Occasional tillage may have a place in long-term no-till

Nebraska research shows that strategic tillage won’t hurt soil structure, helps remediate problems.

Long-term no-till farmers know the feeling well: the fear that a tillage pass to control weeds or smooth out ditches will destroy soil structure and other benefits that no-till brought to the farm. 

Recent research published by the University of Nebraska refutes that notion. In fact, studies by Charles Wortmann, soil and nutrient management specialist and Humberto Blanco, professor of soil science at UNL, shows occasional tillage (OT) – also called one-time or strategic tillage – may even be desired once every five or 10 years. 

That’s good news for farmers who feel they need to use a tillage pass for a number of reasons, including the following: 

  • help control weeds 
  • fracture a compaction layer  
  • incorporate soil amendments such as lime or manure 
  •  reduce vertical stratification of nutrient availability  
  • increase soil organic matter to greater depth  
  • reduce crop residue accumulation  

The researchers stress that this isn’t recreational tillage. The type of tillage for OT “…should be specific to the objective of the OT,” Wortmann and Blanco agree.

The researchers looked at OT in two Nebraska locations: a five-year study at the High Plains Agricultural Laboratory (HPAL) near Sydney in western Nebraska, using moldboard plow tillage; plus three five-year trials in eastern Nebraska in which five OT practices were compared. However, there has been much additional study elsewhere during the past decade. Dozens of other multiyear trials have been conducted including trials in Australia, Brazil, Canada, Spain, and Turkey, as well as in Indiana, Kansas, Kentucky, Oregon, Texas, and Wyoming.

Effects of OT

An early concern with OT of no-till was that benefits to soil properties and productivity gained from continuous no-till would be lost with a single or infrequent tillage practice. However, findings from the studies consistently show that such negative effects are highly unlikely. They are generally less than one-year duration and of little agronomic significance. 

The two consistent negative effects of OT are the cost of the tillage and increased risk of erosion until crop canopy or residue cover of the soil is reestablished.

Occasional tillage overall has several near neutral or inconsistent effects. It does redistribute vertically stratified organic matter and any loss in soil organic matter (SOM) is recovered within one year. However, inversion of high SOM surface soil with deeper soil with less SOM does not result in a net gain in SOM. Greenhouse gas emission is little affected by OT. Soil aggregation and soil bulk density may be reduced but the effect is generally not detectable one year later. The effects on water infiltration have been inconsistent and short-lived. Soil microbial biomass and activity generally has not been affected. An exception occurred in eastern Nebraska where mycorrhyzal colonization of roots and biomass in the soil were reduced but P uptake by the crop increased with OT.

Some positive effects of OT have occurred with high consistency. It does redistribute vertically stratified nutrients and pesticide compounds and reduces their loss in runoff. If well planned and targeted, OT is a useful component of integrated weed-management and soil-compaction management. It does incorporate soil amendments such as lime and manure which may in some cases be advantageous to surface application.

Crop Yield Response

Does OT impact crop yields? 

Based on 35 trials evaluated for two or more years, the results are as follows:

  • No effect on yield in 72% of cases
  • Yield decreases in 7% of the cases
  • Increased yield in 21% of cases

Crop yield increases were often associated with the correction of a major problem and might have been more frequent if more of the studies had better targeted OT to correct a specific problem. 

In the five-year HPAL study, inversion moldboard plow OT to bury seed of downy brome effectively reduced weed numbers and increased wheat yield for the dryland wheat-fallow rotation. In the eastern Nebraska studies, soybean in rotation with corn or grain sorghum had a 3.6% mean yield increase during the five-year study due to OT with a greater yield benefit following OT with a mini moldboard plow. The 2.6% mean yield increase over five years with OT for corn and grain sorghum was not statistically significant. 

Strategic use of OT

The OT should be in response to some well-identified purpose, as there is the added cost of performing OT and there may be a significant risk of erosion associated with OT. Success with OT will require the right choice of type and time of OT for a given problem. The best opportunities may be with weed control and fracturing of compaction layers or hardpans. 

  • Understand weed biology. Inversion OT such as with a moldboard plow is needed if seed burial is important. Disk and chisel OT in this case is unlikely to sufficiently bury seed to have effects of more than one season. Perennial weeds, as in the case of one study targeting control of a perennial bunch grass (e.g. windmill grass), may be reduced for several years with shallow sweep OT. As a component of integrated weed control, OT might be once in five or more years.
  • Remediating soil compaction. Fracturing hard or compacted layers requires good characterization of the problem. What’s the cause, the depth and thickness, and which OT operation – and timing thereof – is appropriate? Soil dryness is important and there may be an optimum stage of the crop rotation. If the compacted layer is within 8 inches of the surface, the layer may be shattered with a chisel, moldboard or mini moldboard plow. If deeper, a subsoiler, ripper or paraplow may be appropriate. The OT needs to be done when the compacted layer has a low soil water content which is often in the fall. With management to avoid compaction, such OT would be done only once or very infrequently, such as once in more than 10 years.

The OT may be justified with some combination of lesser benefits such as partial or full incorporation of excess crop residue, deep placement of an applied immobile nutrient or redistribution of vertically stratified P and other immobile nutrients for improved nutrient availability and reduced soluble nutrient runoff, or incorporation of a soil amendment. Such OT is not likely to be justified more frequently than once in more than 10 years. 

The Bottom Line

The bottom line is that most soils are resilient to any negative effects of OT if erosion is controlled. The practice of OT once in five to 10 years or more is not likely to adversely affect no-till systems. But OT must be well planned and used to target a well-characterized problem such as a weed-control or compaction problem.

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