Hugelkultur is based on the decomposition of deadwood. As it decomposes, it becomes spongy and holds water for long periods and in a manner accessible to plants.
I am moving soil into my raised beds and rocks to raised bed 6. As I brought these rocks down from uphill, I dropped them in front of a raw bank, filled the gap with small deadwood picked up during cleanup, and plant matter as well, then topped with sandy soil.
On the top piece of wood, the white threads running through the wood are mycelium. On the bottom piece of wood are plant roots and mycelium. Notice the soil clinging to the wood is darker than the soil below and is damp. Where these are sitting, the soil is dry and sandy for the top 12 inches. Where the deadwood was buried, the soil was still damp 2 inches below the surface. I have not had more than 1/2 inch of rain in 4 weeks or so and this area faces south.
The first year of hugelkultur does not have the richness of later years, and this was only some small deadwood branches a couple inches in diameter, but it is still keeping moisture in the soil and enriching the sand that holds neither water nor nutrients.
Every small fruiting shrub or tree I plant uphill has deadwood buried in its planting hole and a semicircle of rocks on the downhill side to slow water enough to soak in. I give them an advantage to start, then leave them to nature.
I opened up this tiniest of hugelkultur beds, and see the same benefits in 8 inches that I will see in my 24 inch beds. I have seen photos of 4 foot hugelkultur beds made with logs. When I lived in Oregon I saw massive logs lying on the forest floor serving the same purpose.
Not the best examples, but they show what is occuring inside the hugelkultur beds I am creating. I get on average 16 inches of moisture per year. It is enough to grow a lot of plants without supplemental water, and using what I have available to me. After 11 years of drought, one thing I have in abundance is deadwoood.