Here is what the experts say:
Soils and bark amendments added:
Q: Recently, I read that wood mulches can deplete the soil of nitrogen as they break down. Is this true? If so, what can I add to the soil to compensate for this nitrogen depletion? Or is there another kind of mulch you would recommend that is readily available? Would it be OK to mulch using the fallen leaves of live oak trees? -- Karen Shaw
A: "The nitrogen is not actually depleted, it's temporarily tied up. This can happen when organic materials low in nitrogen, such as shredded wood chips or pine bark, are incorporated into the soil. The microorganisms that utilize the low-nitrogen organic matter as food (decaying it) must obtain the nitrogen they need from somewhere else. They take nitrogen from the soil and, in doing so, tie it up in their bodies as their populations surge. When the organic matter is largely decayed, and the food runs out, the microorganisms begin to die. This returns the tied-up nitrogen back to the soil, with the added benefit of decayed organic matter. The problem arises during the decomposition phase. At that time, so much nitrogen may be tied up in the bodies of microorganisms that plants are unable to obtain all they need to be healthy. This leads to pale leaves and stunted growth, symptoms of nitrogen deficiency.
To keep this from happening, the gardener can do one of two things. First, the organic matter can be composted before use. That way the decomposition process has already occurred, and the addition of the material will not lead to tied-up nitrogen. Or the gardener could simply add a nitrogen-containing fertilizer at the time the organic matter is added. That way, there will be enough nitrogen to satisfy the needs of the microorganisms and the plants. Granular general purpose fertilizers, such as 15-5-10, or organic fertilizers like blood meal or cotton seed meal would be fine.
Now, for your situation, you can simply ignore everything I just said. That information applies when you incorporate organic matter into the soil. When you use low-nitrogen organic materials for mulch, such as cypress mulch, wood mulch, pine bark mulch, pine straw or fallen leaves (oak leaves make an excellent mulch), nitrogen tie-up is not an issue. The organic matter is applied on top of the soil and interacts with only the very surface of the soil. It decomposes slowly at the surface without affecting available nitrogen levels down in the bed."
All of this may sound mute to some, but it does bother me a bit that the soils are being made and sold with incorrect materials in them...why not just use composted bark to begin with? It might also be a concern that the nitrogen in the soil is being tied up for several years and then it is released and you are still fertilizing and so you trees get a double whammy of nitrogen or something. I would think we would all like to have a more controlled situation than that. I'll talk to Jack.