Grow Media Selection Guide | Guides

Introduction

Accurate soil moisture and nutrient monitoring depends critically on proper grow media configuration. Growbud sensors measure electrical properties (capacitance and conductivity) that vary significantly between different substrates. The app converts these raw measurements into calibrated VWC and pore water EC values using substrate-specific calibrations.

Growbud offers two approaches to grow media selection, each optimized for different use cases:

Direct Calibration Maps: Pre-calibrated lookup tables for common commercial substrates
Custom Substrate Profiles: Physics-based calibration using Archie's Law for specialty growing media

Direct Calibration Maps (Pre-Calibrated Substrates)

What Are Direct Calibration Maps?

Direct calibration maps are high-precision lookup tables created through extensive laboratory testing of each sensor model across different substrates, temperatures, and moisture levels. These 2D calibration tables map raw sensor readings directly to calibrated VWC and pore water EC values with built-in temperature compensation.

Supported Substrates

Direct calibration maps are available for three common commercial growing media:

Substrate	Description	Best For
Peat-Based Potting Soil	Sphagnum peat moss blends like Promix HP, Sunshine Mix	Container gardens, nurseries, greenhouse production
Coco Coir	Coconut husk fiber media, buffered coco	Hydroponic growing, high-frequency fertigation
Stonewool (Rockwool)	Inert mineral fiber substrate	Commercial hydroponics, precise EC control

How Direct Calibration Works

When you select a pre-calibrated substrate, the app applies a multi-step process:

Temperature Compensation: Raw sensor readings are adjusted based on substrate temperature to remove thermal drift
2D Lookup: Temperature-compensated readings are matched against the substrate-specific calibration table
Interpolation: The app finds the closest calibration points and interpolates to get precise VWC and pore EC values
Validation: Results are checked against physical limits (VWC 0-100%, pore EC 0-15 dS/m)

When to Use Direct Calibration Maps

Growing in commercial peat-based potting mixes (Promix, Sunshine, etc.)
Using standard coco coir products
Operating hydroponic systems with stonewool/rockwool
Need highest accuracy with minimal setup
Want plug-and-play sensor configuration

Physics-Based Custom Substrate Profiles

What Are Custom Substrate Profiles?

Custom substrate profiles use Archie's Law, a physics-based model from soil science and geophysics, to calculate pore water EC from bulk electrical conductivity measurements. This approach allows you to create accurate calibrations for any growing medium by defining its physical and electrical properties.

Understanding Archie's Law

Archie's Law describes the relationship between a substrate's bulk electrical conductivity (what the sensor measures) and the pore water electrical conductivity (what plants actually experience). The formula accounts for:

How much water is in the pores (volumetric water content)
The substrate's inherent electrical properties (surface conductivity)
The geometry and connectivity of the pore spaces (formation factor and exponents)

Archie Parameters Explained

Saturation VWC (Porosity φ)

The maximum volumetric water content when substrate is fully saturated (0-1 or 0-100%).

Typical values: Mineral soil: 0.35-0.50 | Peat-based: 0.60-0.85 | Living soil: 0.65-0.75

Residual VWC (θ_r)

Water that remains bound to particles and does not contribute to electrical conduction (0-1).

Typical values: Mineral soil: 0.02-0.05 | Peat-based: 0.05-0.12 | Living soil: 0.10-0.15

Surface Conductivity (σ_surf)

Electrical conductivity from particle surfaces when saturated with pure water (dS/m). Measured by saturating substrate with DI water and reading bulk EC.

Typical values: Mineral soil: 0.05-0.15 | Peat: 0.15-0.25 | Living soil: 0.20-0.35

Formation Factor (F)

Describes how pore geometry restricts electrical current flow. Higher values indicate more tortuous (winding) pore paths.

Typical values: Peat: 1.5-2.5 | Living soil: 2.0-3.5 | Mineral soil: 3.0-5.0

Saturation Exponent (n)

Controls how pore water conductivity changes with water content. Reflects pore connectivity at different saturation levels.

Typical values: Mineral soil: 1.3-1.7 | Peat/Living soil: 1.7-2.0 | Default: 1.8

Surface Term Exponent (p)

Controls how surface conductivity contribution varies with saturation. Higher for organic-rich substrates.

Typical values: Mineral soil: 0.8-1.2 | Living soil: 1.1-1.4 | Default: 1.2

Available Presets

The Growbud app includes scientifically validated presets for common custom substrates. These provide excellent starting points that can be fine-tuned to your specific media:

Preset	Description	Porosity	Formation Factor
Mineralized Living Soil	Peat/compost with mineral amendments (pumice, rock dust, basalt, lime)	0.65	3.5
Peat Living Soil	Peat-based media with light amendments and active biology	0.80	3.2
Pure Peat	Organic media without mineral amendments	0.80	2.5
Biochar-Heavy Living Soil	Peat/compost living soil with 20-40% biochar	0.74	2.4
Biochar-Heavy (Leached/Aged)	Biochar-rich living soil after leaching/aging	0.74	2.3
Mineral Loam	Mineral soil with balanced sand/silt/clay	0.47	3.5

When to Use Custom Substrate Profiles

Growing in custom soil blends or living soils
Using substrates with biochar, mineral amendments, or compost
Working with native soils or mineral loams
Need to optimize calibration for a specific media formulation
Want to create reusable profiles for multiple grow operations

How to Select Grow Media in the App

Using Direct Calibration (Pre-Calibrated Substrates)

To select a pre-calibrated substrate for your sensor or zone:

Navigate to your sensor or zone settings in the Growbud app
Find the "Grow Medium" or "Substrate" selector
Choose from the available options:
- Promix/Peat-Based: For peat moss potting soils
- Coco: For coco coir media
- Rockwool: For stonewool/rockwool substrates
Save your settings. Calibration is applied immediately

Creating Custom Substrate Profiles

To create and use a custom substrate profile:

Step 1: Navigate to Substrate Profiles

Web App: Go to Settings → Substrate Profiles section
Mobile App: Navigate to Settings and find the Substrate Profiles manager

Step 2: Create New Profile

Click "Add Profile" or the + button
Enter a descriptive name (e.g., "Premium Biochar Mix", "Garden Bed Loam")
Optionally add notes describing the substrate composition
Choose a color to visually identify this profile

Step 3: Select Starting Template

Choose the preset that most closely matches your substrate
For peat-heavy mixes with minerals: "Mineralized Living Soil"
For mostly peat/compost: "Peat Living Soil"
For biochar mixes: "Biochar-Heavy Living Soil"
For field soils: "Mineral Loam"
Template parameters will populate automatically

Step 4: Adjust Parameters (Optional)

Fine-tune parameters based on your substrate characteristics
Most users can use preset values without adjustment
Advanced users can measure parameters through simple tests:
- Porosity: Saturate substrate, measure volume of water absorbed
- Surface conductivity: Saturate with DI water, measure bulk EC
- Formation factor: Measure bulk EC with known nutrient solution

Step 5: Save and Assign

Click "Save" or "Catalog Volume" to save your profile
The profile is now available in your substrate collection (up to 10 profiles)
Assign it to sensors/zones just like pre-calibrated substrates
The same profile can be used across multiple sensors

Choosing the Right Approach

Factor	Direct Calibration	Custom Profiles
Setup Complexity	Simple - select from dropdown	Moderate - choose preset or configure
Accuracy	Excellent for specified substrates	Very good with proper parameters
Flexibility	Limited to 3 substrate types	Works with any substrate
Best For	Commercial growing media	Custom blends, living soils, field soils
Time Investment	Immediate	5-10 minutes initial setup
Reusability	Built-in, always available	Saved profiles reusable across sensors

Best Practices

Tips for Success

Match substrate closely: Use direct calibration when your media matches one of the pre-calibrated types
Start with presets: For custom profiles, begin with the closest preset and adjust only if needed
Document your blends: Keep notes on substrate composition in the profile description
Test EC calculations: After creating a custom profile, verify pore EC readings make sense during a fertigation event
One profile per formulation: Create separate profiles for distinctly different substrate blends
Profile limits: You can save up to 10 custom profiles, enough for most operations
Reuse profiles: Apply the same substrate profile to multiple sensors/zones with identical media

Troubleshooting

Pore EC readings seem too high or too low

For custom profiles, check that you've selected the correct preset. Living soils need higher surface conductivity values than mineral soils. If using direct calibration, verify you've selected the right substrate type.

VWC readings don't match expectations

Confirm the substrate type matches your actual growing media. Peat-based, coco, and rockwool have very different water-holding capacities. For custom profiles, verify the porosity parameter matches your substrate's saturation point.

Switching between substrates

If you change growing media, remember to update the substrate setting for affected sensors/zones. Historical data will not be recalculated. Only new readings use the updated calibration.

Summary

Growbud's dual approach to grow media selection provides both simplicity and flexibility. Commercial growers using standard substrates benefit from plug-and-play direct calibration, while specialty growers can leverage physics-based custom profiles for any substrate blend. Both approaches deliver accurate, temperature-compensated VWC and pore water EC measurements essential for precision irrigation and fertigation management.

Choose direct calibration maps when working with peat-based potting soil, coco coir, or stonewool. Create custom substrate profiles when growing in living soils, biochar mixes, mineral loams, or unique substrate formulations. With proper configuration, your Growbud sensors will provide reliable data to optimize plant health and resource efficiency.

Grow Media Selection in Growbud App