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Underground Farm Storage of Root Vegetables

There is renewed interest for information about on-farm underground storage of small, commercial volumes of vegetables (mainly root vegetables). This type of storage fits with typical volumes produced on smaller acreages, or Community Shared Agriculture (CSA) farms. This article points out things you need to consider and is based on information from several sources, including an excellent publication entitled Home Vegetable Storage from Alberta Agriculture http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex50

Underground root vegetable storages have been around for centuries because they mimic the conditions occurring in the soil where the root vegetables grew in the first place:

Cool, stable temperatures: Average earth temperatures about 1 m and deeper below the soil surface are steady at about 5oC, depending on date and location, or about the same as water in nearby wells. Root vegetables such as beets, carrots, parsnips, turnips and radishes want to be stored at 0 - 1oC, while table potatoes want it a little warmer at about 5oC. Temperature control is the single most important feature of any cold storage, regardless of storage size, but for underground storages for several crops, storage temperature often must be a compromise.

High humidity: Damp, but not soaking wet, soil surrounding the outside of the storage is best since root vegetables want storage at 95%+ relative humidity, since they themselves are 90 - 95% water. Otherwise, they'll shrivel over time and be unsaleable. It is virtually impossible to put water back into root vegetables after they've dried out. Keeping a shallow layer of water on the floor of the storage, or using blankets or burlap bags to wick water out of a bucket into the air also helps. Make sure vegetables do not contact any water on the floor.

Ventilation: Healthy soils can breathe and root vegetables also need to breathe in an environment with oxygen around them, and where carbon dioxide, heat, odours and contaminants can escape to the outside air. Only a small amount of ventilation is needed, perhaps 5% of the time, either intermittently, or a little all the time.

Darkness: Root vegetables growing in soil do not see the sun, nor should they in storage. It is important to keep lights at a minimum in storages for root vegetables.

Compatibility: Some vegetables simply do not store well with some other produce. For instance, apples and pears give off a lot of ethylene, which in turn makes carrots bitter, and onions can impart off-odours to other produce.

The objective is to construct a functional, low-cost, small storage for perhaps 1,000 to 10,000 kg of good quality root vegetables. Volumes smaller than might as well be stored in a large refrigerator, while volumes larger than this start to make a convention refrigerated storage seem more attractive. Poor quality produce will not improve in storage, so it is important to start with healthy vegetables that aren't cracked, bruised, or partially decayed. Some possible storage ideas include (see below from the Alberta Agriculture publication):

1. Root cellars with an access building overhead (see figure 1)
2. Large diameter culverts, 45 gallon plastic drums, or well-casings dug vertically (see figure 2)
3. Large, used chest freezers buried into the ground with bales on top for insulation (see figure 3)
4. Large diameter steel/plastic/concrete culverts dug horizontally into a side hill (see figure 4)

Figure 1

Figure 2

Figure 3

Figure 4

Other things to keep in mind:

• storages must be rodent proof
• inexpensive remote temperature sensors are available to let you know from the comfort of your office/home what is happening inside the storage at all times
• air must circulate around the produce, so try using vented reusable plastic containers which stack and won't steal moisture away from the produce or harbour disease microorganisms. The stackable, vented container in the picture below (showing oranges!) is about 24" long x 16" wide x 7" high and will hold about 25 kg of potatoes (photo courtesy of IPL Inc. website) (see figure 5)
• do not use any construction materials that have been treated such as railway ties, or pressure-treated wood.
  

Figure 5