What Is The Best Fertilizer For Root Growth? (3 Things To Know)


A strong root system is important for healthy plants, since they must absorb their nutrients from the soil.  However, certain nutrients and fertilizer mixes are better at promoting root growth.

So, what is the best fertilizer for root growth?  The best fertilizer for root growth is one that is balanced with enough phosphorus. A high-phosphorus fertilizer will promote vigorous root growth when applied over the entire root zone of a plant. Watering in fertilizer will allow a plant to absorb more of the nutrients for stronger root growth.

Of course, a balanced fertilizer that contains all three of the “big three” nutrients (NPK, or nitrogen, phosphorus, and potassium) will provide phosphorus for root growth.

In this article, we’ll talk about the best fertilizers for root growth and how to use them.  We’ll also take a look at some other ways to promote root growth and how to avoid poor root growth.

Let’s get started.

What Is The Best Fertilizer For Root Growth?

The best fertilizers for root growth are balanced and contain plenty of phosphorus.  This is true whether you are trying to promote root growth in potted plants, hydroponics, or in an ordinary garden plot.

root growth
The best fertilizers for root growth are balanced and contain plenty of phosphorus.

Phosphorus is one of the most-used nutrients by plants (along with nitrogen and potassium).  It helps to support strong root systems (among other things).

However, you don’t need to apply phosphorus (phosphate) alone.  You can use a balanced fertilizer that contains phosphorus.

For example, the University of Maryland Extension suggests using an NPK fertilizer that has a ratio of 3:1:1 (that is, 3 parts nitrogen, 1 part phosphorus, and 1 part potassium) for trees and shrubs.

(If you want to learn more about NPK labels on fertilizer packages, read my article here).

Fertilizers that promote root growth are sometimes called “root booster fertilizers.”  Some fertilizers that fit the 3:1:1 ratio include:

If in doubt, use fertilizers with lower NPK ratios to avoid burning your plants.  Better yet: before you use any fertilizer, get a soil test to find out which nutrients are lacking (if any).

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

soil test kit
A soil test will tell you what nutrients are lacking or if the pH is off.

Remember that phosphorus is not mobile in soil.  This means it cannot travel easily through the soil to reach plant roots.

As a result, phosphorus needs to come in close contact with plant roots so they can absorb it.  Water is also necessary for a plant’s roots to absorb phosphorus and other nutrients.

Fertilize just before it rains, or else you will need to water in the fertilizer after you apply it.  This will ensure that the roots can absorb the nutrients.

Also, keep in mind that soil pH also affects nutrient uptake by plant roots.  For phosphorus, a pH of 6.5 to 7.5 is ideal.

Outside of a pH range of 6.0 to 8.5, phosphorus availability drops off rapidly for plants (you can see this illustrated in the following graphic from the University of Massachusetts).

They also suggest that some plants (like evergreens with needles) respond well to nitrogen with higher amounts of phosphorus.  According to the University of Missouri Extension, tree and shrub growth is most often hindered by a lack of nitrogen.

snow on tree
Evergreen trees with needles respond well to nitrogen fertilizer when high amounts of phosphorus are present.

Watch for the symptoms of a lack of phosphorus in plants.  For example, stunted, purple-colored plants can indicate a phosphorus deficiency.

What Nutrient Encourages Root Growth?

As mentioned earlier, the nutrient phosphorus encourages root growth in plants.  There are lots of artificial fertilizers that will supply this important nutrient in varying amounts.

However, you can also find plenty of natural sources of phosphorus for your plants.  Here are some natural high-phosphorus fertilizers to encourage root growth in plants:

  • Hair – contains 26 percent phosphorus by weight
  • Enriched Rock Phosphate – contains 17 to 30 percent phosphorus by weight
  • Bone Meal – contains 12 to 16 percent phosphorus by weight
  • Burned Cucumber Skins – contains 11 percent phosphorus by weight
  • Bat Guano – 4 to 8.6 percent phosphorus by weight
  • Fish Meal – contains 4 to 6 percent phosphorus by weight
  • Cottonseed Meal – contains 2.5 to 3 percent phosphorus by weight
  • Worm Casting – contains 2.5 percent phosphorus by weight
  • Blood Meal – contains 1.5 percent phosphorus by weight
  • Manure – contains 0.2 to 4 percent phosphorus by weight
  • Compost – contains 0.5 to 1 percent phosphorus by weight
hair
Hair contains lots of phosphorus, but don’t use it for vegetable gardening if it was treated with chemicals.

You can learn more about these high-phosphorus fertilizers in my article here.

Does Bone Meal Promote Root Growth?

Bone meal can help to promote root growth, since it has high levels of phosphorus (12 to 16 percent by weight).  However, this assumes that the phosphorus is available to plants in the right amount.

Bone meal is a slow-release fertilizer, meaning that it releases phosphorus and other nutrients over a long time period (several months).

bone meal
Bone meal contains lots of phosphorus and is a slow-release fertilizer.
Image from: https://commons.wikimedia.org/wiki/File:%D0%93%D0%BE%D1%80%D1%81%D1%82%D1%8C_%D0%BA%D0%BE%D1%81%D1%82%D0%BD%D0%BE%D0%B9_%D0%BC%D1%83%D0%BA%D0%B8_2014-05-07_13-57.jpg

However, bone meal has a relatively high percentage of phosphorus by weight. Washington State University suggests that too much phosphorus (from bone meal or other sources) can slow the growth of mycorrhizal fungi, which can slow down a plant’s growth.

So, don’t apply too much bone meal in one place all at once.  Apply near the root zone of plants, and spread it out over a large area to allow more of the roots to absorb the nutrients.

How Else Can You Encourage Root Growth?

In addition to using balanced fertilizers with enough phosphorus, there are some other ways to encourage root growth, including:

  • Use a root trainer (this encourages roots to grow straight down, instead of circling around in a small seed starting cell).
  • Repot as needed (this prevents the plant from getting root bound in container that is too small for it).
  • Loosen & aerate soil (compacted & waterlogged soil slows root growth).
  • Water deeply but less frequently (this encourages a more extensive root system).
  • Add rooting hormone (often used for cuttings when propagating plants).
tomato plant roots
Rooting hormone, root trainers, and proper soil, watering, and aeration all help to promote healthy root growth.

Rooting hormone can come as a powder (such as this one) or a gel (such as Olivia’s Cloning Gel from Gardener’s Supply Company).

You can learn more about how to promote root growth in plants in my article here.

For trees and shrubs, apply fertilizer in early spring, when the roots are actively growing.  Be sure to spread the fertilizer over the entire root zone to improve nutrient uptake.

For trees, the root zone extends 1.5 times the distance from the trunk to the tips of the branches.  So, if the branches extend 10 feet from the trunk of the tree, then the root zone is 10*1.5 = 15 feet from the tree trunk (that is, a circle with a 15-foot radius and the tree at the center is the root zone of the tree).

Do Roots Grow Faster In Water Or Soil?

The roots of some plants can grow faster in water than soil.  However, it will depend on several factors, including:

  • Soil and water pH: if the soil or water is too acidic or too basic (alkaline), then the roots may not be able to absorb nutrients for growth.
  • Nutrient levels: a lack of nutrients (or toxicity in the case of too much) can also affect root growth speed.
  • Soil density & compaction: roots need air and water to grow properly.  Soil that is too dry will not provide enough water for roots to absorb nutrients.  Soil that is too wet will not have space for air, which will deprive roots of oxygen.
  • Water aeration: roots in water need air to grow properly, so aeration with a pump is necessary in large-scale hydroponic systems.

What Causes Poor Root Growth In Plants?

Cold, wet soil is one thing that causes poor root growth in plants.  This weather is more likely in early spring (at the start of the gardening season) or late fall (at the end of the gardening season).

root rot
Cold, wet soil causes poor root growth and may even lead to root rot (this causes roots to turn brown and mushy).

As long as you don’t plant too early or too late, you should be able to avoid this problem.  If you have a short growing season, consider starting seeds (such as tomato, pepper, and others) indoors in late winter or early spring to get a head start.

Fertilizing your plants during a drought without watering in the fertilizer can also cause poor root growth.  For one thing, plants need water to absorb nutrients, so dry soil will slow their growth.

watering can
Water in after you fertilize, especially during a drought. Plants need water to absorb nutrients from the soil.

In addition, too much fertilizer in the soil without water can burn plant roots that touch the undissolved fertilizer.  This damages the roots and slows overall growth.

Finally, letting a plant become root bound will also cause poor root growth.  This happens when a plant is allowed to grow in a small pot for too long.

Before this happens, repot the plant by transferring it to a larger container.  If you see roots starting to grow out of the bottom drainage holes of the pot, it is time to transplant (or it will be soon enough).

root bound
Repot plants to a larger contianer before they become root-bound.

Conclusion

Now you know how to promote root growth in plants and which fertilizers to use.  You also know what to avoid if you want to prevent poor root growth.

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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