Always fun when you find a research paper that confirms what you’ve suspected all along. I ran across a paper last week in the Annals of Applied Biology entitled ‘Fertilisers and insect herbivores: a meta-analysis’ (Butler et al. 2012. Ann Appl Biol 161: 223–233). I’m interested in the topic because in recent years a dogma has emerged that if you fertilize a landscape tree it will be immediately devoured by insects. In this study the authors conducted a meta-analysis (basically a compilation of studies on a given topic and then combining and analyzing the aggregated results) and looked at dozens of studies of the response of insect herbivores to fertilization to answer the question, does fertilization increase insect damage? The answer was absolutely no surprise to me: It depends.
What does it depend on? First, what type of insect. Secondly, what kind of fertilizer. For example, fertilizing with nitrogen greatly increases populations of sucking insects. This makes sense when you stop to think that aphids and other sucking insects have to consume a lot of phloem sap –which is essentially sugar water – in order to get sufficient nutrients. Nitrogen fertilization did not significantly increase populations of chewing insects, however. This could be related to off-setting effects of improved nutritional quality of leaves versus increased presence of defense compounds or leaf toughness. For other fertilizer elements Butler et al. found that phosphorus decreased insect populations in 2/3rd of the studies (14 out of 21) and that potassium decreased insects in 7 out of 10 cases. As with nitrogen only, complete fertilizers (NPK) tended to increase insect populations, especially for sucking insects.
I should hasten to point out some limitations of the study as it relates to tree fertilization. First, of course, is the British spelling of fertilizer. Second, the study mainly dealt with fertilization in agronomic crops, not trees. Lastly, the authors only included studies on insect adults. In many cases insect larvae, not adults, are the most damaging life stage, especially for insects that affect trees. Nevertheless, the study highlights the difficulty of making generalizations when discussing host stress and insect interactions. In addition to type of insect and type of fertilizer, we could have added nutritional status of the plant before fertilization to the ‘It depends’ list. My rule of thumb is that trees shouldn’t be fertilized unless a problems is noted by visible symptoms, a soil test, and/or a foliar test – and preferably by more than one of these.
Bottom line: Before you buy into the notion that fertilizing a tree is going to increase insect problems make sure you know what type of pest you’re dealing with, what type of fertilizer and the current nutrient status of the tree.
Great article. Thanks for sharing it.
May I beg if we could please publish it in our next ROSA Annual to share with about 800 rose lovers in South Africa?
Kind regards
Stefanie Seydack
“First, of course, is the British spelling of fertilizer”
LOL!
Thanks for another great article.
Stefanie: By all means. As long as there is attribution to the Garden Professors blog, I consider everything here fair game.te>December 04, 2012
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Bert CreggSylvia: Most scientists aspire to be Steven Jay Gould; my role model is Dave Barry!December 04, 2012
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Karen JeannetteWe have a question from our Extension Master Gardener Facebook page, perhaps you could address?
“I have heard that if you fertilize your plants, the mycelium that would normally help them take in nutrients is hindered. I’m feeling very conflicted about fertilizers at the moment.”December 04, 2012
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Linda Chalker-ScottIt’s certainly true that mycorrhizae do not grow well under high nutrient conditions, especially phosphorus. As Bert says, fertilizer should be added only when indicated soil tests and/or foliar symptoms suggest a deficiency.December 04, 2012
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Terry EttingerIt’s also important to remember that soil nutrient status is only half the issue (if not less than half), as soil physical conditions often dictate the availability – or not – of existing or applied essential elements. Likewise, soil physical conditions largely dictate the level of activity and diversity of soil microorganisms that play a role in nutrient availability. The bottom line is that in all my 30+ years as an extension agent, professional landscape/arboriculture consultant, and radio/TV host, I can’t ever recall once making a recommendation to fertilize anything in a landscape planting – except maybe bedding plants at installation and on a regular schedule throughout the growing season in very high-end settings. The reason for this is that I think it’s impossible to peg poor performance of landscape plantings specifically on soil nutrient status at a given moment in time – i.e., when a soil sample is collected.December 05, 2012
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Bert CreggKaren:
I concur with Linda’s comment. The main issue with fertilization and mycorrhizae is high phosphorus levels depressing mycorrhizal colonization by endo-mycorrhizae. for a list of ecto- vs endo plants go to: http://www.inoq.de/download_Gehoelzliste_e.pdfDecember 05, 2012
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Bert CreggTerry:
I agree and disagree. How’s that for being wishy washy?! I wrote the decision tree that produces the tree and shrub fertilizer prescriptions for the MSU Soil Test Self-mailer program. http://www.msusoiltest.com/ Homeowners buy the self-mailer, collect a soil sample, send it to the MSU soils lab and get a computer generated output based on the results. The default is always ‘Do not fertilize’. If the homeowner is dealing with a chlorotic red maple, pin oak, rhody or azalea the program recommends elemental sulfur based on soil pH. For other trees of other species that are chlorotic the program will recommend a low rate N and addition of other elements based on soil test. The program was launched this past year so we haven’t had a chance to evaluate yet. My expectation is 60-70% will come back “Do not fertilize’, 20% will come back ‘Adjust pH’ and 10-20% will need fertilization. Here’s the bulletin I wrote that homeowners receive with their results. http://www.msusoiltest.com/files/trees_shrubs_nutrition_and_fertilization.pdf
December 06, 2012
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Terry EttingerNot to beat a dead horse, but . . . precisely my point, Bert. Your estimate of 60-70% of test results will recommend no fertilizer, plus 20% “adjust pH” (which isn’t adding an essential element so much as altering the form of the existing elements in the soil to make them more readily taken up by plant roots) suggests that at least 80-90% of all soils are capable of providing adequate levels of essential elements. Then, add in a few percentage points for poor soil physical conditions, a few more for poor planting practices, one or two for “failure to thrive” because we’re working living organisms, and a couple for improper siting (e.g., attempting to grow rhododendrons and azaleas in calcareous soils), etc., and the end result is that very, very, very rarely will the application of fertilizer to landscape plantings have any true benefit over doing nothing! Plus, I didn’t include the fact that root systems are not uniformly distributed throughout the soil, so some percentage of the nutrients in any fertilizer applied will never come close enough to roots to be taken up? (By the way, Bert, you probably are already aware of this, but your 60-70% “do not fertilize” estimate is strongly supported by the extensive analysis of thousands of soil tests submitted to the Cornell Nutrient Analysis Lab between 1995 and 2006 (http://nmsp.cals.cornell.edu/publications/soilsummary.html).
Sylvia: Most scientists aspire to be Steven Jay Gould; my role model is Dave Barry!
We have a question from our Extension Master Gardener Facebook page, perhaps you could address?
“I have heard that if you fertilize your plants, the mycelium that would normally help them take in nutrients is hindered. I’m feeling very conflicted about fertilizers at the moment.”
It’s certainly true that mycorrhizae do not grow well under high nutrient conditions, especially phosphorus. As Bert says, fertilizer should be added only when indicated soil tests and/or foliar symptoms suggest a deficiency.
It’s also important to remember that soil nutrient status is only half the issue (if not less than half), as soil physical conditions often dictate the availability – or not – of existing or applied essential elements. Likewise, soil physical conditions largely dictate the level of activity and diversity of soil microorganisms that play a role in nutrient availability. The bottom line is that in all my 30+ years as an extension agent, professional landscape/arboriculture consultant, and radio/TV host, I can’t ever recall once making a recommendation to fertilize anything in a landscape planting – except maybe bedding plants at installation and on a regular schedule throughout the growing season in very high-end settings. The reason for this is that I think it’s impossible to peg poor performance of landscape plantings specifically on soil nutrient status at a given moment in time – i.e., when a soil sample is collected.
Karen:
I concur with Linda’s comment. The main issue with fertilization and mycorrhizae is high phosphorus levels depressing mycorrhizal colonization by endo-mycorrhizae. for a list of ecto- vs endo plants go to: http://www.inoq.de/download_Gehoelzliste_e.pdf
Terry:
I agree and disagree. How’s that for being wishy washy?! I wrote the decision tree that produces the tree and shrub fertilizer prescriptions for the MSU Soil Test Self-mailer program. http://www.msusoiltest.com/ Homeowners buy the self-mailer, collect a soil sample, send it to the MSU soils lab and get a computer generated output based on the results. The default is always ‘Do not fertilize’. If the homeowner is dealing with a chlorotic red maple, pin oak, rhody or azalea the program recommends elemental sulfur based on soil pH. For other trees of other species that are chlorotic the program will recommend a low rate N and addition of other elements based on soil test. The program was launched this past year so we haven’t had a chance to evaluate yet. My expectation is 60-70% will come back “Do not fertilize’, 20% will come back ‘Adjust pH’ and 10-20% will need fertilization. Here’s the bulletin I wrote that homeowners receive with their results. http://www.msusoiltest.com/files/trees_shrubs_nutrition_and_fertilization.pdf
Not to beat a dead horse, but . . . precisely my point, Bert. Your estimate of 60-70% of test results will recommend no fertilizer, plus 20% “adjust pH” (which isn’t adding an essential element so much as altering the form of the existing elements in the soil to make them more readily taken up by plant roots) suggests that at least 80-90% of all soils are capable of providing adequate levels of essential elements. Then, add in a few percentage points for poor soil physical conditions, a few more for poor planting practices, one or two for “failure to thrive” because we’re working living organisms, and a couple for improper siting (e.g., attempting to grow rhododendrons and azaleas in calcareous soils), etc., and the end result is that very, very, very rarely will the application of fertilizer to landscape plantings have any true benefit over doing nothing! Plus, I didn’t include the fact that root systems are not uniformly distributed throughout the soil, so some percentage of the nutrients in any fertilizer applied will never come close enough to roots to be taken up? (By the way, Bert, you probably are already aware of this, but your 60-70% “do not fertilize” estimate is strongly supported by the extensive analysis of thousands of soil tests submitted to the Cornell Nutrient Analysis Lab between 1995 and 2006 (http://nmsp.cals.cornell.edu/publications/soilsummary.html).