From Ecology Action
Question: I am on an atoll island in the Java sea between the island of Java and Sumatera in Indonesia. We have very little soil, what's here is sandy and acidic. I want to grow veggies and have been bringing in compost etc. Do you have any recommendations how to build the soil we do have? Someone said to add gypsum to build up the soil. Any suggestions would be awesome.
Answer: Thanks for your inquiry to Ecology Action on building soil in your area!
First, I must tell you that with such limited info, it is challenging to give you any specific recommendations; however, I can give you some general suggestions that should help. If it is possible to test your soil (what soil you have), that would be immensely helpful to give you more specifics. Still, even without that information, we can do a lot.
My first recommendation is to see if there is anyone there on the atoll that is farming successfully, as they will have a wealth of information and advice for you. Second, keep making compost and adding it to your soil. It will help immensely with bringing your soil to a better pH, will help your soil hold onto its minerals, and minimize erosion as well as other benefits. But be careful about what you are adding to your compost piles. You don't want to add too much animal manure, as you could add excess salts to your soil. So it is best to go with plant sources; a mixture of dried material and green material should work well.
I would not add gypsum to the soil. While it has calcium (and likely your soil does not have enough calcium), it also has sulfur that will tend to acidify your soil further. If you have access to it, I would suggest adding lime (calcium carbonate) in order to add calcium to your soil and increase the pH (reduce acidity). If you have access to dolomitic lime, that is probably as good or better than calcium carbonate. Dolomitic lime has calcium and magnesium, and where you are, it is likely that your soil could use both (but we can't be sure without a soil test). I would be very cautious about adding any sort of fertilizer to your soil if you don't know what nutrients it are lacking, since it could do more harm than good. Again, if there is anyone farming near you that may have similar soil, they can tell you what has worked for them, and that could work for you too. But—I can't emphasize this enough—if you can get a soil test, that will help you immensely for years to come.
Bottom line, keep making compost mostly with plant materials but use manures if you have them available to you, just not too much (say no more than 1/8th or so of the initial volume of your compost piles). This will be your future soil. You can mix it into your current soil to some extent, but I would not bury it or mix it too deeply.
You will also want to consider using some dried plant material as mulch. This will protect the soil you're building during the rainy seasons and minimize the risk of all of your hard work being washed away.
Then, be sure to keep your soil always growing something, particularly deep rooted crops like grains but also rotating other crops in—just don't leave it fallow. This will also help to create soil, as plant roots hold the soil and eventually decompose, adding important organic matter to the soil that will feed soil organisms, just like what you are doing when adding compost.
Another possible resource to check out is Barbara Fahs book on gardening in Hawai'i, which is going to be similar to your conditions. Her book, "Super Simple Guide to Creating Hawaiian Gardens," includes information about gardening even without dirt on solid lava!