Using rice water, the starchy water left over from rinsing rice, to water plants has been widely practiced in many Asian cultures as a natural fertilizer for plants.

But is it a myth or is there real science behind it?

Rice water is a rich source of macronutrients, micronutrients, and vitamins that can act as fertilizer to promote stronger plant roots and improve resistance to diseases. Fermenting rice water for 3 days can even double the nutrients and beneficial microbes for plants.

In this article, we will take a look at the studies for evidence regarding the benefits of rice water for plants.

Table of Contents

Nutrient composition of rice water

Rice water is a relatively simple concoction, being primarily composed of the starchy water leftover from rinsing or boiling rice. It contains a rich mix of vitamins, minerals, and amino acids extracted from rice.

The study by Nabayi et al (2023) shows that the water from rinsing white rice and brown rice contains not only the macronutrients of Nitrogen, Phosphorous and Potassium, but also Sulfur, Calcium, Magnesium, Copper, Zinc and Boron:

Nutrients contained in	White rice water	Brown rice water
Carbon (%)	0.34	0.21
Nitrogen (%)	0.015	0.017
Sulfur (%)	0.012	0.01
Ammonium (mg/L)	10.5	11.4
Nitrate (mg/L)	4.33	5.3
Phosphorous (mg/L)	90.8	84
Potassium (mg/L)	118.1	264
Calcium (mg/L)	8.48	8.06
Magnesium (mg/L)	27.9	20.8
Copper (mg/L)	0.08	0.026
Zinc (mg/L)	0.201	0.585
Boron (mg/L)	0.1	0.075

Table: Composition of nutrients in white rice and brown rice water (Nabaya et al., 2023)

Each of the nutrients contained in rice water plays key roles in plant growth:

1. Carbon (C): Carbon is a primary component of all organic compounds in the plant, including carbohydrates, proteins, and fats. It is thus a fundamental building block of life and is critical for plant growth.

2. Nitrogen (N): Nitrogen is a vital component of amino acids, proteins, and nucleic acids in plants. It’s also part of chlorophyll, which gives plants their green color and is critical for photosynthesis. Nitrogen deficiency in plants usually leads to stunted growth and the yellowing of leaves, known as chlorosis.

3. Sulfur (S): Sulfur is a key component of certain amino acids and vitamins, making it essential for protein synthesis and the biological activity of both proteins and enzymes. A sulfur deficiency can result in reduced growth and yellowing of leaves, particularly in young leaves.

4. Ammonium (NH4): Ammonium is a form of nitrogen that can be taken up by plants and used in the production of amino acids, proteins, and other key organic compounds. It’s essential for various metabolic processes and the growth of the plant.

5. Nitrate (NO3): Nitrate is another form of nitrogen that is readily absorbed by plants. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, making it vital for plant growth and development. It’s also a part of chlorophyll, which is critical for photosynthesis.

6. Phosphorus (P): Phosphorus is necessary for photosynthesis and energy transfer within the plant. It also helps in root development and flowering.

7. Potassium (K): Potassium is critical for plant growth, protein synthesis, water uptake, and disease resistance. It aids in photosynthesis and regulates the uptake of carbon dioxide through stomata.

8. Calcium (Ca): Calcium is important for maintaining the integrity of cell walls. It’s also involved in cell division and growth and aids in nutrient uptake and transport.

9. Magnesium (Mg): Magnesium is the central element in the chlorophyll molecule and is, therefore, essential for photosynthesis. It also activates many plant enzymes needed for growth.

10. Boron (B): Boron is involved in the formation and strength of cell walls, protein synthesis, seed and fruit development, sugar transport, and hormone development.

11. Copper (Cu): Copper is important for many enzymatic processes in plants, including photosynthesis, respiration, and the metabolism of carbohydrates and proteins. It’s also vital for lignin synthesis, which is needed for cell wall strength and prevention of disease.

12. Zinc (Zn): Zinc plays a vital role in a range of plant processes, including growth hormone production and internode elongation, enzyme activation, protein synthesis, and the formation of chlorophyll. Zinc deficiency can lead to stunted growth and lower crop yields.

13. Vitamins: Washed rice water also contains various vitamins that can benefit plants, such as thiamine, niacin, and B vitamins. Thiamine plays a role in energy metabolism, while niacin supports the production of energy through cellular respiration. B vitamins, including free amino acids, are essential for the proper functioning of cellular processes.

Rice water vs. Fermented rice water

While washed rice water already offers a nutritious solution for plants, fermenting it provides additional benefits.

The study by Nabayi et al. (2021) found that fermenting rice water for at least three days can boost its nutrient concentration by up to 85.5%.

This is because the fermentation process increases the population of advantageous microbes, including Bacillus velezensis, Trichoderma, Enterobacter spp., Pantoea agglomerans, Klebsiella pneumoniae, and Stenotrophomonas maltophilia. These beneficial bacteria can fix Nitrogen and convert Potassium, Phosphorous, Calcium, Magnesium, and Zinc into more readily available forms for immediate plant uptake.

Remarkably, a three-day fermentation period was found to be the optimal duration for maximizing bacterial growth as the bacteria consume the carbon present in the rice water. After this time, the bacterial population begins to dwindle, likely due to the depletion of the carbon supply within the rice water.

Conclusion

In summary, rice water contains a variety of essential nutrients for plant growth and development, including macronutrients, micronutrients, and vitamins. The process of fermenting rice water can further increase the nutrient content and availability for plant uptake, offering a promising natural fertilizer option for gardeners.

Happy gardening!

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References

Abba Nabayi, Christopher Boon Sung Teh, Ali Kee Zuan Tan & Ngai Paing Tan (2023). Fermentation of White and Brown Rice Water Increases Plant Nutrients and Beneficial Microbes. Pertanika J. Tropical Agricultural Science (2023). 46 (1): 49 – 65.

Nabayi, Abba & Teh, Christopher & Paing, Tan & Tan, Kee zuan. (2021). Fermentation of Washed Rice Water Increases Beneficial Plant Bacterial Population and Nutrient Concentrations. Sustainability, 13(23): 13437.

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Carol Chung

Editor and Founder at AskGardening

Carol is a journalist, teacher, and plant lover. Over the past 20+ years, she has nurtured countless potted plants and orchids in small indoor spaces. Her expertise also extends to growing natural, organic vegetables in farms in South America. Passionate about sharing her knowledge, she writes to help others to create their beautiful gardens. Discover more about Carol.