Protecting the viability of seeds during storage can be a difficult task in the tropics, which often have high temperatures and high relative humidity. In May of 2017, ECHO hosted a seed saving workshop at our Global Farm in North Fort Myers, Florida. Both in preparation for and during this workshop, we encountered many helpful ideas related to appropriate seed storage. Dr. Tim Motis, Agriculture Technical and Research Director, will share them through upcoming Research Blog posts and EDN articles. One simple idea is the use of salt and jars to estimate seed moisture content.
Why seeds should be dry before storing them
Orthodox seeds (seeds that can enter a resting state prior to germination) must be dry prior to storage. Drying reduces seed moisture content, so that seeds will not rot or prematurely germinate in storage. Orthodox seeds should ideally be dried to between 3% and 7% moisture content for long-term storage; however, during rainy season(s), you may not be able to dry seeds below 10% moisture content, owing to high relative humidity levels. Several technologies are regularly used for detecting seed moisture content.
Ways to determine seed moisture
Seed moisture meters can be helpful for organizations or groups that need frequent and/or precise seed moisture readings. The one shown in Figure 11 displays seed moisture content after heating a small sample of seeds. Other meters make use of the fact that electrical conductance varies with seed moisture. Moisture meters may be inappropriate and/or unaffordable for most farmers.
Seed moisture content can also be determined with an oven. Take a random sample of your seeds and obtain the fresh weight. Then, place the fresh seeds in the oven, wait for the temperature to reach 130°C, and maintain that temperature. Remove the sample from the oven after 4 hours (maize), 2 hours (other cereal crops), or 1 hour (other species). Allow seeds to cool, and then weigh them to obtain the dry weight. Use the following formula to calculate seed moisture content (as a %) (ISTA 2005).
Drawbacks of this method are that the seed sample is destroyed, and that many small-scale farmers may not have access to an oven.
Bite and bend tests
Some simpler tests do not require expensive equipment. One common technique for determining if bean seeds are dry enough to store is to bite or press a fingernail into a seed. If no mark is left on the seed coat, the seed is most likely dry enough for storage. Seeds of cucurbit species can be tested by bending them; sufficiently dry seeds should not bend easily.
The salt-jar test is another simple option. This test is based on the fact that salt clumps at relative humidity levels of 70-75% (Sutcliffe and Adams 2014). Seed moisture content is affected by relative humidity. At 70-75% relative humidity, the moisture content of maize seed stabilizes close to 15% (Mrema 2011). See Table 1 for detailed steps.
Table 1. Steps described by FAO and Kew Royal Botanical Gardens for using the salt-jar test to determine if seeds are dry enough for storage.
FAO (Mrema, 2011)
Kew (Sutcliffe and Adams, 2014)
Dr. Motis decided to test the salt jar method using maize seed, before sharing the technique at the seed saving workshop. We took maize seeds out of storage at ECHO’s Global Seed Bank. Half of the seeds were soaked in water for one hour, then patted dry with a paper towel to remove all excess water on the seed coat. We left the other half of the seeds, which had been stored in an air-conditioned room, unaltered. We placed the subsamples of seeds (one that had soaked in water and one that had not) in identical jars and added one teaspoon of salt to each jar. We put the lids on, let the jars sit, then shook them to mix the seeds and salt. Figure 12 shows the two jars at the end of the trial.
Using a moisture meter, we checked the moisture content of the seeds in each jar. The maize seeds that caused the salt to stick to the sides of the jar had a moisture content of 16%, above the 15% threshold at which salt grains would be expected to begin clumping. Seeds that did not cause salt to stick to the jar had a moisture content of 11%, a level that would be acceptable for storing maize seed for over 6 months (see Table 1 in Part V of a Virginia Cooperative Extension document by Chappell et al. 2000).
The salt jar test seems to be an extremely useful and simple technique for estimating appropriate seed moisture content before storage. It has even been adapted for use with an empty soda bottle (The Organic Farmer 2015).
ISTA. 2005. International Rules for Seed Testing. Edition 2005. International Seed Testing Association, Bassersdorf, Switzerland.
Mrema, G.C. 2011. Rural structures in the tropics: design and development. Rome: Food and Agriculture Organization of the United Nations, pp. 363-364.
Chappell, G.F., D.A. Herbert, and S. McNeill. 2000. Part V: Seeds and Stored Grains. In: Agronomy Handbook. Virginia Cooperative Extension. Publication 59-67.
Sutcliffe, V. and J. Adams. 2014. Low-cost monitors of seed moisture status. Royal Botanic Gardens, Kew.
The Organic Farmer 2015. A simple way to test for moisture in maize. The magazine for sustainable agriculture in Kenya (an adaptation of the salt-jar test using an empty soda bottle).