I have been told for many years that adding brushwood and logs to soil depletes it of nitrogen so should be avoided. I also avoided adding it to compost heaps because it looked dry and decomposed so slowly.
It seems reasonable to conclude that adding lots of carbon in the form of wood does reduce nitrogen but most people forget that adding nitrogen to the soil reduces carbon. Furthermore, adding air to the soil increases the activity of aerobic bacteria and fungi and reduces both nitrogen and carbon. Both these elements can leave the soil as greenhouse gas molecules.
I have been criticised in the past because burying logs and brushwood produces carbon dioxide and methane; both greenhouse gases. Although this does occur, it is a much slower process than burning it and some of the carbon could be sequestered in the soil for many years.
As logs decay, they form a very sponge like crumbly structure that can absorb and retain water. While there is a slow release of trapped water within the wood, there is also an increase in the soil's ability to drain relatively quickly and avoid waterlogging. Those with limited rainfall can make hugelkultur swales to retain more water, while people like me that have too much water in their soil can increase the drainage in a similar way.
Together with this water, bacteria and fungi can use carbon and any nitrogen they can glean from the wood and soil to grow and respire. When they die they will release molecules that the plants can absorb and so these elements will be recycled.
To overcome the problem of nitrogen loss, I bury the logs and brushwood at least two and a half feet (three spits) deep in the soil by taking out trenches. Any nitrogen that is available at the depth of my trenches has been leached out of the top soil and is much less available to plant roots so the bacteria and fungi are welcome to it. Nitrogen capture and retention might be another benefit of hugelkultur.
The organic matter that is added to a hugelkultur trench will enable plant nutrients to remain in the soil rather than being leached away. Organic matter and clays have a relatively high cation exchange capacity and this can be used by the plants to obtain nutrients.
I take the opportunity to bury a lot of pernicious weeds like couch grass, Elymus repens, and nettles Urtica dioica and in order to add more nitrogen to the mix, I add lots of grass mowings; animal manures; shredded clippings; weed turfs and any other compostable material I have to hand.
I like to sieve the soil back into the trench to remove large stones and rhizomes of plants like bindweed, Calystegia sepium and Equisetum arvensis. As the soil is sieved back into the trench compost or animal manures can be added. I would suggest that this hugelkultur and sieving procedure produces a deep fertile soil that conventional digging or no digging methods would take ten years to produce.
|My trusty bread tray sieve.|
I have begun to get more interested in developing my skills in making hot beds and, reading the Victorian gardeners handbooks, they used tanner's bark because it decomposed slowly and kept its heat longer than animal manures..
So, covering the logs and brushwood with soil and then building an animal manure hot bed on top might warm the soil for a number of months due to the heat produced by a multitude of microorganisms respiring while decomposing the buried litter. This is what I aim to do, so that I can continue to grow salad plants in the autumn.
Burying the logs and brushwood is easy. Making the hot bed work is much more difficult.