I’m in Grand Rapids this week attending the Michigan Nursery and Landscape Association/Michigan Turf Foundation Great Lakes Trade EXPO. The topic for my talk today was Landscape Tree Fertilization. That might not sound like a subject that would generate controversy, but as with most things, there are camps emerging. There is a rising chorus of folks that suggest that landscape trees should not be fertilized with nitrogen. There are a couple of lines of evidence that bolster this point of view. First, many systematic studies on the growth response of street trees or landscape trees often do not show a response. There are numerous examples of this, for example, in Arboriculture and Urban Forestry (formerly J. of Arboriculture). The second line of evidence for not fertilizing landscape trees relates to the relationship between tree nutrition and susceptibility to insect pests. This argument relies on the ‘growth vs. defense’ hypothesis and suggests that fertilization promotes growth at the expense of defense compounds; essentially making fertilized trees tastier to insect pests.
So, in light of this, why do I suggest that landscape trees should receive 1-2 lbs of N per 1000 sq ft. every 2-3 years? First, we need to understand that nitrogen is constantly lost from landscape systems. In forests, trees take up nutrients from the soil, translocate them to leaves, shed the leaves, and the nutrients are ultimately returned to the soil in a cyclic process. In landscapes, leaves are usually raked or blown and removed from the cycle. Soil nitrogen is also lost due to nitrate leaching. Additionally there are often key weaknesses in some of the papers that purport to show no response to fertilization. For example, Ferinni and Baietto (Arb & UF 32:93-99) found no response of sweetgum trees to two levels of fertilization. However, the data show that the control trees, which were not fertilized, had similar (and fairly high) foliar N levels as the fertilized trees. This pattern can be found in several similar studies. The more appropriate conclusion for these papers should be “Trees that are not nutrient deficient do not respond to fertilization”. Similar issues pervade studies related to the growth vs. defense hypothesis. Why would one presume that a nutrient deficient plant would be better able defend itself against insects attack than a tree that has adequate nutrition? Ideally, fertilization decisions should be based on visual symptoms and soil and foliar samples. Nevertheless, low rates of N from either organic or inorganic sources will make up for losses from the N cycle and maintain tree vigor.
It should be noted that the rates I’m suggesting are considerable lower than those that are found in some older extension literature, which recommend rates of N up to 6 lbs for 1000 sq ft. As a point of comparison, Midwestern farmers apply 150-200 lbs/acre to grow a crop a corn. The 6 lb rate for landscape trees works out to around 260 lbs per acre!
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I’m with you, Bert. Nitrogen is the most commonly deficient nutrient, and this is especially true of urban soils. Trees in turf routinely lose out on the nitrogen battle, since grasses are so much better at scavenging nutrients. Microbes are also heavy nitrogen users. And nitrogen is a key component of many alkaloids and other protective compounds manufactured by plants to reduce pest and disease damage.620
e>January 05, 2010
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Foy Update – Cook. Garden. Write. Repeat.I’m curious how you feel about all the plant stakes that are sold for woodies. Do you think they are effective ways to fertilize trees and shrubs?January 05, 2010
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MicahYes, I’d also like to hear about the tree stakes. I’ve got a couple young landscape trees and I was planning on using the fertilizer stakes.January 05, 2010
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Terry EttingerIt’s well documented that tree roots are not uniformly dispersed in an urban environment (or in a forest for that matter). Also, it’s not unusual for the business end of a large tree’s root system (i.e., the majority of “feeder” roots) to be many dozens if not over one hundred feet from the trunk. In my neighborhood, that means the feeder roots of my Acer rubrum might be three doors down the block!
Where I’m going with this is that it’s highly possible that a significant amount of any fertilizer you might broadcast, hydraulically inject or pound into the ground (as is the case with tree spikes) wouldn’t come physically close enough to feeder roots of a large tree to be taken up by its root system.
Also, all you have to do is take a walk through your neighborhood to see that most of the trees are doing quite well – and probably nearly 10
19e0
0 percent have never been fertilizer!
Finally, it’s also been well documented that urban and suburban soils are extremely compacted, resulting in tree root function being less than ideal. No amount of fertilizer is going to correct this soil physical trait – and it could be argued that a significant amount of fertilizer applied would not be taken up by a compromised root system.
I would be very interested to see any literature that reports on the long term effect of mature (ornamental) tree fertilization. Broadcast fertilization of timber stands may increase yield of board feet, but how does that relate to the “yield” of shade and coniferous trees in landscape/streetscape plantings?January 05, 2010
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Bert CreggIt’s true tree roots are not uniformly distributed but neither are nutrients in the soil. Forest ecology literature has demonstrated that trees are fairly effective in utilizing ‘patchy’ nutrients. It could be argued that limitations in root function by compaction in urban environments may increase, not decrease, the need for fertilziation.
With regard to board ft. yield, in my woody plant physiology class I emphasize that diameter growth is the single best indicator of tree vigor; whether we’re talking about landscape, nursery, or production forestry situations. Why? Because it integrates all physiological functions and the cambium is the weakest sink for photosynthates in the tree; therefore it is the most sensitive indicator of plant stress. Obviously, homeowners could care less about caliper growth but an increment borer can tell you a lot about what’s going on in a tree.January 05, 2010
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Terry EttingerConsidering that a typically sized residential lot ranges from roughly 5,000 square feet to maybe an acre (roughly 200 x 200 feet), and assuming the root system of a fairly mature tree might extend 50 to 100 feet in all directions from the trunk (though randomly dispersed), and assuming you have to exclude the space taken up by the house, garage and paved surfaces, are you suggesting that a homeowner essentially apply one to two pounds N over the entire yard every two to three years just for the tree(s)(including landscape beds where tree roots may actually be more concentrated because of less competition from turf, reduced soil compaction, more moderate soil temps and moisture levels under a layer of mulch, etc.), then more N for the turf? Also, what kind of fertilizer would you apply? A review of soil test results in New York suggests that most lawn, landscape & garden soils tested have more than sufficient levels of P and K. Maybe strategically located one foot deep, two inch diameter vertical mulching cores filled with coarse compost or calcined clay to improve root zone oxygen levels and thus root function might be a better strategy than a blanket application of fertilizer – at least some of which would never be taken up by tree roots?January 05, 2010
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Tim ClancyI am uncomfortable with the broad based recommendation of 1-2 lbs of N per 1000 sq ft. every 2-3 years because 1) It’s a prescription intended to remedy a specific “problem”. However in the absence of a specific diagnosis why are we prescribing anything? First things first here let’s confirm our diagnosis with some quantifiable evidence. Once we have the diagnosis then we prescribe treatment.
2) How did we arrive at a dosage of 1-2 lbs. nitrogen. 2lbs. is twice 1 lb. Why would I apply more than necessary or less than required? Again I think a diagnosis is in order prior to recommending any treatment.
3) Every 2-3 years? That’s a big difference as to treatment options. Why would I apply at 24 months rather than 36 months?
I would like to see more precision in the diagnosis and treatment. January 06, 2010
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Terry EttingerAmen, Tim! In a roundabout way, I was attempting to say what you said very well (in many, many fewer words)! In many instances I see fertilizers being recommended and applied as a cure-all when rarely is there a true soil nutrient deficiency. While there may be some visible response to the application in some instances, at best it’s often simply “masking” the symptoms of a more complex underlying condition. Again, Tim, very well done!January 06, 2010
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Linda Chalker-ScottThe problem with measuring soil nitrogen is that it is so transient. There is no way to tell with any precision exactly what the nitrogen content may be from month to month, or even day to day. However, I’ve noticed in the hundreds of soil tests that we’ve done in Seattle area sites is that all but overamended soils are low in nitrogen. I’m no expert in mineral nutrition, but the book I just published (Sustainable Landscapes and Gardens: good science, practical application) has an entire chapter on the subject by two PhDs in urban soil management. Their work indicates that nitrogen is commonly limiting in urban soils.January 11, 2010
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Bert CreggFollowing up on the N fertilization thread. My recommendation of 1-2 every few years is intended as a general guideline for professional landscapers and arborists with whom I deal. In an ideal world, fertlizer prescriptions would be based in a combination of visiable symptoms, soil, and foliar testing. Yet all of these methods, including foliar testing, have limitations. Landscapers may be dealing with beds that include a variety of shrubs, trees, and perenials. Even if they go the expense of run a folair analysis on each, how do they interpret the results? Oftne only very broad guidelines are available for comparison. More information is always preferable than less but it doesn’t change the fact that tree need N. September 07, 2010
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jimAs a websitedeveloper I can say that your page was pretty informative.Thanks for the insights
I’m curious how you feel about all the plant stakes that are sold for woodies. Do you think they are effective ways to fertilize trees and shrubs?
Yes, I’d also like to hear about the tree stakes. I’ve got a couple young landscape trees and I was planning on using the fertilizer stakes.
It’s well documented that tree roots are not uniformly dispersed in an urban environment (or in a forest for that matter). Also, it’s not unusual for the business end of a large tree’s root system (i.e., the majority of “feeder” roots) to be many dozens if not over one hundred feet from the trunk. In my neighborhood, that means the feeder roots of my Acer rubrum might be three doors down the block!
Where I’m going with this is that it’s highly possible that a significant amount of any fertilizer you might broadcast, hydraulically inject or pound into the ground (as is the case with tree spikes) wouldn’t come physically close enough to feeder roots of a large tree to be taken up by its root system.
Also, all you have to do is take a walk through your neighborhood to see that most of the trees are doing quite well – and probably nearly 10
19e0
0 percent have never been fertilizer!
Finally, it’s also been well documented that urban and suburban soils are extremely compacted, resulting in tree root function being less than ideal. No amount of fertilizer is going to correct this soil physical trait – and it could be argued that a significant amount of fertilizer applied would not be taken up by a compromised root system.
I would be very interested to see any literature that reports on the long term effect of mature (ornamental) tree fertilization. Broadcast fertilization of timber stands may increase yield of board feet, but how does that relate to the “yield” of shade and coniferous trees in landscape/streetscape plantings?
It’s true tree roots are not uniformly distributed but neither are nutrients in the soil. Forest ecology literature has demonstrated that trees are fairly effective in utilizing ‘patchy’ nutrients. It could be argued that limitations in root function by compaction in urban environments may increase, not decrease, the need for fertilziation.
With regard to board ft. yield, in my woody plant physiology class I emphasize that diameter growth is the single best indicator of tree vigor; whether we’re talking about landscape, nursery, or production forestry situations. Why? Because it integrates all physiological functions and the cambium is the weakest sink for photosynthates in the tree; therefore it is the most sensitive indicator of plant stress. Obviously, homeowners could care less about caliper growth but an increment borer can tell you a lot about what’s going on in a tree.
Considering that a typically sized residential lot ranges from roughly 5,000 square feet to maybe an acre (roughly 200 x 200 feet), and assuming the root system of a fairly mature tree might extend 50 to 100 feet in all directions from the trunk (though randomly dispersed), and assuming you have to exclude the space taken up by the house, garage and paved surfaces, are you suggesting that a homeowner essentially apply one to two pounds N over the entire yard every two to three years just for the tree(s)(including landscape beds where tree roots may actually be more concentrated because of less competition from turf, reduced soil compaction, more moderate soil temps and moisture levels under a layer of mulch, etc.), then more N for the turf? Also, what kind of fertilizer would you apply? A review of soil test results in New York suggests that most lawn, landscape & garden soils tested have more than sufficient levels of P and K. Maybe strategically located one foot deep, two inch diameter vertical mulching cores filled with coarse compost or calcined clay to improve root zone oxygen levels and thus root function might be a better strategy than a blanket application of fertilizer – at least some of which would never be taken up by tree roots?
I am uncomfortable with the broad based recommendation of 1-2 lbs of N per 1000 sq ft. every 2-3 years because 1) It’s a prescription intended to remedy a specific “problem”. However in the absence of a specific diagnosis why are we prescribing anything? First things first here let’s confirm our diagnosis with some quantifiable evidence. Once we have the diagnosis then we prescribe treatment.
2) How did we arrive at a dosage of 1-2 lbs. nitrogen. 2lbs. is twice 1 lb. Why would I apply more than necessary or less than required? Again I think a diagnosis is in order prior to recommending any treatment.
3) Every 2-3 years? That’s a big difference as to treatment options. Why would I apply at 24 months rather than 36 months?
I would like to see more precision in the diagnosis and treatment.
Amen, Tim! In a roundabout way, I was attempting to say what you said very well (in many, many fewer words)! In many instances I see fertilizers being recommended and applied as a cure-all when rarely is there a true soil nutrient deficiency. While there may be some visible response to the application in some instances, at best it’s often simply “masking” the symptoms of a more complex underlying condition. Again, Tim, very well done!
The problem with measuring soil nitrogen is that it is so transient. There is no way to tell with any precision exactly what the nitrogen content may be from month to month, or even day to day. However, I’ve noticed in the hundreds of soil tests that we’ve done in Seattle area sites is that all but overamended soils are low in nitrogen. I’m no expert in mineral nutrition, but the book I just published (Sustainable Landscapes and Gardens: good science, practical application) has an entire chapter on the subject by two PhDs in urban soil management. Their work indicates that nitrogen is commonly limiting in urban soils.
Following up on the N fertilization thread. My recommendation of 1-2 every few years is intended as a general guideline for professional landscapers and arborists with whom I deal. In an ideal world, fertlizer prescriptions would be based in a combination of visiable symptoms, soil, and foliar testing. Yet all of these methods, including foliar testing, have limitations. Landscapers may be dealing with beds that include a variety of shrubs, trees, and perenials. Even if they go the expense of run a folair analysis on each, how do they interpret the results? Oftne only very broad guidelines are available for comparison. More information is always preferable than less but it doesn’t change the fact that tree need N.
As a websitedeveloper I can say that your page was pretty informative.Thanks for the insights