Soil Formation

Charlie Parker, a land surveyor who lives in Soldotna, has posed an interesting question regarding the formation of soil: When soil forms, does the earth gain weight through the conversion of the energy of sunlight and atmospheric gases to plant matter and thence to the humus matter which is a part of normal soil?

The answer to this question is bound up in the issue of how soil formation takes place. Some soils are very old--as old as 200,000 years--yet many Alaskan soils and those in neighboring parts of Canada are quite young. At Glacier Bay, Alaska, for example, observations show that bare rock has converted to muskeg-bearing soil in about 2,000 years. Studies of Alaskan glacial moraines show that recognizable traces of soil formation can be detected as little as fifteen years after moraine deposition, that a surface loam layer can develop there in about 250 years, and that 2,000 years will create a full soil profile. Soil at the agricultural project near Big Delta, Alaska, is all post-glacial, and so, is typically younger than 10,000 years--some of it much younger.

Soil is a variable mixture of weathered minerals and decayed humus matter. The weathered minerals come from the in-place physical and chemical breakdown of rock or of material transported to the site by water, wind or glacial action. At most Alaskan sites where agriculture takes place or is contemplated, water, wind and glaciers have carried the parent material to the site, which explains the comparative youth of these soils.

In forming soil, plants play a role that goes beyond adding humus to the mixture. Plant photosynthesis injects hydrogen into the developing soil where the hydrogen helps water percolating through to move components of the soil minerals vertically. The result is recognizable layering in a fully developed soil profile, which may be up to several feet thick. The top soil layer becomes highly weathered and, except in desert regions, rich in humus content. Lower layers are less weathered, have less humus content and may differ chemically from the top layer.

Microorganisms--bacteria, fungi, algae, protozoa and other little critters--are another necessary component of a developing soil because they decompose plant matter to produce carbon dioxide which then combines with soil moisture to dissolve mineral materials. Some bacteria also fix nitrogen, that is, they extract nitrogen from the air and incorporate it into compounds needed for plant growth. Animals do their bit, too. The most important of them are the invertebrates living in the soil; they are insects, roundworms, millipedes, centipedes and earthworms who help decompose plant material by feeding upon it and loosening the soil by burrowing through it, thereby helping to aerate it and make paths for water and plant roots. The bigger animals such as mice, squirrels, hares, caribou, moose, bison, camels, horse, mammoth and mastodons have also left their marks on the soil that exists in Alaska today.

All these processes act in Alaska to produce about an inch of agriculturally suitable soil in 80 to 100 years, which is equal to about two or more tons per acre each year.

And now to a specific answer to Charlie Parker's question: Does all of that soil derive from the earth itself, or is part of it contributed by photosynthetic action that converts air to soil and sunshine's energy to soil mass?

Plant growth does draw carbon, oxygen, hydrogen and nitrogen from the atmosphere. When the plants decay to humus these elements do become part of the soil. For the most part, their residence as a part of the solid earth is temporary, a few years or so, and they cycle back into the atmosphere. But the fact that we do have coal and petroleum attests to a much longer residence of a tiny fraction. By and large there is a long-term balance between the earth and the atmosphere, over time scales of thousands and millions of years. So it is almost, but not quite correct to say that soil building does not add to weight to the solid earth by taking it from the atmosphere. Nor does it gain weight from sunshine since photosynthesis merely converts light energy to chemical energy. Yet without sunshine soil formation would not occur since it is the sunshine that really powers both the physical and the biochemical weathering necessary to soil formation.