The Facts about Hugelkultur and Nitrogen Immobilization

Hugelkultur is an old German and Eastern European soil-building technique that involves burying logs and growing plants over them while they decompose. Many growers have one major concern when they first learn about it:

Will the decomposition of wood in hugelkultur rob the soil of nitrogen and starve the plants?

Nitrogen immobilization technically happens in hugelkultur, but not to a degree that will cause problems for the grower. The decomposition that takes place in a hugelbeet (a bed created through the process of hugelkultur) is dominated by fungi, which are more efficient with nitrogen than bacteria are. What’s more, the fungi will only pull nitrogen from the soil immediately surrounding the wood, and that nitrogen is only locked up for a short period before it begins to be released. 

Why do people worry about nitrogen immobilization?

Growers have long been warned that burying wood chips or sawdust will cause them to “rob” the soil of nitrogen. While both are great mulches above ground, putting their entire surface area into full contact with the topsoil provides a buffet for the type of bacteria that process the wood’s cellulose and hemi-cellulose. Because those bacteria also need nitrogen—something the wood provides very little of—their expanding population pulls it from the surrounding soil. We sometimes describe that nitrogen as being “bound up” in the process of decomposition. 

Some studies show that adding a nitrogen source like blood meal will offset this effect, although others disagree. One thing all sources agree on is that time—a season or less—will remedy the problem.

This all seems fairly clear-cut, but the conversation has been missing two important pieces. First, we need a clear understanding of the differences in breaking down tiny fragments of wood compared to large pieces of it. Second, we need an understanding of how the process of decomposition changes under a no-till practice like hugelkultur. While research on hugelkultur itself is painfully lacking, data from related studies tells us that both of these factors point to one significant change: the dominance of soil fungi over soil bacteria.

Fungal Decomposition vs. Bacterial Decomposition

The scenario discussed above (in which burying wood chips or sawdust causes nitrogen to be bound up in the bodies of decomposers) is based on our understanding of bacterial decomposers. This makes sense, since until recently nearly all studies focused on cropland that had been tilled regularly. Tilling destroys fungi’s root-like hyphae, guaranteeing that bacteria will become dominant. In a hugelkultur system, the absence of soil disturbance encourages a thriving population of fungi. The presence of lots of woody material also encourages fungi—and, inferring from a 2007 study in Plant Soil by van der wal et al., the size of that woody material may make all the difference.

In the study, helpfully titled “Initial decay of woody fragments in soil is influenced by size, vertical position, nitrogen availability and soil origin”, the researchers  were interested in the factors that influence how wood decomposition is initiated, so they buried or placed small birchwood blocks (3 × 0.5 × 0.5 cm) and sawdust in various situations. One of their findings was that bacteria only played a major role with the decomposition of the sawdust. The larger pieces were dominated by fungi instead. Interestingly, the rate of decomposition was also higher in the wood blocks than the sawdust, something that should be encouraging to anyone using hugelkultur for its traditional purpose of creating soil. 

But what about nitrogen? That’s what we’re here to discuss, and it is something that interested the researchers as well. They studied how much nitrogen was “bound up” in the process of decomposition by measuring how much was present in the wood samples at intervals of 15, 25, and 40 weeks. Their results, which showed increasing levels of nitrogen in the wood over that time, did confirm that the fungi required the presence of nitrogen in the soil to initiate the process of decomposition. 

The researchers also wondered if adding nitrogen—something growers are often encouraged to do with hugelkultur—would offset the effects of nitrogen immobilization, so they saturated some of their samples with a nitrogen fertilizer. This did encourage fungal growth at first. In fact, it encouraged it so much that the fungi pulled more nitrogen from the soil. But after 40 weeks the samples with and without added nitrogen were indistinguishable. 

We should always be careful not to infer too broadly from studies that are meant to examine a few specific details. In this case, we need to keep in mind that the researchers only monitored the first 40 weeks of decomposition, before any rot fungi had even begun breaking down the lignin in the wood. Also, the observations I have noted above all come from their samples in abandoned cropland soil. Another set of samples on heathland soil had different results, possibly because of acidity or because of a compound in heather that represses bacteria and fungi.  

Still, if asked to jump to reasonable conclusions based on this data, I would say this: if your goal is to break down as much wood as possible in your first season, buried wood with added nitrogen seems like the fastest route to decomposition (at least in terms of cellulose and hemi-cellulose). If you are mostly concerned about whether your crops will be deficient in nutrients, skip the added nitrogen and simply give your plants enough well-balanced soil on top to keep them busy for the first season without seeking nutrients among the buried logs. 

Are Plants Grown in Hugelkultur Nutrient Deficient? 

The unfortunate truth about hugelkultur is that scientific studies and formal side-by-side trials are currently lacking. One student project from the University of Wisconsin does confirm that hugelkultur does not result in plant nitrogen deficiencies, but author Axel Adams notes that more targeted research is needed in that area, as well as a longer trial to make sure roots have a chance to reach past the layer of added soil. 

However, the weaknesses of that study actually reveal important information for growers. Adams was surprised to find that his plants had no nitrogen deficiencies until he realized that the leaf mulch he used under his topsoil had a carbon-to-nitrogen ratio comparable to that of a mature soil. Given that, it makes perfect sense that his plants had access to all the nutrients they needed, in spite of the high-carbon woody matter decomposing beneath the leaf mulch. 

Adams’ experience is, I would argue, typical of most people who try hugelkultur. With layers of topsoil and mulch at the top of the mound, there is no reason to fear the effects of early decomposition in the wood below. By the time any roots penetrate that far, The decomposition will have reached a state in which nitrogen immobilization is no longer a significant problem. 

Nitrogen in a hugelbeet’s first year

It’s common to see lists of plants that are and are not recommended for the first few seasons of a hugelbeet, based largely on their nitrogen needs. In my experience, these lists are stricter than they need to be. Strawberries love nitrogen, but they have shallow root systems. With a few inches of topsoil on the brand new hugelbeet at Garden@Kimbourne in 2014, they swiftly spread from six plants to a vigorous ground cover. Cucumbers have a long tap root in addition to their surface roots, but they thrived as well. 

The crux of the issue seems to be exactly what Adams experienced in his study at the University of Wisconsin. If you are starting with a mature soil, or with amendments that have the same nutrient balance as a mature soil, the nitrogen used to begin the wood decomposition simply will not cause a noticeable effect.

Something to Consider: Why are you Choosing Hugelkultur?

If the issue of nitrogen immobilization makes you very concerned, it’s worth asking whether hugelkultur is the right technique for you.

Hugelkultur originated as a means to quickly turn excess wood into soil, not as an instant garden bed. Some of us still find ourselves piling debris from a fallen tree without a ready source of topsoil to go on top—see my post on hugelkultur in clay for one example. Those of us in that position are choosing hugelkultur because we have too much wood and not enough soil and we can eventually turn one into the other. 

The benefits of hugelkultur are not the same as, for example, the benefits of a traditional raised bed filled with fresh compost. This practice is not so much a poster child for instant fertility as it is for long-term structure and turning waste into a resource. If you have the means to cover the whole mound in several inches of soil, you may get comparable results to a compost-filled raised bed. If not, you have still created something that will:

• Give you microclimates for heat-loving plants on one side and cold-loving plants on the other, with every degree of variance in between
• Collect water (especially if you started with a pit in the ground) and wick it up through the wood toward your plants
• Encourage fungi, which are central to the health of the soil ecosystem
• Produce, in time, a slow release of nutrients over a long period
• Sequester carbon more efficiently than wood decomposing on the surface does

Further Research

There’s no arguing that we need further research on hugelkultur, and I’d love to do a home trial myself one day of a hugelbeet and an ordinary raised bed created with all possible variables controlled. I do suspect, though, that based on the technique’s origin, more official data may be available in German than in English. And although the word “hugelkultur” (hill-culture, a name for the technique) is much better known in English than “hügelbeet” (hill-bed, a name for an individual bed made using the technique), it seems that the latter may be a better search term for any bilingual growers looking for resources in German.

For the rest of us, here are the resources consulted to write this post:

Bibliography

Adams, A. (2013). Hugelkultur Gardening Technique Does not Result in Plant Nutrient Deficiencies and is a Potential Source Reduction Strategy for Yard Trimmings Wastes. Unpublished Manuscript. University of Wisconsin. Madison, Wisconsin. https://www.wisconsin.edu/waste-research/download/2013_student_reports/13%20MSN%20Adams%20Hugelkultur.pdf 

BorisWombat. (2019). Woodchips v Hugelkultur – fixing nitrogen? [Discussion post]. reddit. https://www.reddit.com/r/Permaculture/comments/a4kb1d/woodchips_v_hugelkultur_fixing_nitrogen/ 

Pleasant, Barbara. Building Garden Soil With Wood. Mother Earth News. https://www.motherearthnews.com/organic-gardening/gardening-techniques/building-garden-soil-wood-mulch-zmaz10onzraw 

van der Wal, A., de Boer, W., Smant, W. et al. Initial decay of woody fragments in soil is influenced by size, vertical position, nitrogen availability and soil origin. Plant Soil 301, 189–201 (2007). https://doi.org/10.1007/s11104-007-9437-8

Winters, Patrick. (2012). Hugelkultur 1st Year Plants Good/Bad [Discussion post]. Permies. https://permies.com/t/14963/Hugelkultur-st-Year-Plants-Good