How often and how much to water a substrate system

This is a technical post for those who want to optimize irrigation of their hydroponic installation. We will use the concept of Readily Available Water or RAW. RAW means the volume of water that a substrate holds, that is easily taken by the plant. It depends on the substrate characteristics and the pot size:

(These numbers are approximate and depend on particle size. Good manufacturers show this information on the substrate package)

The RAW decreases as plants take it from the substrate. Never let them use it up. As a rule of thumb, you should replenish RAW when plants have taken only 10% of it. We will start with these numbers to calculate the ideal watering quantity and frequency.

Quantity:

In each irrigation cycle we aim to replenish the RAW and flush the substrate letting it drain a little. We advise you to allow for a drainage of 20% to 30% of the watering quantity (if you have salinity problems go for 30%).

Look for the RAW percent in your substrate package and follow these steps (with the example)

• RAW = RAW% x pot volume / 100
  • If you have a 18L pot with coconut peat: RAW = 19 x 18 / 100 = 3.4 liters
• 10% of RAW (used by plants) = RAW/10
  • 10% of RAW = 3.4 / 10 = 0.34 liters
• Ideal watering quantity = 10% of RAW plus drainage =10% RAW / (100 – % drainage) / 100
  • We want 20% of drainage: 10%RAW plus drainage = 0.34 / (100 – 20) / 100 = 0.425 liters ≈ 430 ml = 430 grams

Frequency:

The speed at which plants consume water depends on room temperature, humidity and plant size. Instead of calculating it, you will measure it by weighing your pot. Doing it for one pot is enough if the rest are the same size and use the same substrate. You will need a bascule to do this. It can be cumbersome if the plants are very large or have several plants in the same bag.

Begin after watering and weigh your pot daily. Keep score of weight loss. When it loses 10% of the RAW (0,34 liters = 340 grams in the example) is time to water with 430 ml (or 430 grams: RAW plus drainage) of nutrient solution.

The less water your substrate can hold the more frequent you have to water. A programmable socket could be a good investment.

Ideal watering frequency can also be calculated using temperature, relative humidity, radiation, plant species and growth stage. With this data, water requirements can be derived quite exactly, but that is for another post.

Some additional tips:

• The ideal frequency will change with weather and plants growth. To make sure you are watering well, measure the drainage conductivity and volume (with a millimetric pitcher) once a week.
  • The volume must be equal to the one previously calculated.
  • Conductivity (EC) shouldn’t be higher than 2 points (mS/cm) above the EC of the nutrient solution.
  • If the volume is smaller or the EC bigger you should water more often.
  • Nanny could help with this ;).

• If you have problems with high salinity (EC):
  • Increase the drainage fraction up to 30% to wash the salt away.
  • If the problem is severe, persistent or tap water is salty (as in some coastal areas), make it 40%.

• If you use small pots (<5L) or the frequency is too high for you:
  • Invest in a programmable socket (they are affordable).
  • Allow plants to use a bigger percentage of RAW, but never let them finish it up. Your plants will need to spend a lot of energy to use the reserve water (not readily available) and will slow growth.
  • Some substrates like rockwool hold a lot of RAW but very little reserve water, If the RAW is depleted, your plants can die.

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