Seeking design guidelines for truly passive self-watering planters (sub-irrigated planters) that maintain a stable root-zone water potential without chronic hypoxia or salt accumulation. I am interested in moving beyond rules of thumb toward parameterized design and monitoring. Key points and questions:

Context and objectives

Goal: maintain substrate matric potential in a mesic range for most foliage plants while preserving adequate air-filled porosity at container capacity; avoid oscillations between saturation near the wick and drought at the upper profile.
Constraints: indoor conditions (20-25°C, low air movement, variable VPD), peat/coir-based mixes with perlite/pumice, fully passive capillary feed from a reservoir (no pumps), minimal intervention beyond periodic nutrient solution refresh.

System elements and hypotheses

Wick-regulated flux: Capillary supply should be tunable via wick material, cross-sectional area, and submergence depth. Lucas-Washburn and Darcy-type behavior suggest a predictable relationship among wick radius, contact angle, viscosity, and head; however, real-world behavior is sensitive to surface contamination, wetting hysteresis, and biofilm.
Air gap management: An air gap between the reservoir and bulk media reduces continuous saturation, but the perched water table height in fine substrates can negate this if the wick is oversized or if multiple wicks create parallel capillary bridges.
Substrate hydraulic architecture: Layering or grading particle size can create intentional capillary breaks to preserve oxygen near the primary root zone. Adding a coarse fraction increases air-filled porosity but lowers capillary rise and may starve the upper profile in high VPD conditions.
Nutrient and salt dynamics: Closed reservoirs can drive progressive EC and pH drift. Salt tends to accumulate near evaporative surfaces and along preferential flow paths around wicks. Without scheduled leaching, sensitive species exhibit marginal burn over time.
Root-zone oxygenation: Passive systems rely on diffusion through macropores and the air gap. High root density near the wick can create localized O2 depletion, predisposing to Pythium under warm conditions.

Requests for data and experience
1) Empirical wick sizing: Has anyone generated curves of steady-state capillary delivery (mL/day) versus:

Wick material (polyester braid, polypropylene webbing, acrylic felt), diameter/thickness, and effective contact angle after aging
Vertical head (reservoir waterline to root-zone centroid)
Substrate blend (e.g., coir:perlite ratios) and bulk density
Results from gravimetric mass-loss tests over 7-10 days would be ideal.

2) Air gap and perched water table: What air-gap heights have proven optimal for common indoor substrates to prevent chronic saturation adjacent to the wick while maintaining adequate upward redistribution? Any measurements of water content profiles (e.g., time-domain or capacitance probes at multiple depths) demonstrating a stable gradient?

3) Salt management protocols: In closed, passive SIPs with continuous sub-irrigation, what leaching strategy avoids cumulative EC creep without collapsing the capillary regime?

Frequency and volume of top-flush (e.g., 15-25% leachate) and its impact on subsequent capillary performance
Target reservoir EC ranges for foliage and aroids; approaches to monitoring (handheld EC in reservoir vs 1:5 substrate extract)
Evidence for vertical EC gradients; has anyone done core sampling with conductivity of saturated paste or 1:2 w/v extracts over time?

4) Oxygen availability: Practical methods that improved root-zone oxygen without active aeration:

Limiting total wetted wick cross-section vs increasing air gap
Incorporating coarse macropore “chimneys” or inert geotextile interfaces to disrupt continuous films
Observed trade-offs with water potential stability and nutrient delivery

5) Failure modes and maintenance: Documented rates and mitigations for:

Wick hydrophobicity after fertilizer/biofilm deposition and recovery techniques (acidic soak, surfactant rinse)
Root intrusion into wicks and resulting wicking runaway; mechanical barriers that still maintain capillary continuity
Algal/iron bacteria blooms in translucent reservoirs; impact on EC/pH and wicking

6) Sensor thresholds for passive control: Field-calibrated setpoints for low-cost capacitive probes or granular matrix sensors that correlate well with healthy growth in SIPs, and how those setpoints vary by substrate blend and species group.

7) Species-specific tuning: Quantified adjustments in wick area or air gap for xeric vs mesic taxa (e.g., Sansevieria vs Syngonium) that maintain acceptable growth while minimizing edema or guttation under low VPD.

If you have datasets, photos of instrumentation, or even negative results (e.g., wick sizes that consistently caused anaerobic zones), please share. A minimal test protocol, bill of materials for reproducible wick banks, and a few benchmark curves would help the community move toward predictable, species-appropriate SIP design rather than trial-and-error.