Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator would be reported as the percentage of croplands nationally having one of three salinity levels (less than 2 decisiemens per meter [dS/M], 2 to 4 dS/m, and greater than 4 dS/M; see below for discussion/description). In addition, the percentage of croplands with elevated soil salinity (over 4 dS/m) would be mapped on a Major Land Resource Area (MLRA) basis. (MLRAs are aggregations of geographic areas, usually many thousand acres in extent, which are characterized by a particular pattern of soils, climate, water resources, and land use. See http://www.statlab.iastate.edu/soils/MLRAweb/mlra/ for a discussion and map.)

Salinization is the process by which salts accumulate in the soil. Soil salinity hinders the growth of plants by limiting their ability to take up water. Soluble salts, particularly sodium salts, may also harm soils by reducing soil structure, tillage properties, and permeability to water.

Soil salinization is most often associated with irrigated agriculture because when water is applied to the land to nourish crops, much of it is taken up by plants (or evaporating directly from the soil surface) and is returned to the atmosphere. Since only pure water evaporates from the soil surface or transpires from the plant surfaces, the salts are left behind in the soil. Thus, irrigation has the potential to lead to excess accumulation of salts in the soil. The occurrence of saline soils, however, is not restricted to irrigated soils. The same processes of mineral weathering or dissolution and subsequent concentration because of water evaporation often lead to high salt levels in soils of arid and semiarid regions. The scarcity of rain that makes these areas arid restricts the possibility of leaching and thus leads to salt accumulation. A special case of dryland salinity of particular concern to the northern Great Plains is that of saline seeps. A saline seep occurs when water in excess of that required by plants percolates below the root zone and, upon encountering some type of barrier or restricting layer, moves laterally downhill and emerges in a seepage area, having picked up dissolved salts in transit. Saline seeps are often encountered where farmers practice a wheat-fallow rotation; during dry periods, such a rotation may serve to conserve some water during the non-cropped period to aid the following crop, but in somewhat wetter years, the precipitation in excess of that required by plants initiates the process that leads to a seep. Drainage from saline seeps is estimated to affect about 2.5 million acres in the northern Great Plains.

Soluble salts in soils are measured by determining the electrical conductivity of a saturated paste extract; the units of conductance are reported as dS/m. Few plants are affected when the extract conductivity is below 2 dS/m, while some sensitive plants are affected when values are between 2 and 4 dS/m. Many plants are affected when values are above 4, and few plants can survive at values greater than 16 dS/m. Salts are usually most damaging to young plants, but not necessarily at the time of germination, although high salt concentrations can slow or inhibit seed germination. Most plants are least affected by soil salts when in their mature stages.

Reduced permeability to water is a common problem with salt-affected soil. Soil porosity becomes gradually altered and some soils can become completely impermeable. The mechanisms responsible are swelling of clays, which reduces pore sizes, and dispersion of the soil, so that aggregates break down, and smaller mineral and organic particles move with water and begin to fill smaller pore spaces. Dispersion is the most frequent cause of reduced infiltration. The measurement that most accurately determines whether the soil is affected by soluble salts is the exchangeable sodium percentage, which expresses the portion of the total exchangeable cations that are sodium. An exchangeable sodium percentage value equal to or greater than 15 indicates a sodic soil.

The Data Gap

Soil salinity measurements are needed on dominant soils, on cropping patterns, and particularly on water management practices under both irrigated and non-irrigated conditions in arid and semiarid regions. Salinity measurements are often included in routine soil tests. However, there is no unified effort in place to collect and analyze the results over uniform regions. A program that can monitor changes over time as a function of soils and management practices is vitally needed.

Soil salinity measurements should include data on dominant soils, cropping patterns, and, particularly, water management practices such as irrigation and drainage. Gathering together the existing but fragmented data, collecting new data, and analyzing the results to ensure national coverage require a coordinated effort. Satellite-based technologies, while promising, are able to detect only visible salt deposits. Since visible surface salts are incorporated into the soil by tilling, these approaches may be of use primarily to complement soil testing.