A long-term study in southern China looked at different fertilization methods on soil and crop yields. Adding manure increased soil carbon and nitrogen, while inorganic fertilizers alone didn't have as much impact. Grain yields were highest with manure and balanced fertilization, but unbalanced or no fertilization led to lower yields. Soil nitrogen levels were key for high crop yields, more so than available nitrogen. Maintaining soil total nitrogen is crucial for successful farming in the long run.