Living among the steep peaks of the Andes, the Incas lacked level fields for farming. To solve this problem, they developed a system of terraces that they constructed throughout the empire like giant green staircases.
Building stepped terraces help the Incas create farmland, and the clever construction of each terrace gave crops the best chance of survival. The first step in construction was building stone retaining walls. These absorbed heat from the sun during the day and radiated it back out at night, often keeping crops from freezing in the chilling nighttime temperatures. Each terrace was filled with a base layer of medium-sized gravel, which was then topped with a mixture of fine sand and more gravel. On top of this, farmers placed a layer of topsoil into which the seeds of their primary crops -- corn and potatoes were top choices -- would be sown.
Occasionally, the terraces would fail to be productive. If this happened, the Incas would turn to a planting method known as "the three sisters." First, they would plant corn. Then, when the corn reached an adequate height, beans would go in the ground and grow up the stalks of the corn. Finally, they'd plant squash in the remaining spaces. This not only produced three crops from one terrace, but the beans fixed nitrogen -- making it available as a nutrient in the soil -- for the corn. The squash acted as mulch for the soil, keeping it moist and relatively weed free.