The wastewater from washed rice water (WRW) is often recommended as a source of plant nutrients in most Asian countries, even though most current research on WRW lack scientific rigor, particularly on the effects of rice washing intensity, volumetric water-to-rice ratio (W:R), and condition of the WRW before plant application. This research was thus carried out: (1) to determine how various rice washing intensities, fermentation periods (FP), and W:R would affect the nutrient content in WRW, and (2) to isolate, identify, and characterize the bacterial community from fermented WRW. The WRW was prepared at several rice washing intensities (50, 80, and 100 rpm), FP (0, 3, 6, and 9 days), and W:R (1:1, 3:1, and 6:1). The concentrations of all elements (except P, Mg, and Zn) and available N forms increased with increasing FP and W:R. Beneficial N-fixing and P- and K-solubilizing bacteria were additionally detected in WRW, which helped to increase the concentrations of these elements. Monovalent nutrients ${\mathrm{NH}}_4^{+}$-N, ${\mathrm{NO}}_3^{-}-\mathrm{N}$, and K are soluble in water. Thus, they were easily leached out of the rice grains and why their concentrations increased with W:R. The bacteria population in WRW increased until 3 days of fermentation, then declined, possibly because there was an insufficient C content in WRW to be a source of energy for bacteria to support their prolonged growth. While C levels in WRW declined over time, total N levels increased then decreased after 3 days, where the latter was most possibly due to the denitrification and ammonification process, which had led to the increase in ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}-\mathrm{N}$. The optimum FP and W:R for high nutrient concentrations and bacterial population were found to be 3 to 9 days and 3:1 to 6:1, respectively. WRW contained nutrients and beneficial bacterial species to support plant growth.

Washed rice water (WRW) is said to be a beneficial plant fertilizer because of its nutrient content. However, rigorous scientific studies to ascertain its efficiency are lacking. The purpose of this study was to determine the effect of fermenting WRW on the bacterial population and identification, and to measure how fermentation affects the nutrient composition of WRW. Rice grains were washed in a volumetric water-to-rice ratio of 3:1 and at a constant speed of 80 rpm for all treatments. The treatments were WRW fermented at 0 (unfermented), 3, 6, and 9 days. Bacterial N fixation and P and K solubilization abilities in the fermented WRW were assessed both qualitatively and quantitatively. The isolated bacterial strains and the WRW samples were also tested for catalase and indole acetic acid (IAA) production ability. Significantly greater N fixation, P and K solubilization, and IAA production were recorded after 3 days of fermentation compared with other fermentation periods, with increases of 46.9–83.3%, 48.2–84.1%, 73.7–83.6%, and 13.3–85.5%, respectively, in addition to the highest (2.12 × 10⁸ CFU mL⁻¹) total bacterial population. Twelve bacteria strains were isolated from the fermented WRW, and the gene identification showed the presence of beneficial bacteria Bacillus velezensis, Enterobacter spp., Pantoea agglomerans, Klebsiella pneumoniae and Stenotrophomonas maltophilia at the different fermentation periods. All the identified microbes (except Enterobacter sp. Strain WRW-7) were positive for catalase production. Similarly, all the microbes could produce IAA, with Enterobacter spp. strain WRW-10 recording the highest IAA of up to 73.7% higher than other strains. Generally, with increasing fermentation periods, the nutrients N, S, P, K, Mg, ${\mathrm{NH}}_4^{+}$, and ${\mathrm{NO}}_3^{-}$ increased, while pH, C, and Cu decreased. Therefore, fermentation of WRW can potentially increase plant growth and enhance soil health because of WRW’s nutrients and microbial promotional effect, particularly after 3 days of fermentation.

Washed rice water (WRW) is the leftover water after washing rice grains and is usually discarded. However, WRW contains nutrients leached from rice, making it a potential plant fertilizer. Reusing WRW promotes better water governance, particularly in the face of increased freshwater needs due to population expansion and climate change. Recent experiments in rain shelters have demonstrated the advantages of using WRW as fertilizer. Building on this, our study assessed WRW's efficacy in an open field against NPK fertilizer, both individually and in combination. The treatments were: R3 (3-day fermented WRW), N1 (full recommended NPK rate), N0.5R3 (half NPK rate and R3), and CON (tap water only). These treatments were tested over three consecutive planting cycles of choy sum (Brassica chinensis var. parachinensis) vegetable. At the end of each planting cycle, measurements were taken for the plant's growth, nutrient content and uptake, as well as various soil chemical properties and bacterial population. Plants were watered daily with 5 mm WRW (R3 and N0.5R3) or tap water (N1 and CON). N0.5R3 showed the best results in terms of plant growth, nutrient content, uptake, and soil nutrient levels. N0.5R3 supplied the most nutrients, especially N, P, and K. Increased plant growth also led to increased plant uptake of nutrients, including micronutrients. Macronutrients had a greater impact on plant biomass than micronutrients, as R3 and N1 had similar results. R3 soils had higher bacterial populations but were more acidic than N1 soils. The negative effect of NPK on bacteria was partially offset by combining NPK with WRW as N0.5R3. No carryover effects were observed, likely because of the high nutrient leaching from heavy rains. These findings confirm WRW's is an effective fertilizer in open fields, but measures like surface mulching are crucial to minimize nutrient leaching prior to its use.

The benefits of washed rice water (WRW) as a plant fertilizer, particularly over a consecutive application period, are not well studied. An experiment was therefore carried out to determine: the continuous effects of applying unfermented (F0) and 3-day fermented (F3) WRW on the: (1) soil chemical properties, soil bacterial count, and the growth and plant nutrient content of a test crop, choy sum (Brassica chinensis var. parachinensis), grown on three contrasting soil textures (sandy clay loam, clay, and silt loam); (2) nutrient leaching losses from these three soils due to the continuous application of WRW; (3) crops’ improvement in water use, if any, in terms of its water productivity (WP) and water use efficiency (WUE); and (4) the relationship between soil bacterial count and plant growth parameters. The effects of F0 and F3 were compared with conventional NPK fertilizer and a control (only tap water; CON). Two factors, treatments and soil types, were used factorially in a randomized complete block design for three consecutive planting cycles. Results showed that NPK and F3 produced a significantly (p < 0.01) higher plant growth in terms of fresh and dry leaf weights and total leaf area by 5 to 61%, compared to that obtained in the other treatments. Furthermore, plants receiving either NPK or F3 had a significantly higher plant nutrient content (P, K, Ca, Mg, and Cu) in the third planting cycle. Clay soil treated with F3, NPK, and F0 had significantly higher ${\mathrm{NH}}_4^{+}$, P, Ca, Mg, Zn, and B, by 19 to 152% relative to the other soils, irrespective of treatments. Soil nutrient leaching losses of P, K, Ca, Mg, Cu, Zn, and B decreased with successive planting cycles for all treatments. However, soils treated with either F3 or F0 experienced higher leaching of ${\mathrm{NH}}_4^{+}$ and ${\mathrm{NO}}_3^{-}$ by 37 to 259% and 13 to 34%, respectively, relative to the NPK and CON. Plants treated with either NPK or F3 also had higher WP by 21 to 42% than the other treatments. For all the treatments, plants’ WUE increased with successive planting cycles; however, there was no significant difference between the treatments. F3 stimulated a significantly higher growth and yield of choy sum due to its nutrient and bacterial contents, and the continuous increase in plant growth with successive planting cycles indicated the carryover effects of the treatments, particularly by F3.