No figures are available for the Kalimantan Iban. The climate of the Iban region is wet and it is not uncommon for annual rainfall in the interior to reach 180 inches. Heavy rains, flat delta land, and swampy inner coastal regions combine to cause frequent flooding of the best agricultural land. The rainfall pattern is, however, very erratic and its variability presents great difficulties for swidden agriculturalists. Those farmers who, with government assistance, have begun to practice wet-rice cultivation may use herbicides to clear their smaller fields and are thus better insulated from climatic variations.
Soils are generally poor.
Most cleared forest areas can be used only for a season or two, and then must be left fallow for 15 to 20 years. Contrary to what early observers supposed, the shifting agricultural techniques of the Iban were probably the best adaptation to this poor soil, causing the least disturbance and allowing the small cleared areas time to recuperate. Iban are no longer free to move their settlements after exhausting an area, but they still shift their fields every few years to allow the land to regenerate. Rice agriculture is a highly ritualized activity and is really a complete way of life, rather than just an economic pursuit. Nearly all of the religious ritual has to do with insuring the success of the crop.