Translations:Benutzer:Arian/Klett-Mini-Test/924/en

The composition of the crop rotation serves the aim of developing a stable, or better still a positive, humus balance — and with it a positive nitrogen balance. To the latter, root crops and cereal crops contribute relatively little; all the more so the legumes. These live in symbiosis with bacteria in the root zone, the so-called *Rhizobia* or nodule bacteria, and have, in connection with the mother plant, the capacity to fix nitrogen from the air. The same capacity belongs, among others, to the alder (*Alnus glutinosa*), which lives in symbiosis in the root zone with the fungus-related *Actinomycetes* — recognisable by the deep green of its foliage, similar to the legumes. The capacity for nitrogen formation, without any direct relationship to higher plants, is found in free-living bacteria such as *Azotobacter*, which prefer a base-rich, more alkaline soil environment, and *Amylobacter*, which prefer a more acid soil medium.