Effect of different fertilization on the batch yield and morphological structure of Medicago sativa in a Karst area of Guizhou, China

Fertilization has an important effect on the yield and morphology of alfalfa (Medicago sativa), which is a high quality protein forage. This paper has studied the effect of different proportions of P and K on the second batch of yield and morphology structure of alfalfa. The research showed that 1) fertilization with different ratios of P and K can significantly increase the biomass of alfalfa (P < 0.05). When single P fertilizer was applied at 240 kg·ha^–1, the yield of the second batch of alfalfa was the highest; the fresh yield was 18 203.33 kg·ha^–1, and the hay yield was 3 975.97 kg·ha^–1, which were significantly higher than those of the control (88.25% and 78.11%) (P < 0.05). 2) Fertilization with different ratios of P and K had a certain influence on the morphological structure of the second stubble of alfalfa. When P at 80 kg·ha^–1 and K at 60 kg·ha^–1 was applied, the stem to leaf ratio of alfalfa was the lowest (0.98). There was no significant difference (P > 0.05) between this stem to leaf ratio and that when P at 80 kg·ha^–1 and K at 30 kg·ha^–1 was applied (1.03). When P at 80 kg·ha^–1 and K at 30 kg·ha^–1 was applied, the alfalfa plant height, main stem length, and internode length were higher, and showed significant differences from those of the non-fertilization control group (P < 0.05). A single application of P at 160 kg·ha^–1 is beneficial to the branch promotion of alfalfa. 3)We utilized a fuzzy logic membership function to comprehensively evaluate the yield and various indices of morphology structure of alfalfa. The treatment of P at 80 kg·ha^–1 and K at 30 kg·ha^–1 ranked in first place with 0.76 points. Therefore, when planting alfalfa in the karst area of Guizhou, we suggest a treatment of P at 80 kg·ha^–1 and K at 30 kg·ha^–1.