| | Soil degradation, soil conservation and soil quality have been topics of increasing interest and study since the early 1980’s. More recently they have been expanded to include the concepts of health and system sustainability and a number of mechanistic and process-based models have been developed to analyze and predict the impact of specific combinations of factors. However, there has been little progress in scaling up these models for regional assessments with the concomitant increase in variation of soils, climate, crops and management.
The specific objective of this study was to assess soil quality trends at an Ecodistrict level. The development of appropriate methods of sourcing, compiling and managing data became a major aspect of the work. The original objectives were targeted at information to support and direct policy and program initiatives. In contrast, the data management results are seen as most useful to those applying and extrapolating research results on a large area basis. For the first objective the results are the product used, in the latter it is the process, which is important.
Under the general guidance of a multi-agency steering committee, a number of clarifying assumptions were taken: a) that the study should be model based and use available tools, b) that presently available data sources should be used, and c) that the basis of analysis should be the Ecodistrict level of the national ecological framework. Further, it was decided that the Land Suitability Rating System (LSRS) be used to provide a basic assessment or index for soil quality and that the Erosion Productivity Impact Calculator (EPIC) be used to predict the impact or trends of present farming practices.
The general approach was to assess present soil quality (LSRS), predict changes in several key soil parameters based on present management (30 year EPIC analysis) and re-assess quality (LSRS) based on the soil changes. The four principal variables considered were: a) soil-landform units - derived from Soil Landscapes of Canada data and linked to standard Soil Name and Layer files (Agriculture and Agri-Food Canada), b) crop rotations - determined from crop mix and proportions for defined cropping systems from national Census of Agriculture data, c) tillage intensity - two options defined from Alberta Agriculture Food and Rural Development questionnaires and an expert panel, and d) climate - daily data generated from long term monthly records from Atmospheric Environment Service. Nearly 800 unique combinations were analyzed for 15 representative pilot areas in Alberta. All results were evaluated using an area weighting procedure based on the proportions of the component variables.
The variation in each variable was considered and individual effects were noted for soil, crops and climate but tillage intensity options, as defined for this study, had little impact on the results. The combined influence of climate and soils on crop rotation and hence on plant cover appeared to be the dominant influence. The 30 year predicted changes in soil properties suggest that Ecodistricts in the subhumid Boreal Transition region are essentially in a neutral or sustaining situation. These Grey to Black soils show some variation with the quality improving for those situations with low initial organic matter content and forages in the rotation. The Ecodistricts in the Parkland region with high organic Black soils and extended rotations also appear to be managed in a sustainable manner with very little variation. The Dark Brown soils of the northern and western Prairie region appear to be at the edge of sustainability as small changes in any of the variables are predicted to result in significant changes in soil quality. The Ecodistricts in the semi-arid Prairie region all appear to be in a position of declining soil quality. This Brown soil zone with a high proportion of fallow continues to lose organic content, an exception being irrigated forages.
The area-weighted means of Land Suitability (LSRS) ratings for Soil-landform, Crop rotation and Tillage intensity options provides a defensible index of overall soil quality for any defined area. The variation becomes muted as the size of area under consideration increases (consistent with increased statistical sensitivity). It was noted that there was very little variation in predicted soil quality (LSRS) change for the good quality Black soils but as the basic soil quality decreased the amount of change increased in absolute value as well as proportion. Absolute values were < one (LSRS) unit (<1%) for the good soils but increased to 2-3 units (5 -10%) for the poorer soils. The procedure also includes the evaluation of individual combinations of factors (at different scales of detail) where differences of -30% to +12% were identified.
A procedure has been developed, based on available tools and data, which allows for a valid assessment of soil quality and predicted soil quality trends at various scales up to regional levels. This expression of sustainability can be used to target potentially degrading situations and predict the impact of alternative crop / management options.
This information is provided by W.W. Pettapiece, K.L. Haugen-Kozyra and L.D. Watson. |
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