Agricultural methods often vary widely around the world, depending on climate, terrain, traditions, and available technology. Low-technology farming involves permanent crops: food grown on land that is not replanted after each harvest. Citrus trees and coffee plants are examples of permanent crops. Higher-technology farming involves crop rotation, which requires knowledge of farmable land. Scholars and engineers not only use crop rotation and irrigation, but plant crops according to the season, type of soil, and amount of water needed. The challenge is not food shortages but unequal distribution of the world’s food supply. The ratio of population to farmable Landy has favoured some countries more than others. Some experts believe government policies in developed and developing countries have hindered equal food distribution. Droughts, floods, and other disasters continue to cause local food shortages. The challenges of feeding the hungry cannot be met unless the world’s land and water are safeguarded. Agricultural practices in developed and developing countries have led to a severe loss of valuable topsoil, water, and other resources. Many countries need better programs for replanting forests. Overpopulation has pushed a growing number of farmers onto lands too fragile to sustain cultivation. Demand for food has led to increased irrigation worldwide. In some areas, irrigation has caused water tables to drop, rivers to run dry, and wells to go empty. Agricultural chemicals that increase production often contaminate soil and groundwater and disrupt food chains.