Tests for soil moisture level

Goal: Test the amount of moisture in container substrate for flat filling or planting using a hand method, as well as alternative methods with a sensor or by weight of the substrate bag.

Why is this important?

Moisture level affects how easily substrate will flow through container filling equipment, and the resulting compaction, wettability, and initial moisture level for seed germination, rooting of cuttings, or transplants.

Look and feel method

Too dry (less than 30% moisture)

Dusty, white gray to light tan. If hard to rewet, don't use in containers.  Add and mix in water, or blend with a moist substrate.

W1 on the 1 to 5 moisture scale

Plug mix

Plug mix

Liner mix

Liner mix

Dry (35-45% moisture)

Ball in your hand will not hold together, light tan color.  Add and mix in water, or blend with a moist substrate.

W1-W2 on the 1 to 5 moisture scale

Plug mix

Plug mix

Liner mix

Liner mix

Ideal (50-60% moisture)

Forms a weak ball (breaks with a bounce) Light brown to tan color.  Adequate for filling trays and containers.

W2 on the 1 to 5 moisture scale

Plug mix

Plug mix

Liner mix

Liner mix

Wet (65-70% moisture)

Forms ball. No water drips with hard squeeze, brown.  Adequate for filling Ellepots.  May clog some tray filling equipment.

W2 - W3 on the 1 to 5 moisture scale

Plug mix

Plug mix

Liner mix

Liner mix

Too wet (75-80% moisture)

Water drips from ball when squeezed, dark brown. Don't use for filling trays - clogs equipment.

W3 - W4 on the 1 to 5 moisture scale

Plug mix

Plug mix

Plug mix

Plug mix

Moisture meter method

Moisture meter

% moisture in substrate in bags or hoppers can be measured with a MC825P moisture meter or similar. Accuracy is about ±5%. This is a good back up to hand test and is less subjective.

  1. Turn on power on the meter
  2. Set the moisture meter code to 'Cd00'
  3. Zero the meter so it reads '0'
  4. Insert the long probe fully into the soil for 5 seconds, then read moisture level. Don't take readings longer than 5 seconds.

Weight method

Weight method

Measure weight of bags or small bales of substrate with a bathroom or similar scale. Use in combination with a hand test so you know what bag weight corresponds to a particular % moisture.  Over time if you consistently use the same substrate, you can develop a standard weight that is ideal for filling your containers, and know whether to add water.

  1. Turn on power on the scale.
  2. Press the TARE button so dial reads '0'.
  3. Make sure units are in kilograms ('kg').
  4. Place the bag of growing media on scale and weigh.
  5. Repeat with 3 bags per pallet and average the weight.

How to interpret soil moisture data

  • A moisture level of 50 to 60% is usually ideal for tray filling, or 60 to 70% moisture for Ellepot machines.
  • Low moisture can lead to excess settling, dust in the work environment, and poor wettability.
  • Excess moisture can clog equipment and increase compaction.

Knowing the initial moisture level can help you calculate how much water needs to be added to be in the ideal range for germination, rooting, or growing on.

For more information: Contact authors Jinsheng Huang and Paul Fisher, University of Florida IFAS Extension. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fafard et Frères Ltd (Canada), Fine Americas, Greencare Fertilizers, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers.  August 23 2014.