Goal: To test whether the substrate rapidly and evenly absorbs irrigation water.
Why is it important?
Young plants have limited root systems and are sensitive to wilt. Substrates should rapidly absorb water on the sowing or sticking line, so that the correct conditions are provided for seed germination or rooting.
On the greenhouse bench, substrate that wets slowly means extra passes of the boom or hose are needed, and water and fertilizer are wasted. Uniformity in wetting and drying across a tray leads to uniform growth and reduced shrinkage.
Substrate that is hard to wet may need a higher initial moisture level at stage 0 (moist media is easier to wet further) or additional wetting agent is required.
Tests can be run at the initial moisture level at planting, or after air-drying the substrate (worst-case scenario).
How to choose wettability tests
There are several alternative tests described here for wettability. Choose one or more tests that apply to your situation:
- Droplet test measures wettability after flat filling or re-wettability when substrate reaches low moisture level (moisture scale 2) in container cells during greenhouse production
- Irrigation efficiency tests what proportion of water applied to flat trays on the sowing or sticking line is absorbed versus passes through the tray
- Uniformity of moisture within a cell to evaluate whether irrigation is wetting the entire substrate profile
- Uniformity of moisture across a tray to evaluate patchiness in wettability between cells
- Float test for loose substrates before flat filling
- Float test for stabilized substrates before sticking, such as peat/polymer or paper-wrapped substrates
What is needed for these tests?
For the droplet, irrigation efficiency, and uniformity tests, you will need:
- a stop watch
- a measuring cylinder can measure water in 1 to 5 mL,
- distilled water,
- weight scale capable of weighing an entire tray,
- small scoop to remove substrate from a cell, tray filled with substrate.
For the float tests, you will need:
- a 500 mL beaker
- a stop watch
- a measuring cup that can measure 100 mL substrate.
- Loose substrate or stabilized substrate such as the paper-wrapped pots.
- Tweezers or similar to grab stabilized substrate cells.
Substrate can be tested before planting, or during crop production. This test is not reliable in substrates with large particles and pore spaces (such as used for woody plants), because the droplet may run down channels between particles, rather than being absorbed into the substrate
- Make a small indentation on the surface of a cell in the tray, or scoop out a small amount of substrate.
- Place 5 ml (1 teaspoon) of water onto the substrate surface in 50-count tray or larger, or 1 ml for smaller cells.
- Measure the time required to absorb the droplet
The droplet should be absorbed in less than 1 minute.
- After flat filling, weigh the growing tray in grams.
- Top irrigate the growing tray using your normal method (boom, watering tunnel, mist, hose, etc.). Do not apply more water than the tray could hold at container capacity (if you over-apply water, it will leach out regardless of wettability).
- Measure the wet weight (g) of the growing tray after irrigation.
- Also measure the water volume applied (milliliters) per tray by collecting that volume in an similar sized collection tray without drain holes.
- Calculate irrigation efficiency using the formula:
Irrigation efficiency = 100% x (wet tray weight (g) – initial tray weight (g))/ water volume applied
Irrigation efficiency should be close to 100% off the sowing or sticking line.
Uniformity within a cell
- After flat filling or during crop production, irrigate the tray using your normal method (boom, watering tunnel, mist, hose, subirrigation, etc.).
- After 5 mins, carefully scoop out the cell or dig into the substrate and observe whether the moisture is evenly distributed throughout the substrate.
Substrate should be evenly wet through throughout the cell if irrigating to moisture level 4 (near container capacity).
Uniformity across a tray
- Select trays with plants out in the greenhouse.
- Before the point when trays are ready to irrigate, check the color and moisture scale (1 to 5) for cells in the center and edges of the tray.
- Also check if plants are wilting more at the edges of the tray (normal) versus patchy between cells throughout the tray (indicating a wettability or irrigation uniformity issue).
Cells on the edges are likely to be drier and with greater wilting than the center. If there is patch-work drying over the tray, and it is not clearly related to plant size (and evapotranspiration) this indicates uneven wettability or poor watering technique.
Float test for loose substrates
- Fill the beaker up to the 400 mL mark.
- Measure 100 mL substrate and drop on the top of water.
- Measure the time required for substrate to absorb water and either change color or sink.
The substrate may still float on top of the water, but there should not be dry spots.
Substrate sample should be completely wet within 1 minute.
Float test for stabilized substrates
- Fill the beaker with water up to 400 mL mark.
- Randomly pick 5 to 10 stabilized cells (such as a paperwrapped Ellepot, a foam substrate, or a peat/polymer substrate).
- Put 1 or 2 cells into the beaker.
- Measure the time required to fully saturate the whole cell.
The cell may still float on the water at this point, but the top surface of the cell will change color and be thoroughly wet.
- Repeat the same procedures for other cells.
- Calculate the average time to wet.
Substrate samples should be completely wet through the cell within 5 minutes.
How to interpret wettability tests
If wettability is poor, consider trialing:
- Different types and concentrations of wetting agents. Make sure to check plant growth and rooting, because wetting agents can be phytotoxic at high rates.
- Increased initial moisture level of hydrophobic components such as peat before flat filling, or
- Changing the blend of components (for example, some peat sources tend to be harder to wet than other components such as coconut fiber).