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Soils - Part 3: Soil Organic Matter

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Why Does Soil Organic Matter Content Differ From Soil to Soil and Change?

Very few soils started out with substantial amounts of organic matter. This is mainly because most soil parent materials are geologic in origin. That is, they are developed from rock, or at least have been moved some distance before coming to rest to form a “new” soil. Because it takes plant growth to develop most soil organic matter, very little organic matter survives the temperatures, pressures and disturbance involved with geologic processes. Once a parent material comes to rest and plants begin to grow, however, organic matter starts to accumulate.

The two major natural variables which affect how much organic matter accumulates in a soil are temperature and moisture. Temperature affects organic matter accumulation in two ways. First, plants tend to grow faster and produce more total mass as temperatures increase. Everybody’s mental image of a tropical rain forest involves lush, thick vegetation. Secondly, however, and usually overcoming the first point is that, as temperatures increase, microbial activity, including the activity of decomposing microorganisms, also increases. Going back to our mental picture of a tropical rain forest, it is surprising to find relatively little organic matter persisting in the soils of the forest floor. Microbial activity is just too intense to allow organic matter to accumulate.

Moisture, on the other hand, is a little more predictable in its effect on soil organic matter. As could be guessed, as rainfall increases, total plant production of organic matter increases, and so soil organic matter increases. Things get a little tricky, however, when the temperature and moisture effects are put together. In the continental United States, east of the Rocky Mountains, the general trend is that soil organic matter content increases from west to east and from south to north (Figure 1.3).

The west-to-east trend is because of increasing rainfall, while the south-to-north trend is because of lower temperatures preserving the organic matter that is produced. These trends hold up even into the Canadian prairies, despite their lower rainfall.

There are, of course, many natural and human-induced exceptions to these trends. One natural exception occurs where water accumulates to a degree that the soil is flooded for long periods of the year. In swamps or bogs, the excess water produces a shortage of oxygen which the decomposing bacteria need for their work. As a consequence, organic matter builds up regardless of the temperature, until the swamp is drained by natural or human causes. These locations are the source of peat available in garden stores.

One major human-induced factor which can change soil organic matter content is cropping. In Nebraska, the organic matter content of our prairie soils before the coming of white settlers was as high as six percent. Cropping, which involved a change from perennial grasses to annual plants, removed at least some of the organic matter produced in harvest and stirred the soil during tillage. Tillage opens up more of the soil to oxygen and increases microorganism breakdown of organic matter, at least temporarily. Within just a few years, organic matter content of a tilled soil can decrease to half of what it was in its previous prairie state. Managing a cropped soil with less disturbance, mainly by reducing tillage, will allow the organic matter content to rise somewhat, but probably not to the level of a native grassland.

Finally, plant growth and land use change not only the amount of total organic matter in a soil but the way in which that organic matter is distributed with depth. Figure 3.2 shows, in a general way, the difference in organic carbon distribution between a soil developed under hardwood forest vegetation and under grass vegetation.


Figure 3.2. 
Grassland soil profiles contain about twice as much organic matter that is more uniformly distributed through the profile than forest soils under similar environmental conditions.  (Fundamentals of Soil Science, 6th Edition, Henry D. Foth, © 1978, John Wiley & Sons.)

The forest soil has high organic matter content at the surface, where leaf drop has contributed. Tree roots die off very little from season to season, however, so organic matter decreases rapidly with depth. In the native grassland, organic matter content is also high at the surface, but drops off slowly with depth because the fine roots die off year by year, and contribute organic matter to a greater depth than in a forest. In a native grassland soil which has been plowed, the organic matter content would be lower overall, and mixing from tillage would level the content of the top few inches.


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