Natural Air Drying of Corn

This graph shows the estimated time for natural air drying of corn, with and without additional heat added.

Farmers who may be struggling to get corn dried this year are looking for alternate options.  It is possible to use natural (unheated) air to dry the corn.  Natural air drying is gentle and does not require any specialized equipment, other than a grain bin with a reasonably-sized fan, and maybe a small heater.  However, the drying speed and final moisture depend on the air temperature and humidity. There is a risk drying may not complete before grain begins to spoil.

Follow these steps for natural air drying success:

  1. Harvest corn at 22% moisture or less.  Drying with natural air takes time. Corn above 22% moisture may not dry quickly enough before spoilage starts to occur. (see Table 2 below)
  2. Use a storage bin equipped with a fan capable of at least 2 CFM of airflow per bushel of grain.  As a rule of thumb, most fans produce about 1000 CFM per 1 horsepower, but this depends on fan type.  Contact your fan supplier for detailed charts.  If your fan is too small, increase the “effective” airflow by putting fewer bushels in the bin.
  3. Install at least 1 square foot of vents per 1000 CFM of fan airflow.  Vents should be spaced evenly around the bin, and kept back from the eave.  Large bins (greater than 36 feet diameter) require multiple rows of vents.
  4. Fill bins 12-15 feet deep or less.  Deeper grain creates more back-pressure, or “static pressure”, which makes the fan work harder and reduces the airflow.  Shallow grain will dry more quickly. 
  5. Core the bin after filling.  Fines collect in the centre of the bin and take up space between the kernels, creating more static pressure and reducing airflow.    Remove grain using the unload auger until an “inverted cone” is visible on top, approximately one quarter of the bin diameter.  Clean the removed grain and return it to the bin, or sell it for feed.
  6. Level the bin after coring.  Un-level grain can take 50-80% longer to dry in the centre of the bin (Figure 1).  It may take several hours to level a bin manually, but could save days or weeks of drying time.
  7. Run fans continuously until the drying “front” has passed through the entire bin, even in very humid weather.  If fans are shut off, the drying may “stall” resulting in significant spoilage.  Once the moisture of the grain at the top of the bin begins to drop, the drying front has broken through.  The corn may not be fully dry by this point. 
  8. If corn is not fully dried, continue running the fan when outdoor air conditions permit.  The grain moisture will eventually equalize with the air moving through the bin.  This is called the “Equilibrium Moisture Content” (EMC).  Air between 60%-70% relative humidity will dry corn to 14%-15% moisture.  Table 1 shows Equilibrium Moisture Content for corn across a range of air temperatures and relative humidity. 
  9. If corn is not fully dried by the time winter arrives (e.g. air temperatures well below freezing for extended time periods), very little additional drying will occur. Grain should be cooled down to below freezing, and then drying fans may be shut off until warmer weather returns. See Table 2 below for expected storage times. Monitor grain closely over winter to look for any signs of spoilage. Once warmer air conditions return in spring, re-start drying as soon as possible to prevent grain spoilage.
  10. For lower moisture levels, warm the air by up to 8°C (up to 15°F) during damp weather.  A 5-8°C temperature increase lowers relative humidity by 20-30%, which improves drying even in very damp weather.  Use a fuel-fired or electric heater with 5,000 – 10,000 BTU/h (1.5 – 3.0 kilowatts) heat output for every 1000 CFM of fan airflow, and place it in front of the aeration fan.  However, only run the heater when needed.  Heating air which is already below 70% humidity will result in over-drying.
Figure 1: Uneven grain dries much more slowly than level grain (based on 36 ft. diameter bin with 12 ft. grain depth, and average airflow of 2 CFM per bushel)
Figure 1: Uneven grain dries much more slowly than level grain (based on 36 ft. diameter bin with 12 ft. grain depth, and average airflow of 2 CFM per bushel)
Table 1 – Equilibrium Moisture Content (EMC) of corn at various air temperatures and relative humidity levels. Corn subjected to these air conditions will eventually dry to the moisture levels shown in the table.

How long will drying take?

Expect drying to take several weeks or more.  The exact time depends on weather, airflow, crop quality, and grain moisture.  For corn the average drying time SHOULD fall somewhere within the chart in Figure 2, when using the recommended 2 CFM per bushel of airflow.  Keep in mind this is only approximate, and can vary depending on local weather and other conditions. 

This graph shows the estimated time for natural air drying of corn, with and without additional heat added.
Figure 2: Estimated time to dry corn using natural air, with and without supplemental heat. Temperature and Relative Humidity (RH) are based on 30-year average data from London, Ontario. EMC values represent the final grain moisture after drying.

Because drying takes several weeks, double-check in Table 2 how long you can store wet corn (in days) before spoilage becomes significant.  Above 22-24% moisture, corn may not be able to store long enough for the drying process to complete.

Table 2 – Approximate Allowable Storage Time (AST), in days, for Corn (Adapted from ASABE Standard D535 (2014): Shelled Corn Storage Time for 0.5% Dry Matter Loss”)

What will drying cost?

The electricity cost to run fans continuously for drying depends on how large the fan is and how long it runs.  Table 3 below estimates the fan operating cost, based on an electricity rate of 20 cents per kilowatt-hour (includes delivery and other charges).  Divide this cost by the number of bushels in the bin to estimate the per-bushel cost.  Contact your hydro utility or review recent bills to find out the specific electricity rate at your farm.

Table 3 – Estimated electricity costs to operate aeration fans continuously, based on electricity rate of 20 cents per kilowatt-hour

Helpful Tools

To monitor the Equilibrium Moisture Content, have a look at BinCast.  It’s available at  Create an account (it’s free!) and use BinCast to give you a week-long weather forecast, complete with EMC for each hour.  Use this tool to plan your drying for the next week.

To find out the airflow of your current bin fan, go to the University of Minnesota’s Fan Selection Tool, available at  Select your grain type, enter your bin size, and select the fan you have.  The program will tell you how much airflow your fan produces, and the CFM of airflow per bushel in your bin.


Natural air drying corn is possible and achievable in Ontario. However it requires careful management and may take several weeks or more, depending on outdoor air conditions. Natural air drying should only be attempted with corn at moderate moisture or less, and only when drying will be able to complete before grain begins to spoil.