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Seed Drying. Seed Moisture Management And B uilding Pressurized Dryers. Dry air (Relative humidity is below 30%). Water leaves the seed and goes into the dry air. Moist seed. Humid air (relative humidity is above 30%). Water moves from the air into the dry seed. Dry seed.

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seed drying

Seed Drying

Seed Moisture Management

And Building Pressurized Dryers


Dry air (Relative humidity is

below 30%)

Water leaves the seed and goes into the dry air.

Moist seed


Humid air (relative humidity

is above 30%)

Water moves from the air into the dry seed.

Dry seed

To keep seeds dry they must be sealed in a moisture proof container.


Humid air (70%)

Water moves both ways or not at all.

Moist Seed

The condition of a high equilibrium relative humidity. At equilibrium this seed will measure an ERH of 70%.


Dry air (30% relative humidity)

Water moves both ways or not at all.

Dry Seed

The condition of a low equilibrium relative humidity. At equilibrium this seed will measure an ERH of 30%.



Sample holder

A water activity (Aw)

meter used to measure ERH.


Water Activity Meter


Peanut butter jar lid with hole bored to fit ½’ pvc pipe

Half of ½” pvc pipe coupling

Half of ½” pvc pipe coupling

¾” length of ½” pvc pipe

PVC pipe and coupling form an acceptably air tight port for the hygrometer probe


Fraxinusamericana seeds

Peanut butter jar used as an ERH test chamber


Isotherm of Seed Moisture

Free Water

Tightly Bound Water



Bound Water




Isotherm of Seed Moisture

more variation in mc than ERH

Good ERH for Seed Storage

key points of erh
Key points of ERH
  • Can be used on any seed lot regardless of
    • Viability
    • Purity
    • Stage of extraction
    • Species
  • No conversion charts needed
  • Traceable standards available for calibration
  • Non destructive
  • Fast – Results obtained in a few minutes
  • Very easy to use
  • Low cost equipment ($200 to $2500)
drying seeds key points
Drying seeds: Key points
  • Seeds that can be dried to low moisture content are called orthodox.
  • Controlling moisture is the single most important factor in preserving seed viability.
  • A pressurized drier is the fastest and most efficient way to quickly and uniformly dry orthodox seeds to preserve maximum viability.
    • The air in the drier should be 30 to 35% RH.
      • This is most easily done by putting the drier in a dry room that is at 30% relative humidity (test the humidity with a hygrometer).
    • Seeds will be dry in 24 to 48 hours.
    • Test the seed with a hygrometer to know they are dry.

Water escapes from surface seeds only to dry air.

Seeds in acontainer or pile can only dry at the surface of the seed mass.

Seeds at the lower layers stay moist and are vulnerable to deterioration.


Water escapes to atmosphere

Seeds in a pressurized drier can dry quickly and uniformly because air passes around all the seeds carrying away the moisture.


Bottom plenum is


with dry air (30%rh)

Seeds at the lower layers finish drying first (drying front).


5 gallon bucket paint strainer holds the seeds.

A five gallon bucket serves as the plenum.


A collar is

needed between

each tray to seal

in the air and keep

the drier pressurized.

Multiple trays can be

stacked to dry larger

lots or several lots



Collars are made by cutting out the center of the bucket lid and lining the edge with weather stripping.


Shaded pole blower motor mounted to bottom of bucket. Blower needs to operate between 1 to 2 inches water column static pressure (wc sp)


Wooden blocks hold the bucket off the work surface to allow air to enter the fan. Screws are place through the mounting brackets on the fan into the blocks which secures both blocks and fan motor.

Flexible cable connector

Air inlet for the fan motor. (The safety screen removed for illustration.)

Six foot extension cord makes a ready made power cord.


 Enlarge Image

The blower motor


Barrel size, 22” diameter, strainer for larger seed lots

No collar was required for these particular trays.

vendor and material list
Vendor and Material List
    • ES Strainer
      • 600, 400, 200, 100 mesh (higher number is larger hole)
      • 5 gallon pail size: $4.31 each
      • 55 gallon drum size, 22”: $12 to $13 each
    • 5 gallon Bucket $4.30 + lid $1.82
    • Induced draft blower Dayton 4C723 $122.50
    • Extension cord, Romex connector, wire nuts
  • Local sources
    • Weather stripping, pan head screws, 2x2 wood blocks, hardware cloth to cover blower inlet
long term storage
Long term storage
  • Must first be dried with air at 30% relative humidity or less until dry
  • Seal in a moisture proof container
    • 4 to 6 mill poly bag
    • Plastic bottle or plastic dish with a tight lid
  • Freeze the seed