Hugelkultur

Hugelkultur is a growing bed built on top of buried wood. Logs, branches and brush go down first, then leaves, grass clippings, manure and compost, and last a cap of soil to plant into. The wood rots slowly underneath, holding moisture and releasing nutrients into the bed above for years. It also uses brush up rather than burning or hauling it, though a compost heap or chop-and-drop mulch wants the same material.

The moisture benefit is not peculiar to wood. It is the ordinary effect of organic matter on soil: it opens the structure so rain soaks in instead of running off, and holds more of it. What decides whether the bed works is the depth of soil over the wood. At least 25 to 38 cm (10 to 15 in) lets most plants root properly clear of the decaying layer; a shallow cap reverses the advantage. A finished mound runs 30 to 90 cm (1 to 3 ft) high and about 90 cm (3 ft) across where it is reached from one side, wider where both are. Building one needs only hand tools and willingness.

Two things go wrong predictably. The mound sinks as the wood breaks down, a 1.8 m (6 ft) mound settling to roughly 0.6 m (2 ft), so it is topped with compost each year and rebuilt from scratch after five or six, which is why fruit trees and bushes do not belong in one. The second is nitrogen drawdown. Microbes breaking down material woodier than about twenty-five parts carbon to one of nitrogen draw the shortfall from the surrounding soil, so seedlings and transplants look starved in the first seasons unless the wood sits below the roots and nitrogen is supplied. Weeds colonise a fresh mound fast unless it is mulched heavily, and rotting matter can leak nutrients into water below. Nothing separates the mound from the ground, so it is no barrier against contaminated soil. No peer-reviewed research on the practice exists, so its figures come from extension guidance and general soil science.