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.