Phosphorus and potassium fertilizers were studied to see how they affect seed yield and quality of a plant called Onobrychis viciaefolia cv.Mengnong in Beijing. Results showed that using more phosphorus led to higher seed yield, while using more potassium led to lower seed yield. Applying all fertilizers in spring or two-thirds in spring gave higher seed yield than applying half in spring. More effective shoots per area meant more seed yield, but more inflorescence per shoot meant less seed yield.