Dryland an area where the rainfall is so low that the soils remain almost dry throughout the year. Drylands occupy around 35% of the continental surface of the earth. They occur either within the climatic belts of the lower or middle latitudes, with an average annual rainfall of 150 to 250 mm. This quantity of rain is not sufficient to maintain a continuous plant cover on soils.
The main drylands of tropical and subtropical regions cover the vast stretches of the Sahara and the Libyan deserts in North Africa, the Arabian and the neighbouring deserts of West Asia, the deserts of Peru and Chile in South America and the deserts of about half of Australia. A vast area of mid-latitude drylands is located in Central Asia, and includes the famous Gobi desert of Turkistan and the desert of Mongolia. In North America the drylands encompass sections of California, Arizona, Nevada and adjoining areas; in South America the drylands cover portions of Patagonia and adjoining areas.
The soils of drylands are light coloured and are usually coarse textured. Chemical weathering is minimal, while physical weathering is dominant. Rainfall is low and so also is percolation.
The soils of drylands have been classified as aridisols in soil taxonomy. They usually have an ochric epipedon and one or more of the diagnostic horizons are cambic, argillic, natric, calric, gypsic and salic. Aridisols include soils that are called desert soils, red desert soils, reddish brown soils etc. Dryland soil profiles are well oxidised as is evidenced by their low organic matter content and lack of free iron oxide movement. The southern part of northwest Bangladesh (rajshahi and naogaon districts) is the driest part of the country, where rainfall is most unreliable in timing and amount. The general soil types of this region are characterised by calcareous dark grey floodplain soils, calcareous brown floodplain soils, shallow grey terrace soils etc. These soils are better suited for the cultivation of dryland crops. Perennial dryland crops (such as fruit trees and sugarcane) require good soil drainage throughout the year for satisfactory growth. Annual dryland crops need adequate soil drainage during their growing season. Dryland rabi crops may be grown in the dry season on poorly drained land which is flooded in the monsoon. Dryland kharif crops must be grown on well-drained soils. [Md Sultan Hussain]