Bryant RedHawk wrote:Wood, when it comes in contact with soil, activates mechanisms in certain bacteria which then consume nitrogen as food so they can decompose the wood.
If this wood is sitting on top of the soil (fallen tree, dropped tree branch and so on) then these mighty bacteria will become active, but only to a depth of around 5mm.
This means that the answer to the nitrogen "robbing" wood is only involved in a minute area under and around the piece of wood.
Nitrogen is robbed from the soil due to composting woody debris. This generally follows along a 7 year rotation, where the first 7 years, the soil in trying to break down this wood and robs the soil of that ellusive nitrogen, but after 7 years, that nitrogen is given back into the soil.
Nitrogen in a real forest is nitrogen-nuetral because the soil was once 100% wood material. The top most layer is recent fallen leaves, twigs, sticks, etc and has hardly broken down, yet the deeper a person goes it can be readily seen that the wood is already broken down. Because of this constant rotation of new layer of wood to break down, and wood already broken down and giving back to the soil, nitrogen in the soil is both taking and giving. The reason ferns and other nitrogen loving plants thrive is because their roots are down in the already broken down woody debris. Considering it takes 600 years to make just 1 inch of topsoil, 7 years is but a day eqivilent in a forest lifecyle. Their roots are feeding off the nitrogen that is being RELEASED back into the soil, not last years fallen leaves.
Now when I clear a forest and put it into a field, is disrupts everything. Suddenly there is no more layers of soil. I am mixing freshly fallen debris into the soil. Because of the size of wood and the freshness therof, I experience a lot of nitrogen robbing. This shows up in my crops by stunting their growth. To overcome that I have to apply twice as much manure in freshly converted lands, but that over-application gets me beyond the nitrogen that is being robbed. Thankfully after about 7 years the nitrogen reverts back into the soil and for 7 years or so, I get to reduce my manure reirements to get the same yield.
If a person uses wood chips to spread out on their garden they are going to get the same thing. If it is tilled in deep, then the nitrogen robbing will be more accute, but if it is spread on the top of the soil, only the soil it comes in contact with will be robbed of nitrogen. In that case the benefit of wood chips is not fully realized; in that case the wood chips are being used to protect the soil from erosiion, to keep the soil moist, and to help thermally protect the soil from heat and cold. Those are all lofty results. So I am not saying they should not do it, they should, especially if their organic matter is way down.
Myself, I think putting in more wood chips, tilling it in, then adding more manure to compensate for the nitrogen loss is a much better plan. The wood chips will areate the soil, get more organic matter into your soil, and provide all the other benefits too, and what garden does not benefit from more manure?