When used as a mulch or top dressing, wood chips or any other woody product (bark, sawdust, etc.) does NOT deplete or tie up nitrogen. That only happens when the wood product is incorporated or mixed into the soil and the process of natural decomposition starts. It is this process of decomposition that ties up N on a temporary basis.

I'm not sure what you have read but it is not accurate :-)) When used just as surface application there is no tie up or depletion of N except perhaps for a very slight surface issue where the mulch meets the soil. And that is not enough of a issue to be of any concern to 95% of the plants you may be growing in a container.

N tends to be the one primary plant nutrient that is often lacking due to its extreme mobility in the environment so is often supplemented, regardless if you are planting in the ground or in a container, with or without any mulch. And in containers, routine supplementation of plant nutrients via fertilization is very necessary, as container or potting soil is typically devoid of any plant nutrients. And the need for frequent watering - even daily or twice daily in hot weather - will leach away any existing nutrients that might be lingering. These nutrients must be replenished for the plants to thrive and be productive so regular fertilizing - "weekly, weakly" - is the watch word for container gardening.

I have gardened extensively in containers for over 30 years and many of mine are very large to accommodate trees but I have never mulched a container. And my pots tend to be planted fully enough that very little container soil is exposed. Since just a small portion of the container soil is protected from heat with a mulch anyway, I am not convinced how much benefit it provides. But then I do not garden on a rooftop in full all day blazing sun :-))

Go ahead and apply your mulch without concern but make sure you are fertilizing properly as well!

Thank you Gardengal, that's very good news!! 

I am also a big fan of the weekly weakly schedule, that really seems to be the best for potted plants.

I have to mulch because the rooftop turns into a frying pan every summer. I also insulate the pots from the bottom and sides to try to keep the roots as cool as possible. I'm so glad to hear the wood chips won't have any detrimental effect on the nitrogen levels. Thanks again!

Wood chips do not “tie up nitrogen”. The reason that the soil suffers when partially decomposed wood is incorporated into soil is that the wood chip are partially decomposed and contain short-chain organic acids. See ’Compost Utilization in Horticultural Cropping Systems’, pp 113; ”Short-chain organic acids resulting from decomposition of organic matter can inhibit or reduce seed germination and root growth. Organic acids responsible for growth inhibition include acetic, butyric, propionic and valeric acids…”. Short answer; use fully composted matter in the top inch of soil, use wood chips as a mulch over the soil, keep raw mulch away from growing plants. Good luck!

What Pam (Gardengal) said.

Al