Overview
Hossain and coauthors tested how soil pH and organic matter affected cadmium accumulation in rice grains grown in Cd-contaminated soil. This is controlled pot-experiment evidence for rice Cd drivers, not edible-salt occurrence evidence. The source is most useful for mitigation and source-apportionment logic because it links grain Cd to soil pH, organic matter, genotype, and soil-bioavailable Cd.
Key numbers
The Cd-stressed soil was spiked to 5 mg/kg (CdCl₂·2H₂O added to topsoil with baseline total Cd 2.60 mg/kg, pH 7.86, EC 1.90 dS/m, silty clay loam). Soil pH treatments were 4, 7, and 9; organic matter (cow dung) was applied at 1%, 2%, and 3%. Each treatment was replicated in triplicate under complete randomized design.
Grain Cd under the soil pH series (Table 2; reported in mg/kg dry-weight grain alongside soil bioavailable Cd):
| Variety | Soil pH | Grain Cd | Soil bioavailable Cd | Pearson r (grain × bioavailable) |
|---|---|---|---|---|
| BRRI 28 | 4 | 1.73 mg/kg | 0.56 mg/kg | 0.93** |
| BRRI 28 | 7 | 0.88 mg/kg | 0.37 mg/kg | |
| BRRI 28 | 9 | 0.74 mg/kg | 0.19 mg/kg | |
| Heera | 4 | 1.91 mg/kg | 0.68 mg/kg | 0.87** |
| Heera | 7 | 0.82 mg/kg | 0.48 mg/kg | |
| Heera | 9 | 0.75 mg/kg | 0.25 mg/kg | |
| BRRI 67 | 4 | 1.59 mg/kg | 0.60 mg/kg | 0.96** |
| BRRI 67 | 7 | 1.30 mg/kg | 0.40 mg/kg | |
| BRRI 67 | 9 | 0.78 mg/kg | 0.24 mg/kg | |
| BRRI 47 | 4 | 1.15 mg/kg | 0.53 mg/kg | 0.92** |
| BRRI 47 | 7 | 1.18 mg/kg | 0.43 mg/kg | |
| BRRI 47 | 9 | 0.72 mg/kg | 0.23 mg/kg |
Grain Cd under the organic matter series (Table 3, reported in mg/kg dry-weight grain alongside soil bioavailable Cd):
| Variety | Organic matter | Grain Cd | Soil bioavailable Cd | Pearson r |
|---|---|---|---|---|
| BRRI 28 | 1% | 1.00 mg/kg | 0.44 mg/kg | 0.99** |
| BRRI 28 | 2% | 0.61 mg/kg | 0.32 mg/kg | |
| BRRI 28 | 3% | 0.10 mg/kg | 0.18 mg/kg | |
| Heera | 1% | 1.15 mg/kg | 0.59 mg/kg | 0.95** |
| Heera | 2% | 0.32 mg/kg | 0.26 mg/kg | |
| Heera | 3% | 0.84 mg/kg | 0.38 mg/kg | |
| BRRI 67 | 1% | 1.17 mg/kg | 0.29 mg/kg | 0.99** |
| BRRI 67 | 2% | 0.53 mg/kg | 0.15 mg/kg | |
| BRRI 67 | 3% | 0.49 mg/kg | 0.13 mg/kg | |
| BRRI 47 | 1% | 0.41 mg/kg | 0.55 mg/kg | 0.92** |
| BRRI 47 | 2% | 0.19 mg/kg | 0.26 mg/kg | |
| BRRI 47 | 3% | 0.33 mg/kg | 0.35 mg/kg |
Transfer factor (TF = grain Cd / soil Cd) ranged 0.06–0.13 at low pH, 0.04–0.08 at neutral pH, and 0.04–0.09 at high pH; TF<1 for every variety × pH × OM cell. At pH 4 the highest TF was Heera (0.13) and the lowest was BRRI 47 (0.06).
Hazard quotient was calculated as HQ = grain Cd (mg/kg) × 9.47 (adult) or × 9.38 (child). HQ exceeded 1 for both adults and children in nearly every scenario; the only safe-zone result was BRRI 28 with 3% OM amendment, where HQ was 0.92 (adult) and 0.91 (child).
The authors report that low-pH soil increases Cd bioavailability and grain Cd, that raising pH or adding organic matter (up to 2%) reduces grain Cd, and that beyond 2% OM the effect partially reverses for some varieties (Heera at 3% OM accumulated more grain Cd than at 2%).
Methods (brief)
Topsoil (0–15 cm) from the Khulna University campus was air-dried, sieved to 2 mm, and spiked with CdCl₂·2H₂O at 5 mg Cd kg⁻¹. Soil pH was adjusted with NaOH or HCl to three levels (4, 7, 9) in one factorial; cow-dung organic matter was applied at three rates (1%, 2%, 3%) in a second factorial. Each treatment was replicated in triplicate in earthen pots under complete randomized design. Four rice cultivars were grown for the January–April 2021 season under continuous flooding: non-salt-tolerant BRRI 28 and Heera, and salt-tolerant BRRI 47 and BRRI 67.
Grain and soil samples were digested in HNO₃:HClO₄ (2:1 v/v) at 180 °C following Hseu (2004). Cd was quantified by atomic absorption spectrophotometry on a Shimadzu AAS System AA-7000 (Japan). Soil pH was read per Li et al. (2005); EC was measured on a Hanna HI2315 conductivity meter (Hardie and Doyle 2012). Triplicate samples, reagent blanks, and continuing check verifications were used for QA/QC. ANOVA with Tukey HSD at α = 0.05 was performed in STATISTIX v10.0. Transfer factor was reported as grain Cd / soil Cd. Hazard quotient followed Kibria et al. (2022): HQ = grain Cd (mg kg⁻¹) × 9.47 for adults and × 9.38 for children.
Implications
Certification: Not a market benchmark, because it is a pot experiment in spiked Cd-stressed soil. It is useful evidence for mitigation and source apportionment in rice.
Courses: Useful for explaining why soil pH and organic matter are practical levers for Cd uptake.
App: Should not directly set consumer rice occurrence values, but can inform source-risk flags.
Related evidence
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