Great Plains: Degradation of Soil Resources

Degradation of Prairie Soil Resources

"Despite its widespread severity and global impact, soil degradation remains an emotional rhetoric rather than a precise and quantifiable scientific entity."¹ Soil is degraded as a result of processes that reduce its productivity². Such processes usually arise from poor management of the resource. Soils are degraded primarily by erosion, loss of organic matter, salinization and acidification. The first two processes are interrelated. Acidification is not a major problem except in areas where cultivation has been extended into previously forested areas. Salinization is more prevalent.

Soil Erosion

Soil erosion reduces soil productivity through losses of nutrients, water storage capacity and organic matter. The losses in terms of productive capacity are significant though presently obscured by the application of fertilizer. Annual top soil losses in terms of lost productivity on the prairies are estimated to be in the millions of dollars. The present value of the accumulated annual losses is measured in hundreds of millions of dollars, the actual losses being difficult to quantify.

Erosion has taken place over the history of cultivation on the prairies. Both wind and water erosion result in the removal of the finer soil particles, thus leading to compaction of the soil and poor soil tilth. Water erosion is a significant problem on long sloping fields, particularly those summer fallowed or left free of cover. The valuable top soil removed is deposited in depressions, road ditches and waterways giving rise to additional costs. Wind erosion is a major problem when there is little cover by crops or residues as in the spring months particularly during dry seasons. Not only are valuable nutrients lost but the soil deposited elsewhere becomes a problem. Indeed, seedling crops may be destroyed.

Dumanski et al ³ have estimated the annual costs of erosion in the prairie provinces to lie in the range of 155-177 million dollars in the case of water and between 213-271 million dollars in the case of wind. Sparrow⁴ has expressed annual soil losses in different terms. He estimated the annual soil loss on the prairies by wind to be about 160 million tonnes and that by water to be 117 million tonnes, these losses from erosion greatly exceeding the rate of soil formation. The annual loss in potential grain production arising from soil erosion during the period of cultivation he considered to be 171 million bushels or 4.6 million tonnes of wheat. He argued that 15 percent of the lost production could not be recovered by additions of fertilizer and estimated the value of lost production at $129 million annually, lending credence to the title of his report Soil at Risk: Canada's Eroding Future.

The physical effects of erosion are painfully evident on the knolls and sloping segments of the prairie landscape. It has led to a gradual evolution of farming practices from an initial use of the plow, through use of blade type cultivation to use of minimum tillage. This progression has enabled increasing amounts of trash cover to be maintained over time, thereby reducing erosion. Moreover, in more recent years the width of fields has increased as a reflecting the size of the equipment used, which resulted from the economies of size in farm operations. In addition, intensive tillage operations to incorporate herbicides and fertilizers were adopted. These latter events work against the soil conservation otherwise achieved through maintenance of trash cover. Fortunately, a reduction in the cost of suitable chemicals has made the use of chemical fallow more economical, thereby enabling adoption of minimum tillage practices. This change in practice has been driven by the need to minimize production costs, the protection of the land from erosion being a complementary benefit. At the same time, summer fallowing has largely become confined to the brown soil zone as rotations have been extended in the dark brown zone and continuous cropping has become general in the black soil zone. Losses from soil erosion can therefore be expected to be less extensive in the future than in the past.

Decline in Organic Matter

The main chemical constituents of soil organic matter are carbon and nitrogen. The former is the energy source for most soil microbes while the latter is one of the most important nutrients for plants. Other necessary components of organic matter are phosphorous and sulfur. Under the ecosystem prevailing previous to cultivation, mineralization and immobilization processes were closely integrated. Plant growth (immobilization) and mineralization of soil organic matter occurred simultaneously. Therefore losses of nutrients were minimized allowing for the accretion of organic matter over time. Carbon transfer through the soil is basic to the functioning of the soil-plant system. Primary productivity determines the amount of carbon entering the system while the amount leaving is controlled by soil micro-organisms. In natural ecosystems, the amount of carbon transferred by means of both processes appears equal.

Upon cultivation of the soil for the production of annual crops, productivity and mineralization are uncoupled, particularly when the fields are bare of vegetation. Total inputs of carbon to the soil are altered due to differences both in plant characteristics and to allocation of carbon for above and below ground production.

The practice of summer fallowing has hastened the loss of soil organic matter. Frequent tillage during summer fallowing has increased the rate of decomposition of organic matter while at the same time disrupting soil aggregation and increasing its susceptibility to erosion. There has also been a loss of the more readily available nitrogen.

The magnitude of the losses of organic matter under cereal production has been established as being in the range of 20 to 50 percent in the cultivated layer according to the soil classification. McGill ⁵ estimated in 1981 that organic matter losses in the topmost layer of soil averaged 41 percent in the brown soil zone, 44 percent in the dark brown soil zone, 49 percent in the black soil zone and 36 percent in the grey soil zone. Research has established that by improved cultural practices, the decline in organic matter can be halted and indeed reversed. Nonetheless, additional nitrogen is now required to maintain yields on many prairie soils.

Regardless of the productivity losses identified, annual wheat yields after an initial decline following settlement have trended upward over the last 50 years. This trend is illustrated in Figure 1

Figure 1 Trends in Wheat Yields in the Prairie Provinces, 1905-84

Source: Environment Canada, The State of Canada's Environment, Ottawa, and authors' estimate.

Uncertainty remains, however, as to how soils will behave under such changed cultural practices as snow trapping, minimum tillage, green manuring and chemical follow. This uncertainty arises because soil changes occur slowly and the effects of agronomic practices take years to become evident.

Soil Salinity

Even before cultivation, large areas of the prairies contained saline soils. However, an expansion of these areas has occurred under cultivation. The rate of increase in the salinized area is related in part to the increase in ground water over time. Salinization occurs as water containing soluble salts moves upward through the soil horizon. Evapotranspiration concentrates the salts in the soil solution to reach levels in the root zone that are detrimental to crop growth.

Any technique which removes water from the subsurface such as drainage, use of deep rooted perennials, reducing summer fallow or continuous cropping diminishes the danger of salinization. Once salinization has occurred, the remedy is to leach the salts out of the root zone. The most viable way to accomplish this cleansing is to use all available precipitation to grow crops and thus reduce the accumulation of ground water. This indicates the desirability of reducing the acreage in summer fallow on the prairies. Such a reduction would also lead to a significant decline in the leaching of nitrogen into the subsoil.

Salinization of dryland agricultural soil constitutes a major problem in southern and central regions of Alberta and Saskatchewan and in a few scattered areas of Manitoba. Approximately 2.2 million hectares of improved land are considered to be subject to secondary salinity. The economic impact of this salinity is said to be in the range of $104 - $257 million annually ⁶.

Soil Acidification

As indicated previously, acidification is not a major problem in other than a limited part of the prairies. Acidification can be overcome by application of lime. However, heavy applications of nitrogen fertilizers over time have been found to result in soil acidification. Consequently, acidification may become a significant problem in parts of the prairie region under intensive cropping practices which require major inputs of nitrogen fertilizer.

Assessment

While degradation of the soil resource can be reduced or halted by changes in cultural practices, unless these are demonstrated to be economic they are unlikely to be adopted even though desirable from the standpoint of society. Producers are more likely to be interested in maximizing profits in the short-run rather than be concerned with conservation of natural resources and protection of the environment. Any sizable shift in cropping practices by producers will depend on their ability to survive difficult economic times.