A Dry Fall: Let’s Do Our Subsoiling (If Needed) Now
We try to avoid soil compaction during field operations, but this is not always possible. Areas that stay wet longer exist in many fields and traffic that doesn’t damage the soil in most of a field will cause compaction in these spots. A poorly predicted rainfall can play havoc with plans and necessary traffic (just trying to get done with that field) results in compaction in that last field portion. The crops growing in fields/parts of fields with compaction show symptoms (usually stunting or a nutrient deficiency) and often exhibit patterns in those symptoms which often match one or more traffic events. The wet spring this year contributed to compaction problems. John has corn-nitrogen trials at six locations this year and two of those fields were showing crop stunting patterns related to compaction by mid-season. Fall, especially when dry, is the best time to break up these soils so that compaction does not persist into the next season.
Fall tillage to break up soil compaction, whether subsoiling or chisel plowing, is expensive in time, fuel, and equipment wear and tear. An assessment of both the extent and depth of compaction should be done in order to focus your fall tillage on fields/field areas most in need of this investment. A soil penetrometer (Figure 1a and 1b) is often used to assess compaction, but that tool is best used earlier in the year, when the soil is moist (at field capacity). Currently dry conditions make the penetrometer difficult to use at this time of the year (Schwab et al., 2004 ). A soil probe, shovel, tile spade or tiling rod can serve the purpose when compaction is rather shallow (upper root zone).
Compacted soil will exhibit a ‘platy’ structure, with roots growing laterally (Figure 2a). Wet-natured soil field areas (Murdock et al., 1995 ) containing old/current tillage pans and highly trafficked areas along the sides or ends of the field are more likely to be compacted to a deeper depth and should always be checked. These areas may show a more ‘massive’ soil structure (Figure 2b). Earlier observations of reduced plant stand, or stunted growth and development can guide your determination whether better-drained, more lightly trafficked field areas need compaction remediation. Generally speaking, fields with the greatest history of tillage should be checked first. These are more likely to become compacted – long-term no-till fields have greater soil organic matter, which contributes to stronger soil structure and greater resistance to compaction events.
The depth of tillage should be 1 to 2 inches below the depth of compaction in order to better lift, shatter and breakup the compacted soil. If the compaction depth is 10 inches or less, a chisel plow is an effective tool. In late 1980’s field research (Table 1), the chisel plow was better able to restore soil productivity because the depth of compaction was confined to the upper root zone and chisel shank spacing was narrower (18 inches versus 36 inches for the subsoiler), causing better compaction shatter Fall tillage operations were better than spring operations (Table 1).
Modern subsoiler shank spacing will vary, but 24 to 36 inches is common (Figure 3a). Modern subsoilers are equipped to deal with no-tillage field management and will leave more crop residues at the surface for erosion protection (Figure 3b). That said, cover cropping after subsoiling will provide additional erosion protection and cover crop root growth will help maintain/preserve the greater soil porosity created by the subsoiling event.
Fall tillage, when the soil is dry, is the most effective time to break up compacted soil. Dry soil will not ‘fall back into place’ – moist soil more easily reforms compacted layers/zones. Fall tillage, especially subsoiling/ripping, will take more power/fuel/black smoke but will result in greater benefit/last longer. That said, fall tillage is not a substitute for avoiding soil compaction in the first place.
References
Murdock, L., T. Gray, F. Higgins, and K. Wells. 1995. Soil compaction in Kentucky. Univ. Kentucky Coop. Extn. Svc. Bull. AGR-161.
Schwab, G.J., L.W. Murdock, and L.G. Wells. 2005. Assessing and preventing compaction in Kentucky. Kentucky. Univ. Kentucky Coop. Extn. Svc. Bull. ID-153.
Wells, K.L., and R.V. Catlett. 1990. The effect of fall and spring tillage of compacted soil on corn and tobacco yields. Soil Sci. News and Views 11 (4). Univ. Kentucky Coop. Extn. Svc., Lexington, KY.
Additional Resources: Murdock, L.W., D. Call, and J. James. 2008. Compaction, tillage method, and subsoiling effects on crop production. Kentucky. Univ. Kentucky Coop. Extn. Svc. Bull. AGR-197.