Agricultural land use

At the global scale the distribution of agriculture is most influenced by climate. Large parts of the planet are unsuitable for farming because they are either too cold or too dry. All crops have minimum requirements for heat. Growth usually begins when the dailyair temperature rises above 6°C. Moreover, temperatures have tobe above this critical level for at least 120 days. As you go nearer to thepoles, temperatures fall and the growing season shorten, until cultivation is impossible. This is the main reason why cultivation rarely extends beyond latitude 60° in the northern hemisphere. Crops also have minimum moisture requirements. The world's hot deserts, such as the Saharan, Arabian and Australian deserts, are too dry for cultivation unless water is availablefor irrigation. High mountains such as the Himalayas and Andessupport few farming activities. As well as severe climates they also have steep slopes and thin soils.

In the more developed countries of the world, where scientific knowledge is applied to farming, economic and human factors tend to play a particularly important role. Physical problems can, to some extent, be overcome. In Holland, for example, a lot of the present farming land has in fact been reclaimed from beneath the sea. In Australia, water from the Snowy river has been diverted to irrigate land in the southeastern Australia, so making it possible to increase crop fields there. In Canada and the USA scientific plant rearing has made it possible to extend the cultivation of wheat into areas which were previously thought to have been too dry or too cold.