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 – 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”.
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:
- Top Seven Myths of Tree Care, by ISA: http://www.treesaregood.com/searchResults.aspx?q=deep+root&cx=010107291988267817406%3aanhvlksvvbw&cof=FORID%3a9
- A Review of Shade Tree Nitrogen Fertilization Research, by Daniel K. Struve.
- Photo Source: Larry D. Moore
I find it difficult to believe that root feeding of trees is a waste of money. I’ve done it for years, with a cheap DIY root feeder, and I attribute the success I have had with trees in our yard versus adjoining properties to this feeding. My basis for concluding it is not a waste of money is identical boulevard trees planted by the municipality on our street – the same trees, planted at the same time, in the same soil. Most neighbors did very little, occasionally watering and possibly fertilizing the boulevard grass. One other neighbor had the same interest in his yard as we did, the boulevard grass on both lots was given the same care as the full lawn, regularly fertilized and watered as required. The difference was I root fed, he spread and watered in additional fertilizer around the trees when fertilizing the lawn. Over the years as the trees matured, the two trees on each property were very noticeably out growing others on the street who tended to rely on nature, alone. Forty years later, you wouldn’t know that the four trees on my property and my friend’s (since moved) property had the same start as those on the rest of the street. But the case I make for root feeding is that the two boulevard trees on my property grew faster in height and width in the early years. The difference was noticeable, my friend commented and discussed. He tried drilling holes and putting in granular fertilizer, and I believe tried spikes – this may not have been given adequate time for results as he soon decided it was less trouble to root feed. The trees on both properties are similar now, having fully matured – most other trees on the street did not mature to near the size of the four trees given early care.
My point is, if root feeding is a waste of time versus surface application, why did the root fed trees noticeably outperform the surface fed trees, in otherwise identical circumstances? Another point is that having removed trees, I question that roots below the 8″ do not benefit from fertilizer (and water) that is delivered beyond 8″, by other means. A number of trees have roots below that depth, and although they may be in the minority, they do function and support the tree.
Let’s be clear about the ‘waste of money’ part. Fertilizing may not be a waste of money. Doing deep root fertilization by yourself may also not be a waste of money provided that you are not paying a high price for the fertilizer. Most people do not do it themselves–they hire a company to do it, and it is expensive. When I checked on prices several years ago it would have cost $300 for a large tree on a hill vs $30 of fertilizer spread on the ground.
Thanks for pointing out the DIY process. Which tool do you use?
You can’t really reach any conclusions based on your neighbors trees. Your own comment says they watered very little and maybe fertilized. It shows that proper care of a tree grows a bigger tree. It does not compare top fertilization with deep root fertilization.
Your one neighbor did provide better care. Unless you both made sure that the amount of water was the same, and the amounts of fertilizer were the same, and applied at the same time, you still can’t reach any scientific conclusion. I don’t think you can say that the two trees were grown in “identical circumstances” since all the variables were not controled. Besides that, one tree in each situation is never enough. Scientists are very careful to control conditions so they can reach a conclusion. For this type of experiment they might use 10 or more trees in each set of conditions so they eliminate any variability in the trees.
You say that your tree grew faster, but that now they are the same size. So over a long period of time, both fertilization practices resulted in full size trees.
My conclusions are based on the movement of fertilizer in soil and the fact that I have not been able to find a single research report to confirm deep root fertilization works better than surface fertilization. I even checked with the arborist societies who promote the idea of deep root fertilization, and they did not provide a source for such research. Several people have contacted me with a strong belief in deep root fertilization, and none of them were able to produce such a report. At 10 times the cost, I see no reason to use the process.
Roots do go deeper than 8 inches, but the deeper roots are mostly there for stability. The real feeder roots are very small and deeper roots have few feeder roots. It is the feeder roots that take up the majority of nutrients and water. But let’s say that deeper roots do benefit from water and fertilizer. Fertilizer and water that is applied to the surface, moves down through the soil layers. In particular nitrogen moves quite quickly and soon it is below the 8 inch level. Water does the same thing. But if you use deep root fertilization, and you go too deep, you miss the feeder roots. Water and nitrogen don’t move up nearly as well as down.
I thought I would add the quote from the reference in the first paragraph of this blog post. This is what the International Society of Agriculture says:
MYTH #7: The root system of a tree is a mirror image of the top.
Fact:
Many people envision a large, branching taproot growing deep into the soil. Actually, taproots are very
uncommon in mature trees. If taproots do develop, they usually will be forced into horizontal growth when they
encounter hard subsoils beneath the surface. The entire root systems of most trees can be found within three feet
of soil. The spread of the root system however, can be very extensive, often extending two to three times the spread
of the crown. This means 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
Regarding MYTH#7. I am not one of the many that envision the root system of a tree to be a mirror image of the top. I have removed or assisted others in enough removals to know this isn’t the case. As mentioned in this quote, roots are “forced” by the nature of the subsoils. It is also evident, in my experience, that the movement of water and the nutrients it carries is often influenced by the same factors. Water delivered under pressure often moves up and horizontally; those who have used a root feeder will be aware of this, and that it is most likely if they are in an area where there is a relatively shallow layer of hard subsoil – it is not unusual to see an earthworm shoot out of the ground a significant distance from the injection point. Under pressure the water and fertilizer tend to take the path of least resistance, which is very often influenced by the same factors that “force” root development. I would expect if scientific tests were conducted, that in many cases, the loss of water and nutrients out of the root area in root feeding would be on a horizontal route in the soil layer above a hard subsoil layer – rather than down through layers as you depict.
Back to the comparison of costs. Comparing the cost of root feeding with special equipment and usually the services of an arborist to DIY sprinkling of fertilizer on the ground has the elements of comparing apples to oranges. The DIY root feeders may not cover the same area as the special equipment of the arborist – so the DIY amateur may have to poke a few more injection points. ( I could argue that this provides valuable aeration not otherwise provided – but I won’t!)
I am not sure that “roots are “forced” by the nature of the subsoils” is necessarily true. Roots grow where it is easiest to grow and where they get the nutrients they need–they are opportunistic. It is certainly true that most subsoils are less inviting to roots. They tend to have fewer nutrients and less oxygen. Hard subpans will certainly affect where roots grow.
I agree that it is not a fair comparison to compare DIY systems to professional applications. I think the important question is; what are the results of using either system? Is one significantly more effective than the other and how do they compare to surface fertilization.
Poking holes would not add aeration. Poking holes actually compacts the soil around the hole making matters worse.
My comments were triggered by your comment “… deep root fertilization is a waste of money.” My comments are not based on a controlled scientific test, nor on ten trees with measured water and fertilizer applications. But contrary to what you say, I can come to a conclusion based on my neighbors’ trees – generally the two property owners that made the effort to do additional watering and fertilizing were rewarded with noticeably superior results, albeit not under a controlled test. My neighbor and I both agreed that my trees appeared to be growing faster than his, we didn’t measure watering but knew that as it was based on need by the lawn, it was similar. As for fertilizer, I have no idea how the pellets I used compared in analysis to the additional lawn fertilizer that he spread around the trees. My guess is that cost wise, I paid more for a specialized product in a small and captive market.
But my point is that I cannot be convinced that the root application of fertilizer (and water) was a waste of money. Whatever I was doing got results that met (if not exceeded) the results of a neighbor using surface fertilization and watering. The cost of doing this was a Ross root feeder that cost likely $20-30 over forty years ago, and is still being used. The fertilizer cost may be more, but is not unreasonable; and the ease of attending to it makes the extra cost well worth it, to me. You mention that deep watering may be wasting water and fertilizer that misses the feeder roots – this I don’t know. I do know that the area that was available to water was moistened by water that moved up, and that I had no run off of water or fertilizer. Surface application is prone to run off of water, and applied fertilizer goes with it. Applying additional fertilizer around the target tree, can result in burning of the lawn grass (if overdone, and depending on the fertilizer used), and it is obvious that some of the fertilizer is used by surrounding lawn, as there was often a greener area around the target trees – a result I am not looking for. To sum up, my experience, although not scientifically proven, convinces me that root feeding of water and fertilizer is not a waste of money.
The blog was based on using commercial services for the deep root fertilization. At $300 per tree, per fertilization, it is not worth the money.
I agree with you that doing it yourself certainly is a cost effective option and since the trees get fertilized it is not a waste of money as a DIT project. I do plan to look into this more deeply to try and understand how effective a DIY job is. I believe the commercial people use much higher forces to distribute the fertilizer better.
No one questions the value of fertilizing. Now that you have eliminated the cost issue, the remaining question is; is deep root fertilization better? The International Arborist Society is not convinced and neither am I.
Hi, I am a nurse with a 36 year old Silk Oak that has been doing great until this year. It is about 60 feet tall. I put a good well around it many years ago and fill it 3 or so times once a month All the other silk oak trees in the area died long ago. Mine is looking like they did before they died. It started to become chlorotic about 7 months ago, fed it ironite. Fed it nitrogen granules last Wednesday and watered it in well. It is even loosing the beautiful bright green leaves. Someone told me that they get a disease. Is this true? Also, would like to know the lifespan of the silk oak. Our stonefruit usually lasts about 10 to 12 years. So I assume that the silk oak has a lifespan. Please RSVP. Gracie
I am not familiar with this tree, but assume you are talking about Grevillea robusta.
Based on this reference, a 36 year old tree is reaching the end of it’s life span. http://www.ehow.com/about_6588141_silk-oak-tree-lifespan.html
Fertilizing an unhealthy tree is usually not a good idea. The first thing to do is to identify the problem, and then react to the problem. It is not likely a nitrogen or iron problem if the tree has been growing well up to this point, unless something has changed in the soil area to cause a problem.
Chlorosis can be caused by a number of things. Have a look at https://www.gardenmyths.com/chlorosis-deficiency-in-iron/#more-1680 for more details.
I would just like to know if the author actually has any formal training/education in either horticulture or arboriculture or if you are just regurgitating google searches when you’re not painting
I have degrees in chemistry, and biochemistry, and have been gardening for 40 years. I am also a Master Gardener, give horticulture courses at the local university, give gardening seminars, and I operate the largest private garden in the city.
I don’t regurgitate Google searches. I do spend time researching the latest scientific studies on various horticultural topics.
If you don’t agree with what is written then it would be most useful if you could present us with some contradictory research to the findings I present. That way we can have an intelligent dialog.
I think you do tree owners and knowledgeable arborists a great disservice with this post. As a certified arborist who makes a living providing holistic plant care, to suggest that the typical urban homeowner should to replace their lawn with wood chips and organic matter …. this is a joke, right? In an ideal world, sure, everyone would remove their lawn and xeriscape their yard. You should realize that not everyone has five acres in Ontario to putz around with. Most homeowners and commercial property owners I service value their trees *and* their lawns for aesthetics, property value, and quality of life.
When Aaron says “I would use a deep root fertilization for an unhealthy nutrient deficient tree. Otherwise I would just use a slow release granular fertilizer” , and you respond by saying “If the soil is short of a nutrient, and your tree is suffering, a slow release fertilizer is not your best choice …. ” You are either struggling with reading comprehension or you truly don’t know nearly as much about tree care as you would have people believe. I’m really not sure what you’re advocating here… that the average homeowner simply pour fertilizer all over their lawn in response to a nutrient deficient tree? Maybe one that was planted decades ago in heavy clay soil? Just out of curiosity, how do you think the lawn will respond, and how much of the fertilizer will be available for the tree? I know, the response would be “plant the proper tree for the proper soil” . Next will be an article “Don’t let the tree guy convince you to cut down your tree and replace it … it just needs more nutrients!”
You said “to suggest that the typical urban homeowner should to replace their lawn with wood chips and organic matter …. this is a joke, right?”. I never said any such thing and this posting has nothing to do with lawns? In fact I think lawns are very important for homes and the idea of removing all of them makes no sense, for practical and esthetic reasons. I do believe in reducing the amount of lawn.
I never said home owners should pour fertilizer all over their lawns – maybe you should reread the posting. If a tree is suffering from a nutrient deficiency, you should not use a slow release fertilizer. It is better to use a commercial fertilizer that will provide nutrients quickly.
The lawn will also grow better when fertilizer is added. Nitrogen will leach to tree roots fairly quickly with the next rain or watering. P and K will move more slowly, but they are probably not the nutrient that is deficient.
Thanks for the article Robert. Could you please give me some advise on what to do about the above ground roots on my two 14 year old live oaks. They are both very healthy. However, they seem to be getting worst every year, with more of the root system coming above ground. I live in a Cinco Ranch Katy. I’ve been told they need the deep root fertilization. What would you suggest? Thanks
Why are roots above the ground? There are two explanations.
a) The tree was planted in soil that had been amended with lots of organics, and now the organics have been used up so the soil level has become lower. This is why it is a bad idea to amend soil in permanent plantings. It is not that the roots are coming above ground–the top of the ground is getting lower and exposing the roots.
b) The tree likes to have it’s roots above the ground. This is quite common for many older trees, but a bit unusual for 14 year old trees.
In the second case you should do nothing. In the first case it may be too late to do anything. You could add soil (straight top soil), an inch a year to increase the level but this may damage the tree. It would be best to do nothing.
Whoever told you that they need deep root fertilization knows nothing about taking care of trees. Fertilization will not lower the roots.
After some research I found another possible explanation for the high roots. If your soil has a hardpan near the surface it can cause roots to grow above soil level. A hardpan is a hard layer of soil which the roots can’t penetrate. You can dig a hole 1-2 feet deep, and if you have it, you will know. It will be like digging concrete when you get to the hardpan. If you do have a hardpan there is not much you can do about it. It could have been removed or penetrated before planting the trees.
I agree that by applying synthetic or salt based fertilizers via deep root fertilization can be a waste of money. While, inoculating the soils with endomycorrhizal spores and compost tea in areas of hardened clay soils or gumbo will dramatically help stressed trees recover by incorporating fast attaching spores and a nitrogen fixation microbes that increase soil health.
I have dealt with compost teas in a separate posting.
https://www.gardenmyths.com/compost-tea/#more-329
https://www.gardenmyths.com/something-stinks-about-manure-tea/#more-1391
There is no scientific evidence that compost tea works. Just spread the compost on the surface of the soil and you will gain it’s benefits.
I plan to do a post on mycorrhizal fungi, but the short answer is that soil already contains these fungi. The spores of both fungi and bacteria float in the air easily and reach all parts of the soil. If the soil is not suitable for their growth, they will not grow. Adding additional spores will not make them grow. In soils like this it is not a case of missing microbes, but that the ones that are naturally present are not able to grow. The solution is to improve the soil by putting compost, wood chips and other organic mater onto the surface and letting nature improve the soil.
The author is clearly not current on newer products and materials used in tree care.
Can you be more specific so that our readers can better understand your position. What part of this post is incorrect and why?
I would use a deep root fertilization for an unhealthy nutrient deficient tree. Otherwise I would just use a slow release granular fertilizer.
Just because a tree is unhealthy does not mean it is a nutrient deficiency problem. If it is a nutrient deficiency problem, it is still best and most economical to fertilize on the soil surface. If the soil is short of a nutrient, and your tree is suffering, a slow release fertilizer is not your best choice. You really want to use a fast acting fertilizer to get the nutrients to your tree more quickly.
A slow release fertilizer is appropriate for long term fertilization of healthy trees, but it is probably not the most cost effective way to fertilize.
Trees of texas in houston texas has fed my trees on my 300 acre ranch at least once a year since 1984 had they not i believe i wood have a desert. excellent service.
Paying for this expensive service for 300 acres must have cost you a fortune? Try just spreading fertilizer at a fraction of the cost.
Since you have been doing this since 1984, all you can say is that the trees with this service are growing. You can not reach any conclusion about the importance of deep root fertilization compared to any other method including no fertilizer at all. Without a comparison testing more than one method you have no basis for reaching the conclusion that deep root fertilization is better than regular fertilization.
Dear Sir, my soil for durian farm is poor of bio or quality of ground soil, some of farmers have fall-down their durian trees when it is rain and wind. This is a big problem, taproot is low or poor into the deep soil, so I just have a question that which fertilizer (NPK) or compost can help to enhance taproot in the deep ground. Thank you so much. From Cambodia
I disagree with you in slight about this service. It is effective, and I have the results many times.
Is is over prescribed, overused and misunderstood by many arborists? Yes, it is.
Applying fertilizer to the soil surface has serious consequences with root development while applying it too deep is like you said – a waste of money. 4 to 6 inches works wonders.
The main reason it is necessary is because soils in the urban environment are subject to a great deal of chaos (construction turns the profile upside down) and plants in our landscape come from all over the world with completely different needs for ph and nutrition.
I know this so well because where I reside on the west coast our soil is very acidic and some plants won’t tolerate it. A proper plant health care program involves soil ph balancing as fertilzer is a wasted if your ph isn’t addressed.
If you are looking for real experts in plant nutrition, don’t talk to a gardener or aborist but check the hydroponic scene for their advice. They have this science perfected.
Sorry for the late reply – somehow this post got lost.
soils in the urban environment are not subject to a great deal of chaos. Granted that when new s=construction takes place what you say is certainly true. Once the builder leaves and trees are planted, things start to remain more static. Even if chaos exists, that does not mean deep fertilization is more effective than just spreading it on the ground.
When fertilizer is applied on the surface several things happen. Soluble nitrates quickly leach to the 4-6 inch level – providing the same fertilization at a fraction of the cost. Potassium dissolves more slowly, but starts making it’s way to the root level. Phosphorous moves more slowly, but even it is moved down by the action of all the life in the soil. Surface fertilization may not get the nutrients to the roots quite as quickly, but they do get their.
I agree that adding fertilizer of any type without knowing the pH is the wrong thing to do. I do not however agree with pH balancing except in very extreme situations. Trying to combat mother nature and alter the soil pH is a lost cause – mother nature will win. Select plants that like the pH of your soil.
Understanding hydroponics is not the same as knowing plants in the soil. Have a look at this for an example: https://www.gardenmyths.com/removing-bottom-leaves-on-tomato-plants/