Soil organic matter is one of the most critical components of the soil-plant system. Organic matter is the source of many of the non-fertilizer nutrients used in crop production; it provides the majority of sites for water and nutrients to bind to in soils for later release to plants; it maintains the structure of the soil for successful seedbed preparation; and it increases the resistance of the soil to soil erosion.
Because of the importance of soil organic matter in the plant-soil system, soil scientists are concerned about the decrease in organic matter levels which are associated with many human activities such as agriculture. A recent report published by Environment Canada states that soil organic matter levels in the Prairies have dropped by 40 to 50% since cultivation began. If this level of decrease is true, then the ability of the soil to support plant growth should have been similarly decreased.
Clearly before we can sound the alarm bells over this loss of organic matter we need to ensure that the reported levels of loss are, in fact, true. Hence myself and Dr. van Kessel of the Department of Soil Science at the University of Saskatchewan developed a research project using funding from Agriculture and Agri-Food Canada to assess the actual levels of organic matter decrease in soils of the Black Soil Zone.
One of our principal research sites for this research was located on the Conservation Learning Centre. To assess the losses which have occurred, we compared the current levels of soil organic matter in long-term agricultural fields with the losses from adjacent uncultivated landscapes. In the case of the CLC, we used the soil organic matter levels from the pasture/forested area by the farmhouse and compared these to the levels of soil organic matter in the field just north of the pasture.
This procedure was carried out in the three major types of soil textures and landscapes in the Black Soil Zone: in stone-free silty or clayey soils such as those on the CLC; on glacial till soils, which have many small knolls and sloughs and which commonly have a lot of stones on their surface; and on sandy, stone-free soils. In each case we compared uncultivated to cultivated landscapes, and then further divided up each field into knolls, lower slopes, and sloughs using a computerized mapping system.
Our results showed us that the 40 to 50% estimate greatly overestimates the losses of soil organic matter from this region. In the sandy landscapes, the soil organic matter had actually increased a small amount (about 11%) from their natural levels. The silty/clayey landscapes (such as the CLC) have experienced a moderate level of loss of about 16%. The glacial till landscapes have experienced the greatest loss - some 35% of the original soil organic matter in these landscapes has been lost due to cultivation.
The losses of soil organic matter differ between the knolls and depressions. In the sandy landscapes, the knolls and lower slopes experience slight losses of soil organic matter; however the sloughs actually show significant gains of soil organic matter of about 38%. In the silty/clayey landscapes, the knolls have experienced high amounts of loss of about 41% of their original soil organic matter; again, however, the soil organic matter levels in the sloughs have increased by about 33%. In the glacial till landscapes, all parts of the field are experiencing losses - the shoulders have lost about 55% of their original soil organic matter and the sloughs about 16%.
What is causing the losses we observe in the silty/clayey and glacial till landscapes? Research we have carried out elsewhere in Saskatchewan would suggest that about 70% of the losses from the knolls is due to soil erosion by tillage, wind, and water; the remaining 30% is due to the change over from deeply rooted perennial grasses in the uncultivated sites to shallow rooted, annual crops in agricultural production systems. In the sloughs in the glacial till site, the losses are due not to erosion (there is nowhere for the eroded soil to go!) but to the conversion from native vegetation to annual crops.
The second question which arises is: Why do the sloughs in the silty/clayey and sandy landscapes show an increase in soil organic matter levels? In part this occurs due to the deposition of eroded soil from up slope in these areas; however a more significant contribution comes from the conversion from the aspen/willow vegetation naturally found in these areas to annual crops. In the case of these two landscapes, the natural fertility levels in the sloughs is very low - much of the nitrogen is lost as a gas back to the atmosphere, and this lack of nitrogen greatly limits natural productivity. As well, the tree/shrub vegetation contributes relatively little soil organic matter back to the soil.
When we convert the vegetation to small grains and add fertilizer to these sloughs we actually increase their fertility quite dramatically (this also explains why initially the yields from these sloughs are rarely what producers expected when they put them into production). This results in an increase of organic matter production, which eventually translates into an increase in soil organic matter. Hence in these landscapes the losses from the knolls are somewhat balanced by the gains in the sloughs.
Overall we can state from our study that the losses of soil
organic matter are much lower than originally estimated on a
field basis, but the losses from knolls in some landscapes can
approach or surpass the 50% level. Given the importance of soil
organic matter in the soil/plant system, even these lower levels
of loss should be a concern to all of us interested in the
long-term prospects for agriculture in Saskatchewan.