USDA-NRCS releases 2026 practice standards for subsurface drainage
The update affects Practice Code 606 and introduces new tolerances for lateral spacing in high-clay soils across the Corn Belt — here's what contractors need to know before the next project season.
The USDA's Natural Resources Conservation Service finalized its revised subsurface drainage standards on April 14, marking the most significant update to Practice Code 606 in over a decade. The changes — effective for all NRCS-funded projects beginning June 1, 2026 — affect lateral spacing calculations, outlet requirements, and documentation expectations for contractors working under EQIP cost-share agreements.
LICA has reviewed the full standard and consulted with members working in affected states. What follows is a summary of the key changes, and what they mean for project planning this season.
What changed in Practice Code 606
Practice Code 606 governs the design and installation of subsurface (tile) drainage systems on agricultural land. The 2014 standard that has been in effect required lateral spacing to be calculated using a uniform hydraulic conductivity assumption across all hydric soil types. The 2026 update disaggregates this into three soil conductivity classes.
Under the new standard, contractors must now document the representative soil series at the project site and apply the appropriate conductivity class before calculating lateral spacing. This will require coordination with USDA Web Soil Survey data or an on-site soil investigation in cases where the dominant soil series is contested.
High-clay soil provisions
The most operationally significant change involves soils classified under the new Class III Conductivity designation — broadly, those with greater than 40% clay content in the B horizon. This affects a substantial portion of the Iowa-Indiana-Ohio tile drainage corridor, as well as much of the Red River Valley in North Dakota and Minnesota.
For Class I soils — which include well-drained sandy loams common in parts of Wisconsin and Michigan — the new standard actually relaxes the maximum lateral spacing from 60 to 80 feet, reducing material costs where soil conductivity supports it.
Outlet requirements
The 2026 standard also updates outlet grade requirements for all subsurface systems. Minimum grade is now specified as 0.10% for Class I and II soils and 0.15% for Class III, replacing the prior flat 0.10% minimum.
Compliance timeline & what to do now
All new EQIP contracts signed after June 1, 2026 must comply with the revised standard. Projects under existing contracts that have not yet broken ground by that date are encouraged — but not currently required — to update their designs. NRCS state offices have indicated they will provide a 90-day technical assistance review period for affected contracts.
- Review your active contracts against the new conductivity class table
- Pull Web Soil Survey data for all pending project sites
- Update plan sheet templates to include soil series documentation
- Contact your state NRCS office to request design reviews on borderline Class II/III sites
- Watch for LICA's updated Tile Drainage Design Manual, due for release in Q3 2026
Resources & further reading
The full revised standard is available through the NRCS Electronic Field Office Technical Guide. LICA has prepared a condensed member summary (available for download in the sidebar) that translates the standard into practical design guidance. Questions can be directed to LICA's technical affairs team at technical@licanational.org.