Montana Earth Science Picture of the Week

Soil Too Salty for Crops

Photo courtesy of Montana Salinity Control Association

Too salty . . .
This aerial photo was taken over Interstate Highway #15 near Power, Montana (25 miles northwest of Great Falls). The white patches to the right of the highway are areas where the soil has been damaged by saline seep, a problem that has ruined more than 300,000 acres of farmland in Montana. The word "saline" refers to the salts (mostly sodium and magnesium sulfate salts) that build up at the surface, making it difficult for crops to grow there.

In a nutshell . . .
The diagram below illustrates how saline seep typically occurs. Water soaks into the ground at the "recharge area". Excess water that is not absorbed by plants moves (percolates) downward through the soil. On its way it dissolves (leaches) mineral salts. In the diagram, the salt-laden groundwater reaches an impermeable layer and then migrates to a lower area where the water table is at the surface. At this "discharge area", the water evaporates, leaving the salts behind as a white crust on the surface.

"Summer Fallow" is the culprit . . .
Although sodium and magnesium sulfates (salts) occur naturally, saline seep is usually not natural occurrence. In Montana the problem is often related to a "crop-fallow" system of farming. With this type of farming, every other year a strip of land is kept barren of vegetation by plowing and/or using herbicides in order to allow soil moisture to build up and eliminate weeds. On these strips, called "summer fallow", there are no plants to absorb the water from rain or melted snow, so it more easily soaks through the soil, leaching salts along the way and causing an elevated water table.

Term: summer fallow


*Montana Salinity Control Association
*Click here to see a happy ending.
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By Rod Benson
Earth Science Teacher at Helena High School


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