Plants After Fertilizing?

Watering after applying fertilizer will make the nutrients available to plant roots (nutrients need to be dissolved in water so that plants can absorb them).  It will also reduce the chances of burning plants with the salts in fertilizer (especially fast-release fertilizers).

The Oregon State University Extension suggests fertilizing right before rain or watering the area for an hour after fertilizing.

Conclusion

Now you know about when you need fertilizer for your plants and how to find out what nutrients they are lacking.  You also know what to do and what to avoid when it comes to fertilizing plants.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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