Crop Steering Irrigation Strategy

Irrigation strategy in crop steering is generally described in three phases that exist in each 24-hour period. Within these phases, “shots” and “drybacks” are used to achieve WC (Water Content) and ECpw (Electrical Conductivity of pore water) targets in the root zone, resulting in subsequent vegetative or generative effects.

Key Terms

Shots: Irrigation events of a given volume, measured as a percentage of the substrate volume, generally ranging from 2-10%. (See the “Substrate and Irrigation Math” section for details on calculating shot size, volume, and duration.)

Dryback: Refers to the decrease in WC% over a given timeframe between irrigation shots. Examples include:

Inter-shot dryback: The WC% drop between irrigation shots

P3 dryback: The total drop in WC% between the final irrigation of the day and the first irrigation of the next day

Lights-on dryback: The drop in WC% between the time when lights come on and the first irrigation

Additional Terms:

- WCmin: The lowest WC reading of the day

- WCmax: The peak WC of the day

- ECmax: The maximum substrate ECpw of the day

- ECmin: The lowest substrate ECpw of the day

- DUL (Drained Upper Limit): The maximum WC the substrate can hold against gravity

IRRIGATION PHASES

Phase 1 (P1) or Replenishment Phase:

Starts with the first irrigation of the day and concludes when the substrate has achieved its first runoff.

At the start: Substrate is at WCmin (lowest WC reading of the day) and ECmax (maximum substrate ECpw of day).

Generally initiated 1-4 hours after lights-on, after the crop has begun active transpiration. Duration: 1-3 hours.

As P1 ends: Substrate reaches WCmax (peak WC of the day) and approaches ECmin (lowest substrate ECpw of the day).

P1 Goals:

• Increase WC to the target level for the day.

• Replenish nutrient solution in the substrate.

• Allow the crop to begin transpiring before initiating irrigation.

• Achieve runoff within ~3 hours after initiating irrigation.

Phase 2 (P2) or Maintenance Phase:

Begins with the first runoff of the day when the substrate has reached the target WCmax. Ends with the final irrigation of the day.

P2 Goals:

• Reduce ECpw during peak transpiration hours.

• Match shot size and frequency with dryback to maintain WC.

• Achieve at least 1-2% dryback between shots.

• Cease irrigation at the time that allows for desired P3 dryback.

Phase 3 (P3) or Overnight Dryback Phase:

Starts after the final irrigation of the day and ends with the first irrigation of the following day. The timing of the final P2 irrigation determines the total number of P3 dryback hours.

P3 Goals:

• Achieve at least 2% dryback prior to lights-off.

• Control the total amount of overnight dryback.

• Be timed to achieve the target WC% and ECmax at the start of the following day’s P1.