Abstract: Silicon is not an essential nutrient element in the process of plant growth and development, but for many crops it plays a positive role in enhancing photosynthesis, improving yield, promoting the absorption and utilization of other nutrient elements, and appropriately alleviating the adverse reactions caused by excessive nutrient elements. In order to study the effects of silicon fertilizer on alfalfa growth and the absorption and utilization of nitrogen, from 2020 to 2021 the field experiment was adopted and some indexes（such as the height, leaf color value and dry weight）and the other three ones related to the absorption and utilization of nitrogen were studied. During the test period, when the application amount of silicon fertilizer is medium（T3）or high（T4）, the significant promotion effect can be found not only in the plant height, leaf color, dry weight, nitrogen uptake of alfalfa, but also in the total nitrogen content of soil and urease activity（P<0.05）. And under these two treatments, the ability of alfalfa to absorb and use nitrogen is better than the treatments of less or no silicon fertilizer. The recommended application amount of silicon fertilizer is 3.2 kg/667m²（SiO₂）（T3）after comprehensive consideration of multiple factors such as the ratio of production to investment and the impact on the environment.

Key words: silicon fertilizer, alfalfa, physiological indexes, nitrogen, absorption and conversion