Deep Root Fertilization for Trees

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Robert Pavlis

Deep root fertilization is a recommended procedure by many arborists. Does it work? Is it the best way to fertilize trees? Do trees need to be fertilized?

deep root fertilization for trees
Deep root fertilization for trees

Deep Root Fertilization – What is it?

Deep root fertilization for trees is a process where you stick a pipe down into the soil about 8-12″ and then, under pressure, squirt fertilizer into the ground. The theory is that since tree roots are deep down in the ground, the fertilizer would also need to be put deeper in the ground. Since this process requires special equipment, it is usually done by an arborist.

It is interesting that the International Society of Arboriculture (ISA) recognizes this as a gardening myth, and yet many ISA certified arborists still sell the service? This is what the ISA says:

you don’t need to perform “deep root fertilization” to reach their root system-most

of the trees’ fibrous, absorbing roots are in the top eight inches of soil”

A recent review of available literature on tree fertilization in the USA by Daniel K. Struve (ref 1) concluded that “Little difference has been found among fertilizer application methods; broadcast applications are as effective as subsurface applications”.

Plant Science for Gardeners by Robert Pavlis

Note added June 2014: One of the people adding a comment below suggested that deep root fertilization could be done as a DIY (do it yourself) project using a very simple device available from hardware stores. When I first wrote this blog I was only considering the application done by an arborist, which is very expensive. In this situation the original post is still correct. If done as a DIY project, the cost is much less and maybe it is no longer a waste of money. I hope to do a future post looking more closely at DIY deep root fertilization.

Fertilizing Trees

Most fibrous absorbing tree roots are found in the top 2-8″ of soil where water and oxygen are abundant. Fertilizer that is placed below this level does little for the tree and is in fact environmentally harmful.

The key nutrients required by the tree are nitrogen, phosphorus and potassium (NPK). Let’s look at each of these nutrients.

Nitrogen moves through the soil very quickly and is probably the nutrient your tree needs most. An easy way to feed your tree with nitrogen is to just spread it on the ground. It will dissolve in water and flow down to your roots.

Phosphorous does not move through the soil very quickly and so adding it lower down in the soil seems to make sense. However, most soils in North America have plenty of phosphorous in them. Unless a soil test indicates differently, or you know that the soils in your area are phosphorus deficient, you don’t need to add phosphorus.

Potassium moves through the soil relatively quickly, but not as fast as nitrogen. Most healthy clay soils have enough potassium.

Should You Fertilize Your Trees?

If you have healthy soil you probably don’t need to fertilize. The problem in our urban landscapes is that we remove the tree leaves each fall. The leaves are natures way to fertilize the trees and by removing them you are removing the food for future years.

Tree roots that are covered by grass add an additional problem for trees since the grass roots compete with tree roots for nutrients.

In either of these situations it does make sense to feed your trees, but deep root fertilization is a waste of money. Just take normal fertilizer and spread it on the ground. If you are fertilizing your lawn, you are also fertilizing your trees. Let nature move the nutrients to the roots.

References:

  1. Top Seven Myths of Tree Care, by ISA: http://www.treesaregood.com/searchResults.aspx?q=deep+root&cx=010107291988267817406%3aanhvlksvvbw&cof=FORID%3a9
  2. A Review of Shade Tree Nitrogen Fertilization Research, by Daniel K. Struve.
  3. Photo Source: Larry D. Moore
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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

87 thoughts on “Deep Root Fertilization for Trees”

  1. Good information! There are a lot of advantages for feritigation. Most importantly, it gives us the ability to apply nutrients at critical periods of crop demand without having to traverse the field thereby reducing compaction and crop damage, saving time and labor, and reducing fuel and equipment costs.
    Yes we are also considering using fertilizer doser which is driven by water, such like this pattern, https://www.htcn.com/product/chemical-dosing-jet-proportional-injector-pump.html
    Do you think it is the good solution? Also, compared to mixing range 1-10%, 0.4-4% doser is ideal?
    Any suggestions will be highly appricated! Thanks a lot!

    Reply
  2. I suspect specialists on fruit tree fertilization have a mental model of orchards — which is fine for orchard owners, But I only have a few fruit trees and shrubs in my back yard, totally covered with lawn grass. I can’t dig the fertilizer in, and I don’t want to spread tree fertilizer all over the grass — which I rarely if ever fertilize. So, beside spikes and root feeders, what else can I do?

    Reply
  3. Question: I live in Las Vegas, the Mojave desert. Not great soil. I only deep water my trees to conserve water. What would be the best way to get fertilizer to the roots? If I put it on the surface it seems unlikely it would make it into the ground at all. (We don’t get much rain. It’s rained once in the last 280 days. So, water is not permeating into the roots.)
    Thanks

    Reply
  4. Yeah, nah. The ISA has removed that advice and your link is now dead. What you don’t appreciate is that root feeding injects liquid which is replaced by air when the liquid soaks in. That leaves air tunnels providing much-needed aeration.

    Reply
    • The ISA still thinks deep root fertilization is bunk: https://www.treesaregood.org/portals/0/docs/newsroom/SevenMythsDebunked_Feb13.pdf

      Delivering quick release fertilizers to nutrient deprived trees is great, however the fact that they are injected doesn’t actually help anything, just having liquid fertilizer in the soil does. The first time water is applied after fertilization forces any water soluble nutrients not already absorbed by the tree down by gravity anyways.

      If you want to relieve compaction, get an air spade. If you want to fertilize a tree, treat with a quick release fertilizer and amend the soil based on recommendations from a soil test. I would love for anyone to present a study that shows using a soil injector improves the uptake of a quick release fertilizer, or that it actually improves the overall pore space of the soil as I have searched and can’t find a single one. Spike aerators make holes too, but they actually make compaction worse, and I am highly skeptical of these beneficial claims.

      Reply
  5. To me, it appears that one of the most misunderstood subjects about plants is root systems. Not all root systems are equal or the same. Some plants have shallow root systems and some have very deep root systems but I have found it difficult to find scientific studies describing the root systems of various plants. American native prairie plants are a good example of this with some plants having root systems penetrating 12 feet deep and all (?) others penetrating deep enough to survive prairie fires. Most trees on the prairie do not survive the fires and that’s why there are prairies.

    In my limited experiences, I think one of the most important things we can do for trees is protect their roots systems with mulch. Mulch not only protects the tree’s roots from harsh freezing weather and scorching hot dry summers it also helps prevent erosion and it fertilizes the soil at all depths.

    Three other important practices in growing trees is testing the soil periodically, proper regular pruning, and having an arborist inspect the trees once a year for disease(s).

    Thanks for an excellent website that allows readers to weigh in!

    Mike Upper

    Reply
  6. Cool concept for a blog. I’ve also wanted to put down garden myths designed to make the nursery business more money.

    The term deep root soil injection to me is an improper term. For 1 soil is a permeable surface. If you inject a root feeder into bare soil it will all come back up and then soak back down.

    Many trees do benefit from a tree fertilization program which utilizes a slow release nitrogen. This is the opposite of the inexpensive fast release stuff sold in lawn fertilizers.
    Also many urban landscapes already have high levels of phosphates and do not need anymore. In fact if phosphates become too high it can block uptake of micro nutrients and can cause your tree to become chlorotic if they become deficient in micro nutrients.

    A soil analysis can help determine the nutrient needs of your trees and is recommended before starting a fertilization program.

    Reply
  7. To the author. Your reference links are broken so we can’t review the cited information to evaluate the validity of your opinion.

    Reply

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