Melanie has mentioned about Bokashi fertilizer. She is mainly using this type of fertilizer in her garden. It is made of fermented rice husks (and can be made of other types of grain husks). As far as I read, it is organic and easy to make. As husks are often byproduct of producing grain for food, you can get the materials very cheep or for free. Sounds like a good fertilizer Idea!
The Link Melanie sent me is in Japanese unfortunately >> https://ymmfarm.com/cultivation/tech/rice-bran-bokashi/
Maybe you read Japanese, or otherwise you can use online translator tools to translate it. It tell you how to make the Bokashi fertilizer. Otherwise I found this video from JICA in Buhtan. It tells step by step how to make it and use it. It sounds like Bokashi fertilizer can improve the soil condition quite well. As the space we are using were not used for planting specific plants before and the soil condition is very sandy, it will be good to do some soil improvement action.
The original recipes are using rice husks as in Japan, there are a lot of rice husks as byproduct of making rice. And it is often free to take when you go to local rice mills. But in Germany, there are no rice production. Instead, I suspect there are still some wheat production around? I had hard time finding a mill near Berlin… but I eventually found one. It is called LM Lindenberger mill GmbH and is located at the edge of Berlin. We are now contacting to see if they can give us (or sell us) some husks.