Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 272

Relevant FAO Papers/Reports ........................... 323
FAO Soils Bulletins ............................... 324
Questions .................................... 324
References ................................... 325
Salinity is an important land degradation problem. Salinity is more widely known
and refers to the amount of soluble salt in a soil. Sodicity refers to the amount
of sodium in soils. The consequences of salinity have detrimental effects on plant
growth and final yield damage, reduction of water quality for users, sedimentation
problems, and soil erosion. When crops are too strongly affected by the amounts
of salts, they disrupt the uptake of water into roots and interfere with the uptake of
competitive nutrients.
Effective management of salinity or sodicity problem requires correct diagno-
sis of the problem. Understanding the soil-salinization processes and limitations to
crop production is the key to improvements in crop productivity related to salinity
and sodicity. Although several treatments and management practices can reduce salt
levels in the soil, there are some situations where it is either impossible or too costly
to attain desirably low soil-salinity levels. In some cases, the only viable manage-
ment option is to plant salt tolerant crops, or management of farming systems to
minimize their impacts. Use of saline water in agriculture requires some changes
from standard irrigation practices. This chapter discusses all of the issues.
8.1 Extent of Salinity and Sodicity Problem
Land and water are the two most important natural resources for agricultural
development. Land degradation due to soil and water salinity and waterlogging is
threatening the sustainable use of the resources. Saline and sodic (alkali) soils can
significantly reduce the value and productivity of affected land. Excess salt ions in
the soil or from irrigation water can cause salinity and limit agricultural produc-
tion. Saline soils are found almost throughout the world. Soil salinization has been
identified as a major process of land degradation. Based on the FAO/UNESCO Soil
Map of the World, the total area of saline soils is about 394 million hectare and
that of sodic soil is about 434 million hectare (ha). These are not necessarily arable
but cover all salt-affected lands at global level. Out of the current 232 million ha of
irrigated land, 45 million ha are salt-affected soils (about 19.5%) (Ghassami et al.,
1995 ) ; and out of the 1,500 million ha of dryland agriculture, about 32 million ha
(2.1%) are salt-affected at varying degrees.
The majority of human-induced land salinization (secondary salinization) in the
world is associated with irrigation. Countries suffering the effects of secondary
salinization include Australia, United States (particularly the states of Montana,
North Dakota, and South Dakota), Canada (the prairie provinces of Manitoba,
Alberta, and Saskatchewan), Thailand, South Africa, Turkey, India, Pakistan, and
Argentina.