Region 6: Alaska

Alaska’s cool climate and rugged terrain limits the agricultural and forestry-related uses of its soil. Accordingly, less than a tenth of the state has been surveyed beyond the “exploratory” level. The United States federal government owns 65% of the state: a conglomeration of national parks, national forests, and national wildlife refuges.

Although Alaska’s cooler climate limits the number of soil types present there, its diverse topography does allow for 7 of the 12 soil orders to occur. The state has two major mountain chains, a vast interior highland, remote stretches of frozen plains, and more coastline than the 49 other states combined. It is also home to the Aleutian Islands, a volcanic island chain built of basaltic lava and ash deposits that create prevalent Andisols. Of course, the frigid temperatures have an influence on soil regimes, particularly in the interior and on the Arctic coast. Cold temperatures also slow the rate of chemical reactions, while ice can increase erosion. Much more of the state was glaciated in the recent past, though extreme aridity seems to have kept significant swaths ice-free. Ice sheets were predominantly found in the south central and panhandle areas, grinding rock to produce sediment.

Precipitation in Alaska generally decreases to the north, varying from less than 18 centimeters (7 inches) per year on the Arctic coast, to more than 630 centimeters (250 inches) per year in parts of the southeast. This increased rainfall in Alaska’s southern portion and panhandle contributes to the wetlands and peat bogs found there, where poor drainage and slow decomposition leads to the development of Histosols.

Figure 8.15: The location (and types) of permafrost in Alaska.

Figure 8.15: The location (and types) of permafrost in Alaska.

Approximately 332,000 square kilometers (128,000 square miles) of Alaska (nearly twice the size of Washington State) are mountainous, and 39,000 square kilometers (15,000 square miles) of that are glaciated. Other than in parts of the high Rockies, the only permafrost found in the United States is in Alaska. More than 80% of the state has some amount of permafrost—it is nearly ubiquitous north of the Brooks Range but thins to the south, where it is primarily found only in the mountains of the panhandle. Permafrost usually forms anywhere that the average annual air temperature is below -1°C (30°F). The longer the area remains frozen, the deeper the ice can reach, sometimes extending thousands of meters (yards) below the surface. In southern Alaska, where it is too warm to form today, there are still stretches of relict permafrost that remain frozen from the Pleistocene, when conditions were cold enough for it to form (Figure 8.15). Unsurprisingly, the permafrost-associated Gelisols are the most common soil type found here.

Subfreezing temperatures affect soil by altering erosional processes, decomposition, the movement of soil particles, and the types of organisms that live in and on the soil. Most permafrost is overlain by no more than a meter (3.3 feet) of annually thawed soil, known as the active layer. The active layer comprises the O and A soil layers, while the lower beds are frozen, effectively removing them from the soil profile. This is precisely why Gelisols are high in organic content and low in nutrients: minerals are prevented from reaching the surface, and organic materials accumulate above the frozen layers.

As organic-rich Gelisols build up at the surface, older layers become deeply buried and eventually freeze. Once frozen, organic materials cease to decay, effectively sequestering the carbon they contain in the ground. When permafrost thaws, decay begins again, and carbon-based molecules—perhaps most significantly methane—are released. Methane is an extremely potent greenhouse gas, and climatologists believe permafrost thaw will be one of the most important feedbacks driving global warming in the coming decades. The buildup and explosive release of methane-rich gases has also been implicated in unusual landforms such as craters and hollows that can form suddenly in permafrost landscapes.

Thawing and freezing can cause soil to move and mix. For example, cold and dry conditions cause the surface to contract, creating cracks that may then fill with sediment. Warmer and wetter conditions cause the ground to expand, and the sediment in the cracks can be forced up. An area in which this process is repeated creates a network of landforms called patterned ground. (Figure 8.16)

Thawing permafrost can also cause the soil to shift and sink. In some parts of Alaska, these shifts have resulted in trees growing at unusual angles in order to compensate for their changing footing, a phenomenon known as “drunken trees.” (Figure 8.17)

Figure 8.16: An overhead view of patterned ground in Alaska.

Figure 8.16: An overhead view of patterned ground in Alaska.

Figure 8.17: A clear example of drunken trees.

Figure 8.17: A clear example of drunken trees.

Entisols represent the most productive agricultural soils of the state, and are found on floodplains and outwash plains where new sediment is deposited at frequent intervals. The Matanuska-Susitna area is also known as “Alaska’s breadbasket,” due to the fertile Entisols that produce over 75% of the state’s agricultural output.