Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [29]
Montana’s main form of salinization is one that has ruined several million acres of cropland in the northern Great Plains as a whole, including several hundred thousand acres in northern, eastern, and central Montana. The form is called “saline seep,” because salty water building up in the ground in an uphill area percolates through the soil to emerge as a seep in a downhill area up to half a mile or farther distant. Saline seeps frequently become bad for neighborly friendship when the agricultural practices of one farmer uphill cause a saline seep on a downhill neighbor’s property.
Here is how a saline seep arises. Eastern Montana has lots of watersoluble salts (especially sodium, calcium, and magnesium sulfates) present as components of the rocks and soils themselves, and also trapped in marine deposits (because much of the region used to be ocean). Below the soil zone is a layer of bedrock (shale, sandstone, or coal) that has low permeability to water. In dry eastern Montana environments covered with native vegetation, almost all rain that falls is promptly taken up by the vegetation’s roots and transpired back into the atmosphere, leaving the soil below the root layer dry. However, when a farmer clears the native vegetation to practice crop-and-fallow agriculture, in which an annual crop like wheat is grown during one year and the land is left fallow the next year, there are no plant roots to take up rainwater falling in the fallow year. That rainwater accumulates in the soil, waterlogs it below the root layer, and dissolves salts that then rise into the root zone as the water table rises. Because of the impermeable underlying bedrock, the salty water doesn’t drain deeply into the ground but emerges somewhere downhill nearby as a saline seep. The result is that crops grow more poorly or not at all, both in the uphill area where the problem arises and in the downhill area where the seep emerges.
Saline seeps became widespread in much of Montana after 1940 as a consequence of changes in agricultural practices—especially the increasing use of tractors and more efficient soil tilling devices, weed-killers to kill weed plant cover during the fallow period, and more land under fallow each year. The problem must be combatted by various intensive types of farm management, such as sowing salt-tolerant plants in the downhill seep areas to start reclaiming them, decreasing the length of fallow time in the uphill area by a crop schedule known as flexible cropping, and planting alfalfa and other perennial water-demanding crops with deep roots to take up excess water from the soil.
In the areas of Montana where agriculture depends directly on rainfall, saline seeps are the main salt-related form of land damage. But they are not the only form. Several million acres of agricultural land that depend for their water on irrigation rather than on rainfall are distributed patchily throughout the whole state, including in my summering areas of the Bitterroot Valley and Big Hole Basin. Salinization is starting to appear in some of those areas where the irrigation water contains salt. Another form arises from an industrial method to extract methane for natural gas from coal beds by drilling into the coal and pumping out water to let methane escape to the surface. Unfortunately, the water contains dissolved salt. Since 1988, the adjacent state of Wyoming, which is almost as poor as Montana, has been seeking to boost its economy by embarking on a big program of methane extraction by this method, yielding salty water that drains from Wyoming into southeastern Montana’s Powder River Basin.
To start to understand the apparently intractable