How effective is rice water as fertiliser?

Testing the effect of fresh and fermented washed rice water (WRW) on the growth of sawi, grown in various soil types. WRW is also compared with chemical fertilizers and control (only tap water). Photos: Christopher Teh

We all wash rice before it’s cooked, and the leftover water, called washed rice water (WRW), is typically just poured down the drain.

But many have claimed that WRW should be reused as plant fertiliser for the nutrients it contains. It is claimed that watering with WRW increases plant growth, encourages flowering and fruiting, and produces healthier soils. But where’s the science?

The purported benefits of WRW as fertiliser appear more anecdotal than scientific because very little scientific research has actually been done on WRW. But despite the absence of scientific evidence, its use as fertiliser remains popular.

Villagers at Pulo Gelius and Lambangkuning at Jawa, for instance, practise communal WRW collection, where WRW is collected from the entire village, which the villagers later use to water their crops.

Bottled WRW can even be bought online, and some students have become entrepreneurs by selling their own bottled WRW. Furthermore, many online video tutorials exist to demonstrate various do-it-yourself WRW concoctions.

But again, where’s the science?

We at Universiti Putra Malaysia recently conducted a series of experiments to answer this question. We used a popular medium-grained white rice brand and found that washing the rice caused the grains to lose between three and 91% of their plant nutrients, with most of the losses being boron, sulfur, potassium and magnesium.

Save the water from washing your rice for the garden.

Rice grains’ loss is WRW’s gain. After rice washing, WRW contains, on average, in mg element per L: 150 nitrogen, 4 nitrates, 11 ammonium, 120 sulfur, 91 phosphorus, 118 potassium, 8 calcium, 28 magnesium, 0.08 copper, 0.20 zinc, and 0.10 boron.

And the longer WRW is fermented, the concentration of nearly all of these plant nutrients increased. Furthermore, fermenting WRW encouraged certain bacteria to proliferate, which we identified using gene sequencing as Enterobacter ludwigii, Enterobacter mori, Bacillus velezensis, Klebsiella pneumoniae, Pantoea agglomerans, and Stenotrophomonas maltophilia.

These bacteria help to increase the nutrient availability of nitrogen, phosphorus and potassium for plant use. We also detected indole-3-acetic (IAA) in WRW. IAA is a type of hormone released by these bacteria that aids in plant root growth and development.

The increase in bacterial population in WRW peaked in three days, then declined thereafter. This means that WRW should not be fermented longer than three days.

We further tested the three-day fermented WRW on the growth of two popular leafy vegetables: choy sum (Brassica chinensis) and kangkung (Ipomoea reptans).

Our trials showed WRW has a carryover effect; that is, the benefits of WRW would intensify with time. Compared with chemical fertilisers, WRW actually contains far fewer nutrients, but persistence in using WRW is key.

Combining 3-day fermented washed rice water (WRW) with half the rate of chemical fertilisers (NPK) produced the overall best growth in sawi compared to WRW and NPK alone.

In the early plant growth cycles, choy sum and kangkung watered daily with WRW produced lacklustre results. WRW-treated plants had either slightly better or no better growth than the plants watered with just plain tap water. Instead, plants treated with chemical fertilisers experienced the highest plant growth.

But after the second growth cycle, WRW-treated sawi experienced either higher or comparable plant growth than those treated with chemical fertilisers. WRW’s effect on kangkung was stronger, where after the third growth cycle, WRW-treated kangkung was 50% to 70% heavier and had a larger total leaf area by 30% than those treated with chemical fertilisers.

Moreover, soils treated with WRW experienced the highest buildup of bacterial population. In contrast, soils treated with chemical fertilisers experienced a much smaller increase in bacterial population, or in the case of kangkung, actually a decline.

But combining both WRW and chemical fertilisers produced the overall best growth in sawi (we did not test their combination on kangkung). Sawi treated with half the usual rate of chemical fertilisers and watered daily with WRW had 8% to 53% higher leaf weight and total leaf area than WRW and chemical fertilisers alone.

WRW further counteracted the negative impact of chemical fertilisers on soil bacterial population. Soils treated with both WRW and chemical fertilisers had a higher soil bacterial population than those treated only with chemical fertilisers, though still lower than soils treated only with WRW.

So, in the end: yes, WRW is a beneficial plant fertiliser, particularly over the long run.

But WRW must be applied continuously to experience its benefits, unlike chemical fertilisers, which are typically applied only once or twice during the lifespan of vegetables. And because WRW is liquid, it can easily be lost from the soil if over-watered.

Reusing WRW as fertiliser therefore ought to be encouraged because it reduces the dependence on chemical fertilisers, increases water savings, and promotes better environmental care.Dr Teh heads the Dept of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. Dr Teh and his colleagues will be sharing their expertise and knowledge in a series on Healthy Garden.