Biochar is a solid product obtained during the pyrolysis of biomass. It is a porous, carbon-rich material that has a wide range of uses, with soil improvement, remediation, and pollution control playing the most significant roles. In agriculture, biochar is primarily used to increase crop nutrition, plant growth, and soil fertility.

Statement 1 is correct. Biochar can indeed be used as a part of the growing medium in vertical farming. It helps in improving the soil structure and fertility, thereby promoting plant growth.

Statement 2 is correct. Biochar promotes the growth of nitrogen-fixing microorganisms. These microorganisms play a crucial role in the nitrogen cycle by converting atmospheric nitrogen into a form that plants can use.

Statement 3 is correct. Biochar also enables the growing medium to retain water for a longer time. Its porous nature allows it to hold onto water and nutrients, making them readily available for plant roots.

Therefore, all three statements are correct.