Do plants have a soul? Should we use animals for food?
No, plants definitely do not have a soul. Neither do animals have a soul. According to the Bible, human beings were made "in God’s image" (Genesis 1:26). This God-likeness includes having a spirit and a soul. Animals definitely do not have the image of God, according to the Christian view of the world.
There are world views which imply everything in the universe has "soul." Pantheism, the world view held by Hindus and Buddhists, is a world view which sees all of creation has having a sort of universal "soul." To them, even animals such as dogs, mice and fish have soul. It might be better to say that they have soul than that they have "a" soul. The meaning of individuals is reduced in Hinduism, as the goal is to lose our individuality and to be absorbed into the universal soul, known as atman.
Another world view which might view plants as having a kind of "soul" is animism. Many primitive religions see inanimate objects such as streams and trees as having anima or soul. This would include plants. They often make offerings before they eat animals and sometimes even plants to appease or honor the spirit which they are consuming.
Obviously, Christianity does not see the world from an animistic perspective. Angels have individual souls, as do human beings, but animals do not. As a Christian you may eat plants or meat. God allowed the Jews to eat some meats, but not all, for ceremonial reasons, but Jesus declared all foods to be clean (Mark 7:19). Therefore, a Christian may eat animals for food.