Water and agriculture
Depending on water supply, plants fall into different types of plant communities, which are adapted to the water supply of the particular habitat. Water and air—or rather, water and oxygen—compete with one another for space in the cavities of the soil. If there is too much water in the soil, this will limit the oxygen supply, and plant life will suffer damage as a result. Irrigation is complicated in another respect too. It has been found, for instance, in modern irrigation works in the Punjab area that water has leaked out of unlined irrigation channels, and raised the water table seven to ten meters above a level which had been relatively stable since 1835. Groundwater is greatly in demand for irrigation in many parts of the world, particularly in arid regions with few lakes and rivers. Owing to the water requirements of plant life, the agricultural potential of areas with little rainfall is always limited by water supply.