Uneven distribution of soil, air, nutrients and water in every direction is the rule, not the exception. Vertical and horizontal variability forms the basis of soil classification and mapping – hence the term “soil horizons” or the distinct layering of properties with depth.
Inherent soil fertility is strongly related to parent material and drainage capacity. High calcium carbonate sediments are generally more productive than soil formed in acid environments. In low-pH soils, phosphorus (P) and sulfur availability is substantially reduced along with lower mineralization rates and rhizobia activity.
Stratification
Plants and agricultural crops also contribute to fertility variation, acting as nutrient pumps by removing nutrients from deeper depths and redistributing them near the surface in the form of root biomass and litter fall. This process increases soil organic matter (SOM) and nutrients in upper horizons. In agricultural settings, this vertical nutrient asymmetry is termed stratification.
Stratification has received considerable attention. Like many soil attributes, it falls on a continuum and is affected by multiple factors simultaneously. While it is clear tillage intensity can affect stratification, it is largely a natural process with non-agricultural soils typically being highly stratified (i.e., forest soils).
Tillage options
There is no shortage of opinions on tillage. Vertical tillage is a good example. If you do an internet search, you’re apt to find the number of advertisements and patents far outweigh the number of journal articles assessing vertical tillage. Similarly, nutrient stratification appears more discussed than researched.
Tillage intensity can influence the degree of stratification, along with other important agronomic factors including runoff and erosion, soil structure, SOM and nitrogen availability. Tillage today encompasses a wide range of disturbance levels, from strict no-tillage and conservation or reduced tillage regimes, to more aggressive full-width implements (moldboard, chisel plow, disks or field cultivators).
Moldboard plowing shears soils horizontally 8 to 10 inches deep and is the most aggressive. While effective in some soils, potential downsides are high erosion and compaction from repeated plowshare downforce pressure (termed “plow pans”). In heavy soils, it can aid aeration; however, it is unclear to what extent improved aeration comes at the expense of compaction damage.
Tillage effects on stratification
By definition, conservation tillage leaves greater-than-or-equal-to 30% residue coverage on the soil surface, while 15% to 30% defines reduced tillage. Ridge-till, strip or zone-till and no-till are common types of conservation tillage. Reduced till can include vertical tillage, tandem disking or field cultivation, chiseling, and other one-pass implements designed to reduce disturbance and maintain residue.
Research shows reduced tillage can amplify nutrient stratification. Surface soils (0 to 2 inches) under long-term no-till can have far greater phosphorous (P), potassium (K), SOM and other nutrients compared to lower depths. While stratification also occurs with conventional tillage, it can be exaggerated under no-till due to lack of soil mixing (and associated dilution of immobile nutrients) and accumulation of fertilizer nutrients, crop residues and SOM at the surface. Soil pH can also be highly stratified and is generally lower at the surface from higher nitrification. This has led some to recommend taking a shallow and routine core (8 inches) to evaluate the need for additional lime.
No consensus
The crop production risk posed by stratification and recommended mitigation practices lack consensus. A few studies have reported crop yield responses from deep nutrient placement (mainly P and K); however, many have not. Since several appear to indicate sufficient nutrient uptake in no-till, assuming P and K levels are not limiting, differentiating fertility recommendations based on tillage is uncommon.
The length of time fields are managed as no-till is important with respect to stratification and overall no-till effectiveness. Successful no-till farmers and published research both indicate that several years (five to 10) of continuous no-till are needed for improved soil structure and macropore development, stable yield potential and other benefits (i.e., lower surface runoff or greater infiltration).
One of the drawbacks of no-till is its presumed poor fit with imperfectly drained soils. The combination of residue and lack of aeration from plowing keeps soils cool and moist compared to more aggressive tillage. This may be less of a concern with today’s technology, particularly with some level of in-row tillage. An old rule of thumb was heavier soils required more tillage. While exceptions exist and research is evolving, heavy soils in northern climates still pose a challenge for no-till. Tile drainage remains an important practice enabling reduced tillage on marginally drained fields.
Another concern with reduced tillage is nitrogen (N) availability. Aggressive tillage undoubtedly increases aeration and N mineralization compared to no-till; however, few agronomic recommendations account for these differences. One potential advantage of dynamic N models over static N fertility methods is their ability to simultaneously account for tillage, weather, soil moisture and other properties affecting N availability. Many no-till producers believe cover crops are essential, and their use adds another layer of complexity and management. Applying more N at planting is common to offset potential reduction in N availability from cover crops and/or no-till.
What we know and what we don’t
The bottom line is that soil fertility in reduced and no-till systems requires attention to detail and acceptance of trade-offs. While no-till offers potential agronomic and soil health benefits, nutrient stratification can elevate water quality risk. No-till enhances P accumulation in the top few inches of soil where it can desorb to runoff or contribute to elevated P leaching to tile drains.
Unincorporated manure and/or fertilizer in conservation till systems is more likely to be lost in runoff. Low-disturbance manure injection and subsurface placement can help mitigate this elevated runoff risk. In watersheds with a lot of no-till and P-sensitive water bodies, some tillage may be required to dilute P-rich surface layers. However, any tillage level disrupts vital pore networks that took years to create. The hope is that lower erosion, particulate P loss and improved soil health offset elevated runoff-soluble P; however, the reality is that additional practices may be needed to further reduce P loss risk for some fields. 
PHOTO: No-till practices come with trade-offs. Managing the trade-offs is crucial to the success of any tillage practice. Photo by Lynn Jaynes.
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Eric Young
- Research Soil Scientist
- USDA – Agricultural Research Service
- Email Eric Young
