Method for developing and increasing, resp., the fertility of soil which comprises, if necessary cleaning crude water and/or contaminated infiltrating water and subsoil water in a known way by leading it on an aquatic plant stand and thereby making it capable of irrigation, carrying out humus forming and enrichment on the soil to be treated by a soil alga by inoculating it with a soil alga of suitable productivity and propagating the alga with the help of irrigation and optionally enriching the irrigation water with a humus material prior to, during or after the treatment of the soil with the alga, and ensuring the presence of the plant nutrients in the irrigation water and the nutrient supply, resp., preferably continuously in organic material synthesized form.
