Why does the yield of cultivated plants in the fields decrease over time?

Agricultural crops are special ecosystems that have been grown artificially by human hands. They differ significantly from natural ecosystems: agricultural systems require constant availability of fertilizers – mineral and organic; they will perish without a person, since they are not able to receive the substances they need on their own; in agriculture 1-3 species prevail, no more. In the fields, cultivated plants suck nutrients from the soil, and as long as this happens, agriculture will live. But minerals in the soil are not eternal: in the natural ecosystem, this balance is restored by reducers (parasites and saprophytes), which a person poisons on agricultural systems. Thus, the richness of the soil decreases over time, which is why the yield of cultivated plants decreases.