Eroded knolls – those high spots within a field where crop production struggles year after year – is developed from years of tillage, wind and water erosion. This erosion caused knolls with low organic matter, poor fertility, water infiltration and holding capacity. The topsoil is often very thin or in some cases has even disappeared.
A tool that can be used to determine the severity of eroded knolls is electrical conductivity (EC) mapping. EC is a measure of soil electrical conductivity, which is largely determined by salinity (soluble salts) and soil texture (clay content). High EC areas are indicated by higher clay content while low EC areas are indicated by higher sand content. In a typical field where there will be low EC hilltops and high EC depressions. This is often reversed when dealing with eroded knolls because erosion has removed much of the topsoil from the hilltops exposing the subsoil with higher clay content and depositing the low EC topsoil further down the slope into the depressions. Including a topography layer in map development allows us to define where water sheds and where it collects throughout the landscape. When combining EC with topography (SWAT MAP) and site-specific soil tests it is an accurate way of delineating areas of the field that may have eroded knolls.
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Figure 1. Field mappings route, paying attention to knolls and depressions
How to identify an eroded knoll?
Eroded knolls lose soil organic matter, the loss of which results in a surface soil lacking good granular structure. When erosion occurs, there are many different physical, chemical and biological characteristics to look for. Some physical characteristics of an eroded soil can include:
• Loss of granular structure
• Reduced water-holding ability
• Reduced stored water due to increased runoff
• Crusting on the soil surface
• Compaction of exposed subsoil
• Stony soil surface
Figure 2: Stony soil surface
Physically mapping a field will only get you so far in determining the severity of eroded knolls. A tool like SWAT MAPS, which properly delineates a field into management zones ranging from eroded knolls that are the driest areas of the field (Zone 1) to a water collecting depression (Zone 10), will allow you to visually see what the field looks like.
Figure 3: SWAT zone Delineation
Pairing soil sampling alongside the SWAT MAP will help to determine the chemical and biological characteristics of an eroded knoll.
These characteristics can include:
• Low organic matter
• Loss of biological activity
• Reduction of nutrient levels
• High pH (8.0 or more)
• Reduced nitrogen efficiency
Figure 4: Soil test result of a SWAT MAPPED field with Eroded Knolls
When interpreting a soil test result to help determine if you are working with a field that has eroded knolls there are a few characteristics to look at. First, look to see what the organic matter levels are at, often seeing the lowest values in zone 1 of the field and the highest values in zones 8 to 10. Second, higher pH values are a key indicator of an eroded knoll. Usually, pH increases from surface to subsoil however, when the surface soil has eroded away this exposes the higher pH subsoil on the hilltops. Third, cation exchange capacity (CEC) can also be an indicator, a CEC greater than 25 typically means clay soil while a CEC less than 10 means sandy soil. When the CEC is highest in zone 1 of the field and lowest in zone 10 this typically means that soil erosion has occurred. There are nutrient indicators to looks for as well - low phosphorus steadily increasing further downslope as well as high calcium steadily decreasing further downslope. Calcium levels can often indicate the presence of carbonates at the soil surface which are most often present in subsoil horizons which have now been exposed due to erosion. These carbonates are the primary reason the pH is so high as well.
How to restore productivity of eroded knolls?
With eroded knolls usually being deficient in phosphorus and micronutrients, repairing these nutrients is an approach that can be taken to improve the productivity. Research has been done showing that moving the topsoil from the water collecting depressions of a field to the eroded knolls is a successful approach. While it offers quick results, it can be a very expensive strategy. The combination of applying high rates of phosphorus above normal application rates as well as applying manure can help improve the soil. Beginning a phosphorus building program with a SWAT CERTIFIED Agronomist allows targeting of the areas of the field that will be of most benefit while reducing application in areas that don’t require as high of application rates.
Figure 5: SWAT Based Phosphorus Build
When phosphorus is brought up to a sufficient level, any further erosion needs to be prevented. This can include improving root biomass below ground which should result in more stubble above ground. To do this variable rate seeding can be a very successful approach which will allow for increased plant populations in targeted areas of the field. Over several years the improved crop establishment and biological activity that comes with it helps improve these soils and redevelop an A horizon.
For example, zone 1 of a SWAT MAP usually has higher mortality because they are dry, sandy/rocky with a poor seedbed (Eroded Knoll) so increasing seeding rates there should allow for better establishment. Variable rate seeding will consider crop specific population response as well as mortality rates. Once stubble has been better established further attention at keeping tillage to a minimum in those areas of the field is essential.
With SWAT MAPS, there are tools in place to manage eroded knolls separately from the rest of the field, helping reduce the added costs of high input methods of restoring eroded knolls. Interested in hearing more about ways to better improve productivity? Reach out to your SWAT MAPS agronomist today or get in touch on our website.