Salt of the Earth

Natural History, Feb, 1998 by Robert H. Mohlenbrock

Inland wetlands typically receive a supply of fresh water from rainfall or streams and are drained by nearby creeks. They may accumulate salts during cycles of drought, but these minerals are flushed out during wet seasons. Where the land is not drained, however, evaporating water may leave behind salty compounds - especially sodium chloride, calcium chloride, sodium sulfate, and calcium sulfate - on exposed patches of soil. As the cycle is repeated, a whitish crust often accumulates, eventually forming what is termed a salt flat, and any surface water becomes increasingly salty.

During rainy periods, runoff temporarily dilutes the salts, but at other times the salinity of the soil may approach 5 percent. The water table is usually shallow, with the upper inches of silt underlain by a more or less impervious layer of clay. Botanist Irwin Ungar and his colleagues have shown that the salts are generally concentrated in the upper ten to twelve inches of soil. Freshwater marsh plants cannot grow under these conditions.

Several salty wetlands lie in or near Lincoln, Nebraska, and can be distinguished by their salt-tolerant plants from local freshwater wet prairies, where prairie cordgrass grows. One is Arbor Lake, a state-owned and -protected salt marsh at the northwest edge of the city. A twenty-acre depression that fills temporarily following heavy rain - thereby justifying the term "lake" - it is easily accessible from North 27th Street, which forms its western boundary. The state has built a small observation deck overlooking the edge of the marsh.

When I visited in early June, Arbor Lake was fairly dry, with only a few areas of shallow standing water visible around the periphery. From the deck I could see a dense cover of wet prairie vegetation, but I was looking for signs of saline conditions. My gaze fell upon what appeared to be a few patches of bare soil, and I left the deck to inspect them. As I neared one, I could detect a telltale white, crusty film. When I reached it, I saw that the soil was not really bare but carpeted with dwarf plants - all salt-tolerant species. As I walked away, the plants I encountered were not only progressively taller but also species less tolerant of salt. The tallest plants were salt-intolerant weeds and wet prairie species, apparently growing in salt-free soil.

The soil of Arbor Lake salt fiats may be as much as 4.7 percent saline. Near Lincoln, one can also find a number of pools or ponds of standing water, a few of which have a salt content of about 3 percent, similar to that of the ocean. They are usually surrounded by plains bulrush, a three- to five-foot-tall sedge with triangular stems and flowering spikes that are nearly one inch long. Most of the salt ponds contain a submerged, aquatic species called widgeon grass. This plant, with tufts of threadlike leaves attached to much-branched slender stems, is not really a grass but is instead a relative of pondweeds. It is usually found in coastal pools and ditches.

For visitor information write:

Nebraska Department of Game and Fish P.O. Box 30370 Lincoln, Nebraska 68503 (402) 471-5422

Disturbed wet prairie plants grow where the soil's salt content is negligible. Indigenous prairie cordgrass and prairie dropseed are joined by species that have colonized the area because of disturbance, possibly by hikers. The "weedy" plants include annual sunflower, common ragweed, smooth bromegrass, curly dock, yellow sweet clover, and pennycress.

Slightly saline soil, with salt content of up to about 1.5 percent, supports a number of species, including squirreltail barley and marsh elder. The barley dominates this zone from April to late June, but once it has ceased flowering and dropped its seeds, the marsh elder, which germinates in late spring, rapidly grows three or four feet tall. Both species also thrive in the absence of salt, as do some other plants found here, such as kochia, heath aster, panicled aster, and sprangletop grass.

Certain plants appear to depend on the presence of some salt. They include spearscale, whose often triangular-shaped leaves are covered with minute gray scales that give the plant a gray-green appearance, and salt grass. Another is red glasswort, found in the northern Great Plains westward to British Columbia and here, in Nebraska, at the southern limit of its range. It is a member of the genus Salicornia, whose species - variously called saltworts or glassworts or pickleweeds - are major components of coastal salt marshes. They have leafless, jointed, cylindrical stems and bear their flowers in hollows of the thickened upper joints. Like some of its coastal relatives, red glasswort has stems that turn red in autumn.

Salt marsh species that tolerate greater salinity include salt grass, easily distinguished from most other grasses because its short, narrow, rather stiff leaves grow on opposing sides of the stem rather than spiraling around it. Salt grass is also one of a few grasses whose pollen-producing flowers and seed-producing flowers are on separate plants, ensuring cross-fertilization. Salt grass also grows only one or two inches high where the salt is most concentrated, but up to eight inches high where the salinity falls to about 1 percent. This same species is common along the coastal plain of North America, although there are a few minor differences between the inland and coastal forms.