Following placing 26 kg of compost around a tree, with mulch, at the beginning and end of spring, can I top the two layers with 26 kg of compost around a European Beech tree again. I imagine it eventually breaks down, but is too much of anything possibly going to smother the tree system? I’m referring to the response below, and live in west end Toronto.
Thank-you for contacting Toronto Master Gardeners with your follow-up question about compost and mulch around your European beech tree.
If you were to apply additional compost around your tree, the procedure would be to scrape back the mulch, add the compost layer, and then re-apply the mulch on top. The ingredients of the compost should be adjacent to the soil in order to be readily accessible to the tree roots. The purpose of the mulch, which will decompose very slowly, is so provide a protective layer on top of soil, retaining moisture and suppressing weeds.
While the issue, however, is not really suffocating the roots, too much compost is not a good thing. An increase in nutrients can have negative effects on your soil, alter the pH and prevent the plant from accessing key elements in the soil. Increased levels of potassium, in particular, can be a problem. Manure-based compost is especially problematic, but even plant-based compost can lead to high levels of this element. The best way to determine whether your soil needs additives of any kind is to have it tested, but if you have added a good layer of compost as recently as the spring, it is unlikely the soil around your tree requires more.
Please refer to these articles on the issues of over-use of compost and to the link to soil testing resources in Ontario:
University of Minnisota article on excessive amounts of compost
Garden Myths on Too much compost