Why don’t landscape trees respond to fertilization?

As part of my ‘other duties as assigned’, I have taken on an assignment to develop fertilizer prescriptions for landscape trees and shrubs based on soil samples submitted by homeowners to the MSU soils lab.  This has gotten me immersed in two sets of conflicting literature. 

On one hand is a raft of extension bulletins and ISA guidelines on tree fertilization, which typically suggest adding 2-4 lbs of nitrogen (N) per 1,000 sq ft of ground area – in some cases up to 6 lbs of N per 1,000 sq ft (Rose 1999).  To put that last number in perspective, that works out to 262 lbs of N per acre.  A typical recommendation for corn in Ohio is 150 lbs of N per acre.  Landscape trees need more fertilizer than a field of corn?

On the other side are numerous landscape studies, and even some nursery field studies, that suggest landscape trees simply don’t respond to fertilization (Day and Harris, 2007; Ferrini and Baietto, 2006; Harris et al., 2008; Robbins, 2006; Werner and Jull, 2009). 

So, what gives?  I mean, we know trees need nitrogen to make proteins, enzymes, chlorophyll, and all that good stuff.  We rake up leaves and haul them away each fall; short circuiting the natural nutrient cycle.  Urban trees should be starving for nutrients and begging to be fed, yet don’t grow any better when we fertilize them?  There are likely several factors at work.  First, many of the widely circulated recommendations are based, at least in part, on rates that might be applied in production forestry or nurseries where maximizing growth is a primary objective.  Also, as Dan Struve (2002) noted in his review, some recommendations were based on poorly designed studies.   Or, as Dan Herms (2002) hypothesized, trees may be allocating more resources to defensive compounds. 

Another answer may be found in some of the papers cited above.  In many cases, foliar N levels of the control (unfertilized) trees were already in sufficiency ranges.  In other words, even without fertilizer the trees had enough nutrients.  Part of this is a recent emphasis on the question of fertilizing at transplanting.  Trees coming in from a nursery are likely to be pretty jacked up with fertilizer – often referred to as a ‘nutrient loading’ effect.  Trees can be pretty efficient at internal nutrient re-cycling, so this effect can persist for a few years after transplanting. 

So where does all this leave me on my homeowner recommendation?  For some elements, like potassium and phosphorus, we can base a recommendation on the standard soil test included in the program.  Unfortunately there is no simple indicator of soil N availability (see Scharenbroch and Lloyd (2004) for a thorough discussion).  Foliar samples would be the best bet but aren’t part of our testing program.  At this point, I’m leaning toward a simple visual assessment – Do the trees/shrubs look healthy and have acceptable color? (check yes/no).  If ‘yes’, don’t fertilize.  If ‘no’ add 1-2 lbs N/ 1000 sq ft.

Your thoughts?   


Day and Harris. 2007. Arboric. and Urban Forestry 33:113-121.

Ferrini and Baietto. 2006. Arboric. and Urban Forestry 32:93-99.

Harris et al. 2008. Urban Forestry and Urban Greening 7(3): 195-206.

Herms. 2002. Environ. Entomology 31:923-933.

Robbins. 2006. So. Nursery Assoc. Proc. 51:113-117.

Rose. 1999. HortTechnology 9(4):613-617.

Scharenbroch and Lloyd. 2004. J. of Arboric. 30:214-229.

Struve. 2002. J. of Arboric. 20:252-263.

Werner and Jull. 2009. Arboric. and Urban Forestry 35:252-262.

14 thoughts on “Why don’t landscape trees respond to fertilization?”

  1. My unscientific observation of trees in my yard: red maples and a fir were looking ok but not growing well three years after transplant, despite slow release fertilization each fall. After an arborist came in with an airspade and opened up the roots (yes they were large trees planted too deeply), the maples and the fir almost exploded with pent up growth each season after! If landscapers would stop worrying about the perfect balance of expensive extra nutrients and just get some oxygen and loose soil to the roots of most trees that are poorly planted or in compacted areas . . . that makes more difference I think.

  2. I agree, Laurrie. And not to sound like a broken record (ok, I know I do), mulching well with a coarse organic mulch will also loosen soil and improve gas exchange. It’s far easier (and less expensive) than the airspade route.

  3. As an outlier in the arborist community, I often buck the trend of companies who routinely fertilize trees. I had not heard about the studies mentioned above; in fact, I have seen and heard of trees that push new growth in response to N supplements. But
    that doesn’t necessarily make it a good idea.

    My concern has always been that this growth must come at some cost. People want to see results in people time, but trees are on a different schedule. Shigo taught us that trees must divide their resources between growth, maintenance, reproduction, defense, and then a little back to the soil. Forcing some expenditures on growth and maintenance may compromise defense, leading to stress or death. Beyond that, I also wonder if adding N pushes growth too rapidly toward the tips without allowing sufficient time to develop branch taper, root systems, etc.

    Meanwhile, @LC-S, I often recommend mulch for relief of compaction as you suggest, but air spading can help where mulch will not. One way is by removing excess soil from too-deep trees, but it also allows correction of root problems that tend to develop in nursery-grown plants (or natives with added soil).

    As a consumer, I have a healthy skepticism of practices that justify the purchase of an expensive tool/machine (“deep-root” fertilization being a prime example, though I do believe this practice has its place). But the air spade is a vital part of any true arborist’s tool kit.

  4. treedweller: not to worry – the system is less than ideal but at least we can share ideas.
    Clearly there are more to potential tree issues than nutrition. Our goal at hand is to expand our current program for soil testing for lawns to a program to include landscape tree and shrubs and garden plants. We’re trying to develop a decision tree that could process hundreds of soil tests and spit out a recommendation.

  5. Whatever happened to the common wisdom that plants sleep/creep/leap over their first few years anyway? It would appear that their natural inclination is to develop a sturdy root system first, then work on top growth. As treedweller asks, what trade-off does the plant have to make to produce growth to “our” expectations instead of the natural order of things? In my opinion (NOT a scientist, just a student/observer) soil test results should be followed and nitrogen should be applied sparingly if at all. A main reason for caution in the fertilization program is because of what happens to the N and phosphorus if they are not used by the plant community nearby. They often run off to bodies of water that surely don’t need the extra food. (Yes, I’m in the Chesapeake Bay watershed and have to teach/worry about nitrogen pollution.)

  6. I have a challenge that relates to this discussion. I have an old (probably 20 years or more) bridalwreath spirea against my backyard fence. I had planned to take it out and replace with something bigger to provide more screening.

    Instead, I was convinced to shape this shrub to be more upright and provide fertilizer to encourage growth. For two years I’ve provided extra helpings of fall leaves, compost, and slow release fertilizer. However, the results have been quite modest.

    Am I on a fool’s errand here? Should I just yank the thing out?

  7. Is it possible that nitrogen is simply not the “limiting reagent” in tree development?

    Plants don’t excrete nitrogen compounds during metabolism, do they? So the significant need in N arises only in actively dividing cells to build proteins, DNA etc. I suspect that a square mile of a tree plantation produces far less biomass a year than a square mile of a nearby wheat field (I would love to see a mass balance of these two!). Besides, much of a tree is made of cellulose and lignin which are nitrogen-free.

  8. Sounds like you’ve been placed between a rock and a hard place – “recruited” for a project that goes against one of the most basic of all “sustainable” practices which is recognizing that one size – or even numerous sizes – don’t fit many circumstances. To begin, it’s very likely that many/most homeowner/consumer soil samples are gathered incorrectly (being a former extension agent, I know this to be true) – rendering any recommendation of limited value right from the get-go. Second, if all of the consumer “landscape” soil test results generated by the MSU lab were analyzed, I’ll bet the majority show high to excessive levels of P and K. This is precisely what was found when consumer soil test results from all counties in New York were analyzed between the years of 1995-2001 and 2002-2006 (http://nmsp.cals.cornell.edu/publications/soilsummary.html). And, this doesn’t even begin to address the fact that the root systems of landscape plantings are not uniformly distributed (unlike a field of corn, a golf course fairway, a block of trees in a production nursery, etc.), resulting in much of any applied fertilizer not hitting the intended target. I’m absolutely with Laurrie above – focus on the improvement of soil physical conditions and tree and shrub roots will very efficiently utilize existing soil nutrients. After all, well over 90% of all trees and shrubs in “built” landscapes have never been fertilized and the vast majority are growing perfectly well.

  9. I plant a lot of trees in my yard. I for the most part have poor very sandy soil. I worry more about moisture than fertilizing them. All out trees/shrubs are mulched to the drip line (not to much though). We every year continue to expand this drip line. Any fertilizer they would get would be from our once/year lawn application what the spreader would spew out into that drip line. I do look at the amount of growth pushed out each year knowing that my soil type can contribute to less growth. I look at the color of the leaves as well to see if I can detect and potential nutrient deficiency. I think choosing the correct tree and one’s soil type is more important to the long term health of a tree.

  10. I second Laurie’s observation. I didn’t use an airspade, but
    my trees really took off once I corrected their planting depth and mulched around them, rather than having them planted deep with lawn up to the trunk like most landscape trees are. Lawn is really a bad mulch unless you get plenty of rain. Otherwise the lawn will just suck up all the moisture from the soil, leaving young trees gasping for water.

  11. Jason, if the screening is inadequate, I’d yank it out and replace with something that grows bigger. I don’t think spireas get that tall. I’ve had the best of luck with red-twig dogwoods and “dwarf” arctic willows as far as fast-growing big shrubs go. Just provide plenty of mulch and water, but don’t plant too deep.

  12. The only reason to fertilize mature trees is to help them outgrow fungal infection or correct an obvious deficiency. The longest lived trees tend to be the slowest growing ones. I believe that extra N for establishing trees (applied in early spring or late summer) is helpful because in most cases it does accelerate growth while other nutrients will make no visible difference. Of course, accelerating growth comes with certain risks, not the least of which is foliage seems to become more attractive to certain insects such as aphids, scale and mites. I’ve even seen a study where deer are more attracted to N pumped plants. On the other hand, the sooner a tree reaches a certain size the less likely it is to be killed by any number of things, including deer and drought.

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