In early December, I posted my thoughts about fertilizing crops vs. landscapes. An anonymous reader asked if we could follow up by discussing the relationship between excessive fertilizers and plant susceptibility to pests and disease. It’s taken a month to get the scientific literature (and my act) together, but here it is.
There are decades’ worth of articles about the direct relationship between increased nutrient availability and increased susceptibility to pests, disease, and disorders. One of the earliest articles linked the incidence of celery blackheart to over-fertilization. Since that time, researchers have found similar causal relationships in vegetable crops such as rice, onions, and soybeans, ornamental crops including poppies, and perennial orchard crops such as nectarines. Unfortunately, there’s been no research on landscape species.
Happily, the way plants react to excess nutrient levels is generic – so we can apply the findings in the agricultural literature to landscape situations. Just like kids and candy, plants will greedily take up all the available macronutrients their roots can find, especially nitrogen and phosphorus. (It makes NO difference is the fertilizer is organic or inorganic.) Plants in highly nutritive soils respond with lush vegetative growth – and fewer flowers, by way. Less metabolic energy is put into protective compounds, so these succulent new leaves and shoots are prime targets for all kinds of unwanted plant-eaters and foliar pathogens.
As with so many things in life, moderation is the key. For routine landscape needs, use woody mulches rather than fertilizers and nitrogen-rich composts. This “slow food” approach not only benefits your plants, but provides ideal habitat for mycorrhizal species, which have been shown to help restrict root uptake of excessive nutrients, while assisting with uptake of less available ones.
I never knew mycorrhiza helped to restrict uptake of excessive nutrients. You really do learn something new every day.
In native vegetation areas in Australia excess nutrient loading is one of the fundamental issues in ecosystem management, especially with regards to invasive exotic grasses. These grasses usually have short life cycles than their native counterparts, and hence are usually more vigorous to boot. Once they begin to colonise an area, the thatch they leave adds excess nutrients to the soil which build up over time to make the soil more and more fertile. It is then that you see larger shrub or woody weeds encroach into previously high quality bush areas – they otherwise would not have grown their because most in Australia are relatively nutrient deprived and inhospitable to non-local plants.
The Davies lab at Texas A&M has done some work with this on chrysanthemum. Research articles are available online:
http://aggie-horticult
ure.tamu.edu/faculty/davies/research/abstracts/stressabstract.html
Really interesting post, and Jimbo, your comment is a great addition, too. Every living thing is part of some system, and Juicing up a plant with an overabundance of nutrients affects every other part of the system(s) in which that plant lives, for better or for worse. Perhaps that’s ok on a very short-term basis, but as a sustained practice not so good. This three-dimensional view (or four, if you add time in) of horticulture is one of the reasons this blog is so excellent! Thanks.
Charlie, thanks for the link.
I’d also love a reference or some more “further reading” for the bit about less energy going to protective compounds in nitrogen and phosphorous rich soils
Grrrrrrrr! Enough with comparing fertilizer to “food” already! Unless I completely misunderstood the section on photosynthesis in my high school biology class, plants make their own food (simple sugars) through the process of photosynthesis, right? Therefore, fertilizer in no way, shape or form is “plant food!”
Plants consume the components of fertilizers…. nutrients…. and use them to conduct metabolic functions. Thats food in my book.
I use food or nutrients to make fat, which I do too occasionly burn for energy… my own body fat is not food. (at least not for me)… but it is stored energy… just as sugars and starchs are stored energy for plants.
I really enjoyed this column. Because I manage home and estate orchards for a living, nutrient management is something I think about a lot.
Have you looked into the affect of juicing up mature trees (which gives tree care companies something to do on rainy days)? I suspect this practice speeds their death- not just because of insect and disease susceptibility but also due to the excessive growth affecting the architectural integrity of the trees. Trees routinely grow themselves to death even without the extra stimulation.
I believe that one result of excess N is larger individual cells with thinner walls and more water. I’ve always suspected that this is connected to their greater vulnerability to insects and disease and also why over fertilized produce loses flavor. Do you have any info or opinion on this?
I totally agree with your observation,even farmers like me get used to of (fast food ) chemical fertilizers for their wine yards