Two years back I purchased 24 acres of land in one of the worlds rainest spots - Kaiwiki, big island. An average year sees about 300 inches of rain, a wet year 400 inches or more. The previous owner bulldozed about 8 acres clear down to the subsoil, leaving behind only clay. The topsoil and standing biomass was bulldozed into the low spots (seasonal streams). You can imagine the soil loss that occurred. I've been terracing, micro-swaling, mulching and composting. After two years of efforts, most of the cleared area is either swales or overgrown with the local (incredible invasive) wainaku (aka torpedo) grass. So soil loss has been hugely reduced, mostly by natures bandaid (invasive grasses and shrubs) and in small part by human efforts.
Rehabilitating degraded land is something I really enjoy. I also work on a three acre parcel about 5 miles away that gets less than half the rain my land does. Yes the climatic diffrences can be pretty extreme in Hawaii. This three acres has been managed for almost 20 years using permaculture techniques. It is really eye opening to see the diffrences in plant performance between the two sites. To get a tree to even survive on my land means biochar, lime, compost, micronutrient sprays and mulching. To get a tree to blast of like a Titan rocket on the three acres means digging a hole big enough to accept the root ball.
The biggest success I've had with building soil has been a combination of composted biochar (80% compost to 20% char) tilled into the soil along with micros and lime and terraced or swaled. From this mixture I can grow decent crops of corn, squash, taro etc. Plant selection is of course key to success.
In my area many people try to keep animals on pasture without swales or terraces. All of the land is sloping, and when the rains come it's clear to see the pasture degrading and the animals not thriving.
My point being that permaculture techniques work. I wish I could find more resources for managment of soils in high rainfall tropic&subtropic areas, but it seems to be an understudied area of permaculture (and land managment in general). With so many acres of rainforest being destroyed or degraded each year, I feel blessed to be able to learn what I can about these very sensitive soils, and to be able to do what I can to regrow the Topsoil & biomass of these few acres.
Well, hi everyone on this really cool and informative forum, and thanks to Everyone spreading information and experiences.
Aloha jon in Kaiwiki
Big Island, Hawaii, 2,000 ft elevation, 200+ inches yearly rainfall.
Ua Mau Ke Ea O Ka Aina I Ka Pono