If you want to better understand trees and the nutrition necessary for prime health and proper growth, Davey expert and ISA Board Certified Master Arborist R.J. Laverne suggests you take a walk in the forest.
"On the forest floor, we can see there's a lot of organic material on the top layer of soil - broken up leaves and twigs from last year," he explains, picking up a chunk of the soil in his hands. "Going through the soil profile, we see there's little and smaller bits of organic material and lots of organisms in the soil like earthworms and nematodes. Their job is to break up organic materials that accumulate on the ground and incorporate them back into the forest soil."
As Laverne squeezes the forest soil, he describes the texture as "spongy," which is the result of those organisms that help infuse oxygen into the soil. This makes the soil perfect for "absorbing rainfall and allowing oxygen to get to the roots, which is very important for trees," he says.
Trees planted in urban or developed environments, such as those in a typical home landscape, are often exposed to stresses not usually found in woodlands or natural environments. Raking leaves and removing topsoil on construction sites eliminates natural sources of plant nutrients: decaying organic debris. The nutrients from this debris keep trees in the wild from needing any additional fertilization, but trees in cultivated areas often need extra attention.
This is why landscape trees need to be fertilized on at least an annual basis - to give them the nutrients they're lacking compared to their counterparts in the forest environment.
Nitrogen, phosphorus and potassium are the primary nutrients found in fertilizers. A fertilizer's grade lists the levels of each of these three nutrients in N-P-K order on product packaging. "The best combination for N, P and K is not much different than what we find in the natural forest setting," Laverne says. "The right combination should provide nutrients to landscape trees and help to feed the microorganisms in the soil that decompose organic materials and make for healthier soil."
With proper maintenance and some regular fertilization, Laverne says "you can have landscape trees that will be valuable components to your landscape and stay healthy for a long time."