I built a hugelkultur about 4 years ago as an experiment, and as I took it apart this spring in preparation for selling the house (it looked like a grave...) I took a sample of the soil there and sent it in for analysis at my local extension office. The pH, organic matter, nitrate, and potassium are all similar to my sheet mulched beds, but the phosphate is through the roof at 253 ppm compared to 110-119 ppm in my other beds. (The testing service says the "optimum" range for growing veggies is just 47-76 ppm.) What to do about it is a moot point since I'm moving out, but how on earth did it get that way? This hugel bed was built without any manure of any kind, just rotten wood and green plant matter. I didn't notice any nutrient deficiencies (zinc or iron) in the plants that were growing there. Has anyone else seen outrageously high phosphate in a hugel bed? What do you suppose caused it? Thanks in advance.
First off, just because Phosphate tested high (this is water soluble phosphate) doesn't mean you are way out of whack plant growing wise.
Remember that soil tests are done on the premise of chemical fertilizers being used for artificial nutrient availability over those who use nature to grow plants.
What types of wood were installed in the Hugel? were they perhaps phosphate rich woods? It certainly seems that is the likely cause of the difference.
Many of the hard woods will have quite a bit of phosphate bound into the lignin and as the wood rots that mineral becomes free for uptake by microorganisms.
Well built hugels will have a good variety of bacteria, fungi and other microorganisms living in the soil, that means there will be a bacteria that works on the wood to release the nutrients stored in the wood.
You also noted that you didn't see and deficiencies so congratulations you built a super hugel bed.