Recommendation of nitrogen, phosphorus and potassium balanced fertilizer based on soil status/soil availability and economic benefits of alfalfa in salt-affected soils of North China plain

Improving alfalfa forage yieldis closely associated to the use of fertilizers. Generally, with the increase of fertilizer, alfalfa yield also increases correspondingly. However, in saline-alkaline areas, balancing the amount of nitrogen, phosphorus and potassium and achieving a high alfalfa yield is of great importance. In this study, we assumed that the alfalfa yield would reach the maximum at a moderate proportion of multiple fertilizers. A five-year continuous multi-fertilizer experiment with different ratios was carried out in the North China Plain. The experiment used randomized complete block design, tested three fertilizers and four fertilization levels (N0 0 kg ha− 1, N1 5 kg ha− 1, N2 10 kg ha− 1, N3 15 kg ha− 1; P0 0 kg ha− 1, P1 30 kg ha− 1, P2 60 kg ha− 1, P3 90 kg ha− 1; K0 0 kg ha− 1, K1 90 kg ha− 1, K2 180 kg ha− 1, K3 270 kg ha− 1), resulting in a total of 14 different treatments, N0P0K0, N0P2K2, N1P2K2, N1P1K2, N1P2K1, N2P1K1, N2P0K2, N2P1K2, N2P2K2, N2P3K2, N2P2K0, N2P2K1, N2P2K3, N3P2K2. All fertilizers applied twice a year. Results indicated that N2P1K2, N1P2K1 and N2P2K1 treatments had significant effects on increasing alfalfa dry matter and dry to fresh biomass; N2P2K2 treatment had a significant effect on alfalfa crude protein, but crude protein under N3P2K2 and N2P2K2 treatments showed no significant differences. In addition, the relative feed value (RFV) was significantly affected by potassium fertilizer. Under low potassium application, RFV showed a significant increase. N2 (10 kg ha− 1), P2 (60 kg ha− 1), and K2 (180 kg ha− 1) significantly increased the utilization efficiency of fertilizers, agronomic efficiency, and economic benefits, which were associated with decrease in fertilizer expenditure and increase in alfalfa income. The N3 (15 kg ha− 1), P3 (90 kg ha− 1), and K3 (270 kg ha− 1) significantly increased soil fertility, while had no significant effect on hay yield, nutritional quality and utilization efficiency. Therefore, N1P2K1 (5 kg ha− 1 N, 60 kg ha P− 1, 90 kg ha P− 1) treatment in the saline-alkali areas of the North China Plain is recommended for achieving high alfalfa yield, forage nutritional content, and improving the utilization rate of fertilizers by plants, increasing the farmers’ income without compromising soil fertility.