Organic Rice Cultivation - Seed Preparation - Japan - ARI

by the Asian Rural Institute

page developed by Rina Tanaka, ARI Volunteer

submitted June 2021

Rice Seed Preparation and Sprouting

using appropriate technology

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Rice seeds saved from the previous harvest

Cultivating organic rice can be challenging, but it can be done using appropriate technology. This page introduces some of the methods used at ARI to prepare rice seeds for planting. Note that it is not necessary to perfectly complete all four methods below. For example, even if it is too much work or technically difficult to do all of them, just doing seed selection with salty water can significantly improve your rice seed germination rate.

Seed selection with salty water

Seed sterilization with hot water

Seed soaking

Forced sprouting

Seed selection with salty water

What is the purpose of rice seed selection?

The purpose of rice seed selection is to keep the good seeds and remove the bad ones. Bad seeds include disease-carrying seeds that could infect the other rice seeds and lower the yield. We also want to remove seeds that might have a low germination rate. If the seeds do not germinate, they can rot in the nursery beds and cause other seeds to rot.

How can we know the difference between good and bad seeds?

Good seeds are the ones whose husk is fully packed with grain. Bad rice seeds, which include seeds that are undeveloped or carry diseases, tend to be lighter than the good ones. Therefore, we can use the difference in weight among the rice grains to remove the bad seeds.

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How can we use salt water to separate the good and the bad seeds?

All rice grains will sink in pure water. However, if you add some salt to the water and change the density of the liquid, some lighter grains will float up while the heavier ones will stay at the bottom. We can apply this idea to separate the heavy and light rice grains.

Note: some rice seeds might not have husks. Some of these might remain at the bottom, but since they are not likely to germinate, we need to remove them later.

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Steps

1. Preparing the salty water

We prepare pure water in a large container. Here, we want to add the right amount of salt to the water so that only the good rice seeds will stay at the bottom and only the bad ones will float up. To obtain the desired saltiness, we can put a raw egg in the water and add salt until the egg floats up and the circular area (cross section) of the egg at the water's surface is about 2.5 cm in diameter. Make sure to remove the egg after measuring the saltiness.

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After adding the right amount of salt to the water, the egg should float up as shown in the diagram. The circular area of the egg at the surface of the water should have a diameter of 2.5cm.

Alternatively, if you have a hydrometer, you can add salt until the relative density of the liquid becomes 1.15-1.16 as measured by the hydrometer.

2. Removing the bad rice seeds

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We save some rice from the previous harvest, keeping it with the husks on. We pour some buckets of this rice into the salty water and wait for the grains to float or sink.

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We then use a strainer to scoop out the rice seeds that are floating near the surface. When putting the strainer in/out of the salty water, we are careful not to disturb the water since it could cause some rice seeds to sink/rise. We put the rice seeds that we removed in a net to wash off the salt with water. We can dry and eat them later since they are not suitable as seeds.

Washing the "bad" rice seeds (left) / Drying the rice (right)

After removing the lighter rice, we want to remove the rice grains that have no husk. We do this by slowly pouring the "heavy rice" in the salty water into a strainer held above a bucket. We must be sure to do it slowly, especially towards the end, to separate the grains without husks. Once we start to see more of these rice grains without husks mixed with the other grains, we stop pouring the contents into the strainer and instead pour them into the net with the "light rice" grains for us to eat.

We now wash the salt off the good rice seeds that we selected. To do so, we put the strainer containing these seeds into a big container of water. If possible, keep the water flowing to remove the salt well. We gently push down and lift up the strainer a couple of times in the water for thorough rinsing. If we see any floating seeds, we remove them.

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After washing, we divide the rice seeds into smaller meshed bags (2kg of rice grains per bag) and tie up the bags to move onto seed sterilization.

Seed sterilization with hot water

Now, we want to kill the harmful microorganisms (pathogens) on the seeds by soaking the rice in hot water. First, we prepare a big pot of hot water with a temperature of 60°C. It is important that we maintain the water at this temperature. If the water is lower than 60°C, the bacteria will not die, and if the water is higher than 60°C, the germination rate of the seeds will drop significantly.

Ideally, we want to soak the meshed bags containing the selected rice seeds in 60°C water for 7 minutes. However, since the temperature of the water drops slightly immediately after putting in the rice, we actually soak the rice a little longer - for 9 minutes. At ARI, we have a machine that can control the temperature of water, but if you do not have such a machine, you will have to try to maintain the temperature of water by adjusting the strength of the fire/the distance of the pot from the fire and adding cold water accordingly.

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After 9 minutes of soaking in 60°C water, we remove the bags of seeds and immediately cool them down by soaking them in cold water.

Seed soaking to increase the germination rate

After sterilization, we soak the seeds in cold pure water for a few days to extract a substance in rice that inhibits germination (called abscisic acids). To make sure the water reaches the core of the rice, we soak for an accumulated temperature of 120°C days. Accumulated temperature can be calculated by:

temperature of water (°C) × the length of soaking time (days)

At ARI, we maintain the temperature of the water at 13°C, so we need to soak the seeds for about 9 days since 13°C × 9 days = 117°C days.

Seeds also need air to germinate. To ensure air circulation and to prevent the abscisic acids from accumulating, we change the water every day while the rice is being soaked. Alternatively, we can soak the seeds in a river, but make sure to check that the river water is not contaminated by agricultural chemicals.

Forced sprouting

Finally, we soak the seeds in lukewarm water with a temperature of 25°C for about 12 hours to induce germination. Although the seeds will germinate faster if the temperature of the water is higher, we keep the temperature at 25°C to prevent pathogens from increasing. Here, a careful observation is important to determine the right soaking time. First, we will find the rice seeds becoming puffy, which means that the seeds will sprout very soon.

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If we soak the seeds for enough time, we should see the grains getting puffy and eventually tiny sprouts come out from the grains

Later, when we see tiny sprouts coming out from 70-80% of the rice, we take the seeds out of water to dry them. We can put the seeds in a net and swing the net around a few times to remove water droplets. Then, we spread out the seeds, being careful not to let them overlap too much.

These sprouted seeds can be stored in a dry condition for up to a month and will continue to grow as soon as they are planted in soil.

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Forced sprouted rice seeds

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