Vietnamese durian and cadmium: why it happens and what growers can do

Cadmium in durian is linked to acidic soil and phosphate fertilizers. This guide explains the causes, soil tests and a seasonal risk-reduction plan.

Context

In May 2024, a friend who exports durian texted me: "China just returned 30 containers." I think it's probably a paperwork error. Turns out no. The General Administration of Customs of China (GACC) has just issued the first warning: 30 batches of Vietnamese durian exported from May 2023 to January 2024 exceeded the allowable cadmium (Cd) threshold — 0.05 mg/kg according to GB 2762 standard.

It was the first message in a series of bad news that lasted more than a year.

By June 2024, China reported 77 more batches exceeding the threshold, suspending 15 packaging facilities and 18 growing areas. In the first four months of 2025, durian exports to China dropped to 35,000 tons — equal to 20% of the target, down 56% over the same period last year. About 200 containers were returned to Vietnam. Meanwhile, Thailand — which has long built a cadmium control system — quickly regained 86% of the durian market share in China, pushing Vietnam's share from 62% to only 11% in January–February 2025.

This is not a few unlucky batches, nor is it a matter of individual techniques of a few growers. This is the accumulated consequence of a decade of cultivation — and to remove it, we must understand it from the root.

Why is there cadmium in durian fruit?

The first question people often ask: does the durian tree produce cadmium on its own? Are not. Cd is a heavy metal that comes from the soil, is absorbed by plant roots and then travels through the sap flow into the fruit. So where does Cd get into the soil? There are two main paths, and both are directly related to the way we farm.

The first path: phosphate rock and phosphate fertilizer

Most of the world's commercial phosphate ore — the raw material for DAP, MAP, SSP and NPK — comes from ancient marine sediments in North Africa (Morocco, Tunisia, Algeria). The characteristic of these sediments is that cadmium was deposited with apatite over millions of years in the Cretaceous and Eocene marine environments. When the factory crushes ore and processes it into fertilizer, Cd follows, hidden in each grain of DAP fertilizer that growers spread on the orchard.

Cd levels in phosphate ores vary greatly depending on the source:

That is, with the same name as DAP, fertilizer made from North African ore can contain 30–100 times more Cd than fertilizer made from Lao Cai apatite or Kola. Two 50 kg bags look identical on the outside, but their chemical "background" is completely different.

The EU has issued regulations since July 2022 (Regulation 2019/1009): CE-mark phosphate fertilizers must not exceed 60 mg Cd/kg P₂O₅, and "green label" products must be below 20 mg/kg P₂O₅. Vietnam has its own standard — 12 mg Cd/kg product — but because of the calculation based on "product" instead of P₂O₅, direct comparison with the EU is not simple: with DAP (46% P₂O₅), the Vietnamese standard converts to about 26 mg/kg P₂O₅ (stricter than the EU); but with SSP (16% P₂O₅), it is equivalent to about 75 mg/kg P₂O₅ (more liquid). The bigger issue than standards is enforcement — is it really possible to test each batch of imported fertilizer? In May 2025, Vinacam Group sent a document to the Prime Minister, questioning a type of DAP imported from Korea that is commonly used in Tien Giang, Long An, and Dong Thap. The hypothesis has not been independently verified, but the mechanism is consistent with science.

India — a country with a large agricultural base and heavy dependence on imported phosphate fertilizer — has witnessed exactly this phenomenon. A study published in 2025 in Frontiers in Soil Science conducted by a Central Punjab University team on 149 acres of agricultural land in the Bathinda–Mansa region found that DAP was the fertilizer with the highest Cd content among the groups surveyed (an average of about 6.7 mg/kg), while urea and potassium were almost clean. More importantly, long-term data in India shows that after several decades of regular phosphate fertilization, the amount of Cd in topsoil can increase dozens of times. This means: if we keep fertilizing without controlling the quality of the fertilizer, Cd will silently accumulate in the soil — and one day the tree will "pay its debt" with fruit.

The second path: acidic soil makes Cd "come alive"

This is the part that few people pay attention to, but is no less important than the first path.

Cd may already be in the soil, but it is not always taken up by plants. The level of "bioavailable" Cd depends greatly on pH. When soil pH is above 6.5, Cd tends to precipitate or stick tightly to soil glue, making it difficult for plants to reach. But when pH drops below 6 — especially at the level of 4.0–4.5 that alum soils in the Southwest often encounter — Cd dissolves strongly into the soil solution. In the low pH range, just by reducing the pH by 0.2 units, the amount of easily absorbed Cd can increase up to 5 times.

Fertilization → more acidic soil → Old and new Cd both "wake up" → plants absorb more. This is a vicious cycle that growers need to break.

The problem is that farming itself causes pH to decrease. When applying a lot of urea and DAP, bacteria in the soil convert NH₄⁺ into NO₃⁻ and release H⁺ — the soil gradually becomes acidic. Tropical rain washes away alkaline cations such as Ca, Mg, K, making the soil more acidic. Durian orchards in the West and Central Highlands often fall into a pH range of 4.5–5.5 — the worst range for Cd control. Unfortunately, this is also the pH range in which many areas are expanding durian acreage, especially on improved alkaline soil.

A new survey in 2025 by Dr. Duong Minh Vien (Can Tho University) measuring on 7–8 year old durian orchards in Can Tho and Dong Thap showed that total Cd in soil was in the range of 0.2–1.4 mg/kg. This number in absolute terms is not a heavy level of pollution — but when combined with the low pH typical of the region, it is enough to push the Cd in durian pulp above China's threshold of 0.05 mg/kg.

Why durian, and not another tree?

Same question: why in the same orchard, in the same soil, in the same manure, is durian "sticky" while many other fruit trees (at least not yet) have not had the same problem?

There are three reasons worth discussing.

First, durian is a perennial plant. A 20–30 year old orchard means 30 seasons of spreading fertilizer, 30 seasons of tree roots absorbing everything in the soil solution. Cd accumulates in an arithmetic progression. Short-day plants of 3–4 months have much lower exposure.

Second, durian "eats unicorn" a lot. A orcharder in Tien Giang can fertilize 5–6 kg of phosphorus/tree per crop to stimulate off-season flowering — many times the norm for seed plants. But as mentioned, each kilogram of phosphate fertilizer is a small potential dose of Cd.

Third, at the molecular biology level, Cd "tricks" plants by imitating the structure of useful metal ions such as Zn²⁺, Fe²⁺, Mn²⁺. Transport channels in the root cell membrane — belonging to the Nramp, ZIP and HMA families, which have been described thoroughly in rice and Arabidopsis — do not distinguish Cd from Zn or Mn, so they draw Cd into the roots like absorbing useful trace minerals. When the durian fruit is growing, the phloem stream transfers Zn and Ca to the fruit pulp, and Cd follows. This is also an important clue for a treatment measure: if we "feed the tree enough Zn", Cd will be outcompeted and significantly reduced in the fruit. Will discuss in detail below.

Prevention and treatment: from lime to foliar spray

There is no silver bullet. An effective approach is to layer 3–4 treatments, each cutting 20–60% of Cd, which combined can bring Cd in fruit below the threshold in 1–2 seasons. This is also the direction that the Tien Giang pilot model is running according to Decision 116/QD-TT-VPPN (March 20, 2023).

I prioritize "cost/benefit ratio" — from cheapest, easiest, to more specialized.

First thing: raise pH with lime or dolomite

This is the cheapest, most effective, and the foundation for other measures to be effective. Apply 1–3 tons of dolomite [CaMg(CO₃)₂] or limestone CaCO₃ per hectare, bringing soil pH to 6.0–6.5. Dolomite is a priority for Central Highlands orchards because basalt soils often lack Mg.

Important: apply lime at least 2–4 weeks before fertilizing. If mixed at the same time, the two sides can "fight" — lime sticks to urea, causing loss of N, or precipitates with phosphate fertilizer, making it unable to be absorbed by plants.

Quantitative evidence comes from many places. A classic experiment in tobacco fields in Greece (Tsadilas et al., 2005) applied 3 tons of Ca(OH)₂/ha — pH increased by 0.8 units, effective Cd in soil decreased by 40%, Cd in leaves decreased by 35%; After 4 years the cumulative effect is more than 60%. A 7-year long-term study on rice soil in Southern China, applying lime twice per year, recorded a 51–90% reduction in grain Cd without yield loss (Plant and Soil, 2023). Simple mechanism: pH increases, Cd is "locked" in the soil, plants cannot absorb it.

Change to low Cd phosphate fertilizer

This is where Vietnam's fertilizer industry has real opportunities. Currently, there is no way for growers to know how much DAP they buy is high or low in Cd — there is no label, no declaration. If a manufacturer proactively tests and declares Cd content ≤ 20 mg/kg P₂O₅ on the packaging, that product will immediately have a different value in the export growing market.

Specifically, there are three "safe" phosphorus source options:

• DAP, MAP made from Lao Cai apatite — domestic ore with extremely low Cd.
• Fertilizer imported from Russia (e.g. PhosAgro) or sources that use magma ore.
• Fused phosphate fertilizer (CMP — calcium magnesium phosphate): this product not only lowers Cd, but also raises soil pH when applied. A 2024 study on lettuce published in Toxics showed that at the same dose of P₂O₅, DAP increased Cd in vegetables by 73.7%, SSP increased by 90.9%, and CMP reduced Cd at high doses. This is "two in one": both providing phosphorus and improving acidic soil.

Vietnamese fertilizer businesses that proactively announce low Cd on fertilizer bags will have a real competitive advantage with export growing areas — not only durian but also coffee, pepper, dragon fruit, and mangosteen.

Biochar — utilize existing waste products

Vietnam has surplus raw materials to make biochar: rice husks, coffee husks, sugarcane bagasse, corn cobs. Pyrolysis of these materials at 400–600°C under anaerobic conditions will create biochar with a large surface area, bearing many functional groups -COOH, -OH capable of retaining Cd through ion exchange and surface complexation. Apply 3–5 tons of biochar/ha to the canopy area, can be mixed with organic fertilizer to increase efficiency.

A meta-analysis published in Science of the Total Environment in 2024 (combining data from many experiments from 2000–2023) shows that sawdust biochar reduces Cd in rice grains by an average of 25%; Rice husk biochar decreased by 13%. A field experiment (Zheng et al., Scientific Reports, 2022) showed more impressive results: applying rice husk biochar at 2% of soil mass, combined with limestone, reduced Cd in wheat grains by 38.5% and increased soil pH by 28%. On heavily acidic soils, there are studies reporting that Cd in brown rice is reduced by up to 58%.

Note: biochar efficiency depends on raw materials, calcination temperature, and initial soil pH. In general, biochar with a pre-alkaline pH (such as roasted rice husks around 7.5–8.5) will be most effective on acidic soils.

Intercropping — lessons from India

I especially like this part because India did it long before Vietnam and has a lot of real data. The concept of "phytoremediation" — using plants to treat contaminated soil — is widely used in India on land polluted by industrial wastewater, and the "superstar" of this industry is the Indian mustard (Brassica juncea).

Two articles by Goswami & Das group (2017 in International Journal of Phytoremediation; 2019 in Environmental Pollution) are worth reading in their entirety. In the 2019 study, they planted 80 different canola varieties on Cd-Pb contaminated soil, the IM-25, IM-13, IM-65 varieties achieved a translocation factor of 3.38 — meaning that Cd in the stem and leaves is more than 3 times higher than Cd in the roots, meaning the plant "pulls" Cd from the soil and then stores it in harvestable biomass. In a 2017 study, intercropping canola with stinging nettle (Urtica dioica) at a density of 50 plants/m² in just 60 days reduced total Cd in the soil by 38%.

The Tien Giang model is intercropping mint, water mimosa, pennywort and Chinese broccoli between durian rows. This is not a spontaneous initiative but rather an application of the Indian-style phytoremediation principle. There is an extremely important point: post-harvest intercrop biomass absolutely cannot be used as food for humans or livestock, because Cd is in it. Must be treated as special waste — either incinerated at high temperatures, or buried in a controlled area.

Limitations of phytoremediation: can only "wash" the topsoil ~20 cm, and it takes 3–5 years to see significant results. This is a long-term strategy, not an immediate fix.

Spray leaves with Zn, Se, Si — a final measure before the fruit ripens

This is the intervention closest to harvest, directly affecting Cd in the fruit. Principle: plants need Zn for metabolism, if Zn is lacking it will "mistakenly take" Cd through common transport channels. Spraying Zn leaves when the fruit is developing will make the tree "satisfied" with Zn, closing the Cd intake. Similarly, Se competes with Cd at the cellular level, while Si strengthens the cell wall and reduces Cd translocation through the xylem.

A meta-analysis in the Journal of Hazardous Materials (2024, pooling more than 1,600 paired observations) showed that selenium supplementation — especially low doses of nano-Se — reduced Cd in rice grains by 37%. Spraying 0.5% ZnSO₄ during grain formation reduced Cd in wheat grains by 14–22%. Nano-silica sprayed at flowering reduced Cd in rice grains by 31–65% (Chen et al., Environ. Sci. Pollut. Res., 2018). Small organic acids such as citric acid, asparagine, glycine, foliar spray also reduces Cd by 14–38%.

Recommendation for durian (no direct experiments on durian, so extrapolate cautiously from researched trees): spray ZnSO₄ 0.4–0.5% combined with sodium selenite 50–100 mg/L, possibly adding nano-Si, at three times — 30 days, 60 days, and 90 days after fruit set. This is the most feasible intervention to reduce Cd in durian flesh right in this season, while waiting for soil measures to take effect in the long term.

Symbiotic microorganisms and fungi

The last layer is microbiology. Arbuscular mycorrhizal fungi (AMF) — such as Funneliformis caledonium or Rhizophagus intraradices — create filamentous networks around plant roots, secreting glycoproteins that chelate Cd, preventing Cd from entering the roots. In wild apple Malus hupehensis, AMF reached 62–71% root colonization and significantly reduced Cd in the aboveground part (Zhuang et al., 2025). Some bacteria such as Pseudomonas and Bacillus also have similar abilities. Tien Giang is using microbial products in Solution 4 of Decision 116.

A set of actions for a 1 hectare orchard

To make it easier to share with growers, I packaged it into a seasonal calendar:

Before the dry season: Take soil samples at 4–6 points/ha, measure pH, EC, total Cd and absorbable Cd (extracted with DTPA), organic carbon. This is the baseline data needed to understand the orchard.

Dry season: Apply 1–3 tons of dolomite/ha (depending on initial pH) + 3–5 tons of biochar/ha to the canopy area. Wait at least 2–4 weeks before applying phosphate fertilizer — if possible, change to a certified low-Cd fertilizer or use CMP. Intercrop mint, spinach, and mustard greens between rows of plants.

Rainy season, fruiting stage: Foliar spray ZnSO₄ 0.4–0.5% + sodium selenite 50–100 mg/L 30, 60, 90 days after fruit set. Apply AMF or Bacillus products when the tree buds. Divide urea dose into smaller doses, use balanced NPK with trace amounts of Ca, Mg, B, Zn.

Before harvest: Take fruit samples at least 7 days before harvest. Discard batches with Cd ≥ 0.04 mg/kg — placing a 10% safety margin below China's 0.05 threshold. Fully recorded in the digital traceability system according to Decision 5272/QD-BNNMT (December 13, 2025).

One last thought

Cadmium is not a post-harvest problem. Cadmium is not a border laboratory problem. Cadmium is a problem of soil chemistry and fertilizer quality — accumulating silently over decades, then erupting when a major market suddenly tightens standards.

Sadly, this crisis could have been avoided if the fertilizer industry and the crop industry had talked to each other sooner. Thailand has been doing it for a long time: hundreds of orchard-level labs, province-by-province Cd maps, strict imported fertilizer testing. They keep the market while we are having to rebuild from scratch.

The good news is that the science is there, the tools are available, and most solutions are cheap. Applying lime, biochar from rice husks, intercropping vegetables, spraying Zn leaves — nothing fancy, nothing that requires imported technology. The difficulty is organization: who tests fertilizer? Who issues the certificate? Who is responsible when a 30 year old orchard has accumulated Cd?

Vietnam's fertilizer industry has an opportunity: any business that proactively declares low Cd on its fertilizer bags will win the trust of export growing areas — not only durian, but also coffee, pepper, dragon fruit, and mangosteen are waiting. As for growers, there are only three things: apply lime before applying phosphorus, check the soil, not just the fruit, and don't ignore cheap but effective things like biochar from your rice husks.

Vietnamese durian still has the chance to hold a billion-dollar position in China. But that door is opened by soil science, not by luck.

Source notes

Compiled trade data from Vietnam Ministry of Agriculture and Environment, VietnamPlus, Produce Report (2024–2025). China standard GB 2762-2017/2022; EU Regulation 2019/1009. Phytoremediation research referenced from Goswami & Das (2017, Int. J. Phytoremediation; 2019, Environmental Pollution); Cd survey of fertilizers and soil in Punjab published in Frontiers in Soil Science (2025). Meta-analysis of biochar and foliar Zn/Se/Si: Wang et al. (2024, STOTEN), Sun et al. (2024, J. Hazardous Materials), Chen et al. (2018, ESPR), Tsadilas et al. (2005, Comm. Soil Sci. Plant Anal.). Durian soil Cd survey in Can Tho – Dong Thap: Dr. Duong Minh Vien, Can Tho University, announced in mid-2025. Technical numbers in the article should be verified from the original source before using for official purposes.