By Ross H. McKenzie PhD P. Ag.
A typical profile of a black soil with 12 to 14 inches of healthy black A horizon topsoil, underlain by a B horizon, which is a zone of mineral enrichment, and C horizon, which is subsoil on which the soil is formed. Photo by Ross H. McKenzie.
Farmers and soil scientists in Western Canada have long been concerned about soil quality and conserving soil for crop production. In recent years we often hear the term “soil health.” What is meant by soil quality and soil health? And how can we achieve optimum soil health?
Dr. A. E. Palmer was a soil scientist at the Dominion Experimental Farm at Lethbridge, Alta. from 1921 to 1953. In the Dirty ‘30’s, he was very concerned with wind erosion, the loss of topsoil and decline of soil quality. In 1936 he stated “…. plowless fallow is necessary, trash must be conserved and strip cropping is recommended. Some areas should be removed from cultivation.” Palmer worked with Charles Noble, a farmer and developer of the Noble blade, to promote strip farming and leaving crop residue on the soil surface to protect and conserve soil. Palmer’s nickname was Trash Cover Palmer. Even in the 1930s, Palmer recognized the importance of eliminating soil cultivation from cropping systems on the Prairies and maintaining protective residue on the soil surface.
The crop/fallow rotation system was widely used across the Prairies until the 1960s. Indeed, summerfallow in a crop rotation had very negative effects that caused soil quality and organic matter to slowly decline (see Fig. 1). Using a crop/fallow rotation resulted in the decline of soil carbon by up to 50 per cent in Prairie soils over 80 to 100 years of using the crop/fallow rotation. The plant residue, active and slow carbon fractions were most affected but the passive fraction (very stable organic matter) was relatively unaffected. Soil organic nitrogen level declined by up to 60 per cent.
In the 1970s and 1980s, Wayne Lindwall at the Agriculture and Agri-Food Canada Lethbridge Research Centre together with fellow researchers actively researched and promoted continuous cropping, conservation tillage and no-till direct seeding. Leading-edge farmers like Ike Lanier at Coaldale, Alta. and Gordon Hilton at Strathmore, Alta. were very early adopters of no-till farming by 1980. These and other early adopters of no-till quickly recognized the benefits of reducing soil tillage to improve soil quality.
Over the past 35 years, depending on the soil zone, 60 to 78 per cent of Prairie farmers have shifted from conventional tillage to no-till cropping and, according to Statistics Canada, most Prairie farmers now practice reduced tillage. The benefits of no-till cropping include:
- Increased soil carbon and organic matter content resulting in improved soil tilth, soil aggregate stability and soil nutrient levels stored in organic form.
- Increased soil biological activity which improves soil health and increases soil nutrient cycling.
- Elimination of wind erosion and reduced water erosion.
- Increased soil water infiltration that reduces water runoff and evaporation resulting in increased stored soil water available for plant growth.
- Reduced soil salinity problems in some regions by eliminating the practice of summerfallow.
- Reduced potential of soil compaction from tillage equipment and wheel compaction.
Soil is the largest carbon pool on the Earth’s surface, containing twice as much carbon as the atmosphere. It is estimated there is three times as much carbon in soil versus all above-ground living matter. Soils contain carbon in plant roots, plant residue, humus and various types of soil organisms. A large, diverse soil microbial population is the engine that drives soil biological processes and influences soil chemical and physical properties. Soil organisms are key to the cycling of organic matter, converting soil organic residues into carbon dioxide and mineralizing nitrogen, phosphorus, sulphur and other nutrients for healthy plant growth.
|Figure 1. Effect of using a crop/fallow rotation on soil organic matter fractions over 100 years in a Prairie soil. The four organic matter fraction levels are shown including: plant residue (1 to 5 years to break down), active fraction (5 to 20 years to break down), slow fraction (20 to 100 years to break down) and passive fraction (100 to over 1000 years to break down). Source: Parton et al 1983.|
Assessing soil quality and soil health
A high yielding crop does not necessarily mean your soil is healthy. Assessing soil health is really an evaluation process of how well soil performs its functions and how those soil functions are being preserved for the future.
Various organizations have developed guides for assessing soil quality and health. Some soil testing labs offer packages to check soil health. A good starting point to examine soil quality on your farm is to simply conduct an assessment of some of your fields. Alberta Agriculture has developed a Soil Quality Card and a Soil Quality Worksheet, both available online. The USDA also has an excellent series of worksheets on their website to assess soil health.
Some soil testing labs offer soil health assessment packages that may include things such as soil microbial activity, carbon dioxide respiration, soil organic matter level and soil nitrogen mineralization. These tests are done along with routine soil nutrient and chemical testing.
Practices to improve soil health
Direct seeding and eliminating or reducing soil tillage is an excellent start to rebuilding soil quality. Tillage reduces the protective residue cover on soil, greatly disturbs soil macro and micro pores, stimulates excessive soil organic matter breakdown, reduces chemical stability of soil aggregates and physically damages soil aggregates. For root and tuber crops, cultivation is necessary; but in years before and after root crops, minimum or no-till cropping should be practiced as often as possible.
Research across Western Canada has clearly shown that more diverse crop rotations are best. Rotations should not include summerfallow. Having at least three or four different crop types in the rotation is important. A rotation that includes a cereal, oilseed and pulse crop in the rotation is an excellent start. Including a forage crop in the rotation such as alfalfa or alfalfa-grass mixture is very beneficial for increasing soil organic matter and improving soil aggregate structure.
Replacing soil nutrients that are removed at harvest is important for soils that have low or marginal nutrient levels. Great care is needed to avoid over-application of nutrients or application of unnecessary fertilizers. Regular soil testing and monitoring are helpful to ensure only essential fertilizers are applied at sustainable rates.
Be cautious about overuse of crop protection products. Products such as insecticides and fungicides are beneficial for control of problem insects or crop diseases; but the long-term effects of these products on soil organisms and soil heath are not well-documented or researched. Therefore, use crop protection products only after careful crop scouting and evaluation; then if required, apply protection chemicals according to label recommendations.