From the linked report: - "Excessive nib Cd levels occur due to several main factors: 1) Theobroma cacao is a natural Cd accumulating plant species compared to

From the linked report: - "Excessive nib Cd levels occur due to several main factors: 1) Theobroma cacao is a natural Cd accumulating plant species compared to others grown on the same soils; 2) topsoil Cd increases from deposition of litter and harvest debris on the soil surface; 3) Because cocoa roots remain in the topsoil, tillage to incorporate soil amendments harms the trees; 4) Most cocoa soils in South America and the Caribbean (LAC) are strongly acidic with pH as low as 4.5, so low that soil Cd is highly phytoavailable; 5) Some cocoa soils were historically Cd contaminated by mining wastes (minor part of land in cocoa production); and 6) some cocoa soils are naturally Cd enriched from parent rocks (e.g., marine shale) which makes these have high Cd:Zn ratios and thus have very high Cd phytoavailability. In these soils, beans can reach 10 mg Cd/kg rather than the 0.8 mg Cd/kg limit usually enforced on bean imports in the EU. One location in Colombia has 27 mg Cd/kg topsoil, 100-fold background levels. Other areas with high Cd:Zn problem soils growing cocoa were identified in at least Ecuador, Honduras, Peru and Trinidad & Tobago." - "Cocoa plants absorb Cd into roots on a ZIP-family Zn transporter, not on the NRAMP5 transporter used by rice. Zn2+ in soil solution strongly inhibits Cd2+ uptake; thus higher Zn2+ in soil can reduce Cd in cocoa beans. The Cd:Zn ratio in the soil solution strongly affects Cd accumulation in cocoa trees and beans. Unfortunately, surface applied ZnSO4 does not rapidly leach into the rooting zone soil, and foliar Zn sprays are ineffective in reducing Cd in beans. Surface applied ZnEDTA as a Zn fertilizer could leach Zn into the rooting depth soil. But ZnEDTA costs much more than ZnSO4."