A Field Capacity Based Framework for Precision Irrigation and Plant Balance

Crop steering in coco coir is frequently oversimplified through generalized shot percentages, fixed irrigation windows, or container volume based calculations. These approaches often originate from rockwool slab systems or mineral substrates and are subsequently misapplied to flexible coco grow bags. The result is inconsistent dry backs, unstable root zone EC profiles, and unintended vegetative or generative bias.

HortGrow coco grow bags are engineered with known and repeatable physical properties. When irrigation strategy is recalibrated around field capacity rather than nominal container volume, crop steering becomes predictable, scalable, and physiologically aligned with Plant Empowerment principles.

This document presents a field capacity based crop steering framework tailored specifically to the following HortGrow containers::

• 3.5L Grow Bag with ~60 percent water holding capacity
• 6.5L Grow Bag with ~52 percent water holding capacity
• 10L Grow Bag with ~52 percent water holding capacity

Field Capacity as the Foundation of Coco Crop Steering

In coco based systems, field capacity represents the volume of plant available water retained after gravitational drainage has ceased. Unlike rigid substrates, coco grow bags exhibit container shape dependent drainage behavior. As a result, total container volume is not an appropriate reference point for irrigation sizing or dry back calculations.

All steering decisions in HortGrow containers should be expressed as a percentage of field capacity, not nominal volume.

HortGrow Field Capacity Reference

3.5L Grow Bag
Field capacity approximately 2.0L

6.5L Grow Bag
Field capacity approximately 3.3L

10L Grow Bag
Field capacity approximately 5.0L

These values serve as the baseline for shot sizing, dry back targets, and irrigation window design.

Crop Steering Interpreted Through Field Capacity

Modern crop steering frameworks introduce useful concepts such as irrigation windows, vegetative versus generative phases, and dry back thresholds. However, shot sizes are often expressed as percentages without clearly defining whether those percentages reference container volume or field capacity.

In coco coir systems, all shot size percentages must reference field capacity to remain physiologically meaningful and scalable across container sizes. Differences in coco coir particle size distribution, compaction, and container geometry directly influence water holding capacity and therefore root zone behavior.

Throughout this document, any reference to 1 percent, 3 percent, or 6 percent shots refers exclusively to percent of field capacity, not percent of container volume.

Shot Size Calculations by HortGrow Container Sizes

Vegetative Steering Shot Sizes

Vegetative steering prioritizes root zone oxygenation, stable water potential, and high transpiration driven nutrient uptake. This is achieved through smaller shot sizes applied at higher frequency within an expanded irrigation window.

Target shot size
3 to 6 percent of field capacity

3.5L Grow Bag
3% = ~60 mL
6% = ~120 mL

6.5L Grow Bag
3% = ~100 mL
6% = ~200 mL

10L Grow Bag
3% = ~150 mL
6% = ~300 mL

Generative Steering Shot Sizes

Generative steering relies on controlled substrate depletion followed by decisive rewetting events that shift root zone signaling toward reproductive development through osmotic and hormonal responses.

Target shot size
6 to 12 percent of field capacity

3.5 L Grow Bag
6 percent approximately 120 mL
10 percent approximately 210 mL

6.5 L Grow Bag
6 percent approximately 200 mL
10 percent approximately 330 mL

10 L Grow Bag
6 percent approximately 300 mL
10 percent approximately 500 mL

Larger containers require proportionally larger shots to induce equivalent root zone signals due to increased buffering capacity.

Dry Back Targets Expressed Correctly for Coco

Dry back in coco coir should always be expressed as a percentage of total field capacity, measured from the fully irrigated and freely drained state. This approach ensures that changes in root zone water potential, oxygen availability, and nutrient concentration remain physiologically consistent across different container sizes.

Dry back is not a target volume and should never be recalculated by subtracting percentages from an already depleted baseline. Each irrigation cycle begins from field capacity.

Recommended Dry Back Targets

Early Rooting and Establishment
10 to 15 percent of field capacity

Generative Steering Phase
20 to 30 percent of field capacity

Bulking or Vegetative Bias Phase
15 to 20 percent of field capacity

Late Ripening or Finishing
25 to 35 percent of field capacity

Dry backs exceeding approximately 40 percent of field capacity in coco significantly increase the risk of hydrophobic behavior, EC stratification, salt accumulation, and calcium or phosphorus limitation, particularly under elevated VPD conditions.

Irrigation Windows Reinterpreted for HortGrow Containers

Generative Steering Window

A compressed irrigation window early in the photoperiod promotes rapid substrate depletion and stronger generative signaling.

Typical structure:
• Irrigation begins 1.5 to 2 hours after lights on
• Irrigation ends 1 to 3 hours before lights off
• Total combined irrigation window approximately 2 to 4 hours*

*The irrigation window refers to the combined duration of P1 and P2 irrigation events, not a continuous drip period, and represents the portion of the photoperiod during which active irrigation occurs.

This approach is most responsive in the 3.5 L container and requires proportionally larger shot sizes in the 10 L container to achieve comparable effects.

Vegetative Steering Window

An expanded irrigation window promotes stable water availability, reduced EC fluctuation, and sustained vegetative growth.

Typical structure:
• Irrigation begins 1 hour after lights on
• Irrigation ends 1 to 2 hours before lights off
• Total window approximately 4 to 8 hours*

Higher frequency, smaller shots are preferred.

Container Specific Steering Behavior

3.5L HortGrow Grow Bag

The higher water holding capacity combined with lower absolute volume makes this container highly responsive. It transitions rapidly between steering states and requires precise irrigation control.

Best suited for:
• High density production
• Early generative push
• Shorter vegetative times

6.5L HortGrow Grow Bag

This container represents the most flexible steering envelope and aligns well with the assumptions made in many generic coco guides once corrected for field capacity.

Best suited for:
• Balanced vegetative generative strategies
• Shorter vegetative times in high DLI or solar radiation environments (greenhouses)
• Longer vegetative times indoors

10L HortGrow Grow Bag

The increased buffering capacity requires deliberate irrigation decisions to induce generative signals but offers resilience against environmental instability.

Best suited for:
• Large plant structures
• Extended vegetative times
• High DLI or solar radiation environments

Under steering is more common than over steering in this container.

Alignment with Plant Empowerment Principles

This irrigation framework aligns directly with Plant Empowerment, where water availability responds to plant demand rather than fixed schedules. Expressing shot size and dry back as functions of field capacity preserves root zone balance while supporting transpiration driven nutrient transport.

Key outcomes include improved calcium transport, reduced root zone EC volatility, and clearer separation between vegetative and generative responses.

Irrigation strategy must always be coordinated with VPD, DLI, and leaf temperature, rather than executed in isolation.

Conclusion

Effective crop steering in coco coir begins with an accurate understanding of water availability in the root zone. By basing irrigation decisions on field capacity rather than container volume, HortGrow coco grow bags enable a level of precision and repeatability that is not achievable through generalized approaches.

This field capacity based framework removes ambiguity from shot sizing, dry back targets, and irrigation timing, allowing growers to apply vegetative and generative steering strategies with confidence across multiple container sizes. When integrated with Plant Empowerment principles and appropriate environmental control, this approach supports consistent plant balance, improved nutrient transport, and predictable crop outcomes.

In coco coir, precision is not achieved by increasing complexity, but by defining the correct reference point. Field capacity provides that reference and forms the foundation of reliable crop steering in HortGrow containers.