Abstract: The comparative importance of nitrogen, phosphorus and potassium fertilizer addition in alfalfa forage production, was studied using an incomplete orthogonal regression design. The forage yield and total protein yield of alfalfa were included in a fertilizer effect model in this study. It was found that the contribution of N, P and K to the yield of a two year old alfalfa stand was K>P>N, and the contribution to the yield of three year old alfalfa was P>K>N, while the interaction effect in two and three year old alfalfa stands ranked N-P>N-K>P-K. For total protein yield the corresponding rankings were N>K>P, and N>P>K, respectively, for two and three year old alfalfa stands, indicating that nitrogen is the key limiting factor in alfalfa protein production. For total protein yield, the N-P interaction and the N-K interaction effects were significantly greater than that of the P-K interaction. By using the frequency analysis method, the optimal fertilization amounts of N, P and K were calculated as 56.27-67.51 kg·ha^-1, 77.69-90.48 kg·ha^-1 and 76.43-87.18 kg·ha^-1, respectively, when the target dry matter (DM) yield of two year old alfalfa is higher than 17522 kg·ha^-1 ; as 46.75-57.66 kg·ha^-1, 80.15-92.28 kg·ha^-1 and 57.79-69.74 kg·ha^-1, respectively, when the target DM yield of three year old alfalfa is higher than 19234 kg·ha^-1; as 66.35-77.48 kg·ha^-1, 79.34-98.87 kg·ha^-1 and 73.68-85.38 kg·ha^-1, respectively, when the target total protein yield of two year old alfalfa is higher than 2116 kg·ha^-1; and 68.44-79.50 kg·ha^-1, 72.74-85.96 kg·ha^-1 and 50.68-61.61 kg·ha^-1, respectively, when the target total protein yield of three year old alfalfa is higher than 2656 kg·ha^-1.

Key words: alfalfa, fertilization model, yield, total protein yield