Cassava grown intercropped with coffee in Machakos County, Kenya, had higher cadmium, copper, and lead in its soil and tubers than single-cropped cassava, and tuber cadmium exceeded the WHO limit cited by the authors.
Authors
- Kasyoka EM, Mbugua GW, Wanjau RN, Nambafu GN, Ndiritu JN., Applied and Environmental Soil Science. 2024
What was studied
This original research article measured cadmium (Cd), copper (Cu), and lead (Pb) in soils and cassava tubers (Manihot esculenta) from farms where cassava is grown alone (single-cropped) and farms where it is intercropped with coffee, in Kathiani Subcounty, Machakos County, Kenya. Coffee farms in the area receive regular applications of fertilizers and pesticides, and agrochemicals can carry heavy metals as impurities. The study asked whether growing a food crop among coffee bushes raises the metal content of the soil and of the tuber. Soil and tuber samples were collected in a zigzag pattern of five cores per farm, the tubers were divided into bark, epidermis, flesh, and pith, and metals were measured by atomic absorption spectrometry (Shimadzu AA-6200). Differences were tested by one-way ANOVA with Tukey's test at α = 0.05.
Who was studied
No people were studied. The samples came from cassava farms in four localities of Kathiani Subcounty: Kaewa, Kalunga, Kombu, and Kaliluni. In each locality, soil and cassava tubers were sampled from single-cropped farms (the controls) and from farms where cassava was intercropped with coffee; in the intercropped farms, soil was taken close to the coffee bushes, between the rows. Kaewa, Kalunga, Kombu, and Kaliluni are sampling locations, not cassava varieties; the authors note that the age and variety of the cassava plants were not considered.
Most important findings
| Critical Point | Details |
|---|---|
| Intercropping effects on soil metal content | Soils from intercropped farms had higher levels of all three metals than soils from single-cropped farms: 0.25–0.83 vs 0.09–0.59 mg/kg for Cd, 4.9–10.1 vs 4.1–4.6 mg/kg for Cu, and 0.022–0.037 vs 0.021–0.032 mg/kg for Pb. All soil values were within the WHO limits for agricultural soil cited by the authors (0.8, 36, and 85 mg/kg for Cd, Cu, and Pb). |
| Metal levels in whole tubers | Cassava from intercropped farms accumulated more metal. Cadmium was significantly higher in intercropped tubers (5.38 vs 3.7 mg/kg dry mass), as was lead (0.14 vs 0.037 mg/kg). Copper was higher in intercropped tubers (5.4 vs 4.4 mg/kg), but the difference was not statistically significant. |
| Distribution within the tuber | Metals were highest in the pith and bark and lowest in the flesh. In intercropped tubers, Cd averaged 7.8 mg/kg in the pith and 2.8 mg/kg in the flesh, and Pb averaged 0.20 mg/kg in the pith and 0.06 mg/kg in the flesh. The authors attribute the high bark levels to direct contact with soil and the high pith levels to the vascular tissue that carries metals through the plant. |
| Comparison with limits | The authors report that copper and lead in the tuber tissues were within the WHO limits they cite, while cadmium in all tuber tissues exceeded the WHO limit they cite. Cadmium and lead differed significantly between the two cropping systems. |
| Likely cause | The authors conclude that agrochemical use in coffee farming, including phosphate fertilizers that naturally contain Cd and copper-based pesticides, could explain the higher metal levels in intercropped soils and tubers. |
Key implications
Intercropping cassava with coffee was associated with higher, not lower, cadmium, copper, and lead in soil and in cassava tubers, and cadmium in the tubers exceeded the limit the authors applied. The authors recommend monitoring heavy metals in agricultural soils to protect the environment and human nutrition, and they recommend that farmers be discouraged from intercropping cassava with coffee. For heavy metal testing, the study shows that a crop's growing context, such as nearby agrochemical-intensive crops, can raise metal levels in the edible part. The authors note that they did not account for plant age or variety, seasonal variation, or soil physicochemical properties.
Citation
Kasyoka EM, Mbugua GW, Wanjau RN, Nambafu GN, Ndiritu JN. Levels of Cadmium, Copper, and Lead in Soils and Cassava Tubers Grown in Machakos County, Kenya. Applied and Environmental Soil Science. 2024;2024: Article ID 9063314. doi:10.1155/2024/9063314
Cadmium is a persistent heavy metal that accumulates in kidneys and bones. Dietary sources include cereals, cocoa, shellfish and vegetables, while smokers and industrial workers receive higher exposures. Studies link cadmium to kidney dysfunction, bone fractures and cancer.
Lead is a neurotoxic heavy metal for which no safe blood lead level in children has been identified. It contaminates food, consumer goods and drinking water, and exposure is linked to cognitive deficits in children, adverse pregnancy outcomes such as preterm birth and low birth weight, and cardiovascular disease. HMTC’s rigorous lead testing applies ALARA principles to protect infants and consumers and to prepare brands for tightening regulations.