Many plants have natural substances in their roots, flowers, leaves, etc., that actually repel and/or attract insects. In some cases, they can even have an affect on the growth rate and flavor of other plants. By planting “companion”plants near each other, you can help to naturally balance the eco-system in your gardens, allowing nature to do its job, without spending a dime. Planting certain plants together “and other plants far away from each other” can enhance everything you grow in your garden, without the use of chemicals or fertilizer.
Golden Harvest Organics has a wealth of information on its website about companion planting and which plants are well-suited for one another.