Agar is the best solid agent in solid culture. Its main function is to solidify the liquid culture medium. Some reports mentioned that filter paper, glass balls, glass wool, quartz sand, vermiculite, gelatin, silica gel, acrylamide, foam, even pumpkin and potato were used to replace agar. However, until now, no more convenient and better support than agar has been found. By the way, agar itself does not provide any nutrition. It is a high molecular carbohydrate, which is extracted from algae such as red algae, only dissolved in hot water to become a sol, and solidified into a solid gel after cooling (below 40°C). The so-called "boiling" culture medium is to dissolve agar in hot water (above 90°C).

The solid culture medium made by adding agar has many advantages, such as simple operation, easy support and ventilation of culture. It is convenient for regular observation and research. However, it also has some disadvantages, such as small contact (i.e. absorption) area of the culture in the culture medium, slow diffusion of nutrients and hormones in agar, and different diffusion speeds, which makes the supplement of nutrients slow and the proportion of components change. At the same time, some metabolic wastes discharged from the culture gather on the absorption surface with high concentration, which inhibits the metabolic activity of tissues. In addition, agar inevitably contains impurities, which makes the growth rate of the culture in solid medium significantly lower than that in liquid medium.

The coagulation ability of agar is not only related to the raw materials, the processing mode of the manufacturer, but also related to the temperature, time, pH value and other factors during autoclaving. Long-term high temperature will reduce the coagulation ability, and excessive acid and alkali combined with high temperature will hydrolyze the agar, but if it is stored for too long, the agar will turn brown and gradually lose its coagulation ability.