The hidden heavy metals in rice: Can agroecology help?

For decades, chemical fertilizers have helped farmers increase crop production. But some of these fertilizers can also leave behind substances that build up in the soil over time.

Among them are cadmium and arsenic, heavy metals that can be absorbed by crops such as rice. Because rice is a daily staple for millions of Filipinos, even low levels of contamination can become a food safety concern when exposure continues over many years.

The issue is especially important in places such as Catanduanes, where limited farmland must support a large population. Repeated fertilizer use, combined with the province’s dependence on rice, raises questions about how to protect both soil health and the food people eat.

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This report examines how cadmium and arsenic can enter rice-growing areas through chemical fertilizers and how agroecology-based organic farming may help reduce the risk while supporting food production.

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Cadmium and arsenic are present as contaminants in many chemical fertilizers, especially phosphate-based fertilizers. Over time, repeated application can lead to their accumulation in agricultural soils, where crops such as rice, wheat and vegetables can absorb them.

Natural rock phosphate deposits contain 3 to 150 milligrams per kilogram of cadmium, depending on their origin. Because cadmium is not removed during superphosphate manufacturing, it can be transferred directly to farms through fertilizer use (Roberts, 2014).

Long-term use can increase cadmium levels in soil, while rice plants can absorb the metal efficiently. Chronic exposure to cadmium has been linked to kidney damage, bone demineralization such as itai-itai disease, and increased cancer risk (Kubier et al., 2019).

Arsenic, meanwhile, is present in some fertilizers and can also be mobilized by irrigation water. In flooded rice paddies, arsenic can become more available in the soil and accumulate in rice grains.

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Health effects associated with long-term arsenic exposure include skin lesions, cardiovascular disease and increased risks of lung, bladder and skin cancers.

The issue is particularly important in the Philippines, where 70% of rice areas have received phosphate fertilizers for more than 40 years since Masagana 99.

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In Catanduanes, where only 3,770 hectares are irrigable and the population is 262,919, repeated application of four bags of complete fertilizer per hectare per cropping adds about 0.8 to 2.4 grams of cadmium per hectare per year.

Over 20 years, this translates to an additional 16 to 48 grams per hectare in the topsoil, enough to exceed the European Union limit of 1 milligram per kilogram for agricultural soils.

Because rice consumption is 119 kilograms per capita per year, even low-level accumulation in grain can become a chronic exposure pathway for children and pregnant women.

Studies have shown that cadmium and arsenic levels in soils rise significantly in areas with intensive chemical fertilizer use, contributing to long-term soil degradation and food safety risks that cannot be reversed simply by stopping fertilizer use for one season (Kubier et al., 2019).

The concern is both a public health and an agricultural issue.

The displacement of local sugar by 1.1 million tons of alternative sweeteners in 2025, combined with concerns over contamination in chemically grown rice, raises questions about the quality of food security being pursued.

Catanduanes has also set a target of 10 tons of palay per hectare per year, equivalent to 6.5 tons milled, plus 90 kilograms of rice and 120 kilograms of root crops per capita to achieve 100% rice sufficiency and 103.6% overall food sufficiency using only 1.7% of its 149,216 hectares for root crops.

Achieving those targets will depend not only on productivity but also on maintaining healthy and safe agricultural soils.

The report focuses on three areas: the sources and mechanisms of cadmium and arsenic contamination from phosphate-based chemical fertilizers in Philippine paddy soils; the ways agroecology-based organic agriculture, particularly composting and maintaining high soil organic matter (SOM), can reduce the availability and uptake of these metals; and a possible pathway for Catanduanes and similar island provinces to transition to agroecology-based systems.

The analysis draws on literature on cadmium content in rock phosphate and phosphate fertilizers, as well as secondary data from the Fertilizer and Pesticide Authority (FPA) on fertilizer use since the 1970s.

Annual cadmium loading was computed based on standard application rates in Catanduanes.

The report also examined how SOM can reduce the availability of heavy metals through several processes, including binding metals, increasing cation exchange capacity (CEC), buffering soil pH and supporting microorganisms that can help immobilize contaminants.

READ: ‘Food Nationalism’: The politics of what Filipinos eat

Key experimental evidence was drawn from a long-term paddy trial by Fang et al. (2026) in Hunan that compared soils with 1.1% and 3.2% SOM, as well as a Healthy Babies Bright Futures (2025) market survey of 145 rice samples comparing organic and conventional rice.

For its assessment of possible farm-level applications, the report used outputs from the two-day Agroecology-Based Rice Farming Training in Catanduanes on Sept. 11-12, 2026.

The training involved 47 farmer-leaders who designed diversified farms with at least 20 species under the bahay kubo model, compost production at P360 per sack, deep plowing to 20 centimeters, and SRI using planting windows from May 15 to the first week of June and from mid-December to the first week of January.

Food sufficiency modeling was based on a standard of 0.41 hectare per person.

How cadmium and arsenic build up in rice fields

Phosphate fertilizers are a major human-caused source of cadmium in agricultural soils globally, accounting for 54% to 58% of cadmium inputs in Europe and Asia (Roberts, 2014).

Unlike some other fertilizer components, cadmium does not disappear through volatilization. Instead, it can accumulate in the upper layer of the soil where crops grow.

In flooded rice fields, soil conditions can also affect how easily cadmium and arsenic are taken up by plants.

Cadmium becomes more available in acidic soils. A pH of 5.2, which the report says is common in Catanduanes after decades of ammonium sulfate use, can make it easier for rice plants to absorb the metal.

Arsenic behaves differently. Under flooded conditions, iron compounds in the soil can release arsenic into the soil water, where it becomes available for uptake by rice roots.

Agroecological practices such as organic farming, composting and maintaining high SOM can help reduce the availability of cadmium and arsenic to crops.

Organic matter contains substances that can bind with these metals, making them less mobile in the soil and less likely to be absorbed by plants.

Raising SOM from 1% to 3%, which the report says can be achieved with 5 tons per hectare of compost made from water hyacinth, abaca waste and carabao manure, can also increase the soil’s capacity to hold nutrients and contaminants.

The report says such an increase can raise CEC by 30% to 50%, providing more sites where metals can be held in the soil rather than taken up by crops.

Compost can also support beneficial microorganisms that help immobilize cadmium and improve soil pH from 5.2 to near-neutral 6.5, further reducing the metal’s availability to rice plants.

Unlike chemical fertilizers that may introduce additional contaminants, compost made from local biomass does not add new heavy metals if the materials used are clean.

What the studies show

Fang et al. (2026) found that soils with 3.2% SOM had 42% lower cadmium and 35% lower arsenic levels in rice grains compared with soils containing 1.1% SOM under the same level of arsenic in irrigation water.

The study attributed the difference to the ability of SOM to bind metals around rice roots, limiting their movement into the plant.

Healthy Babies Bright Futures (2025), meanwhile, reported that organic rice systems had, on average, 32% lower cadmium and 28% lower inorganic arsenic than conventional rice.

It also estimated that shifting to organic rice systems could avoid billions in health care costs by reducing chronic disease burdens associated with early-life exposure.

The findings cited in the report suggest that farming systems with higher SOM and without phosphate fertilizer inputs can help limit further accumulation while reducing the amount of existing contaminants taken up by crops.

From farm practice to food security

Agroecology-based organic agriculture addresses the cadmium and arsenic problem in three ways.

First is source elimination. Replacing phosphate-based chemical fertilizers with compost would prevent additional contamination from fertilizers. The report cites locally produced compost priced at P360 per sack and containing negligible heavy metals.

Second is legacy remediation. Increasing SOM to 3% or higher can bind metals already present in the soil and reduce the amount absorbed by plants.

Third is dietary diversification. Moving away from rice monoculture toward the bahay kubo model, with at least 20 crop species, could reduce dependence on a single staple.

The model includes root crops such as camote and gabi, which the report says accumulate less cadmium than rice. Diversifying production could therefore reduce dietary exposure while contributing to overall food sufficiency.

For Catanduanes, however, the issue extends beyond individual farm practices.

The report argues that long-term implementation will require support from local agricultural institutions because projects led by civil society organizations or nongovernmental organizations may end once their funding cycles are completed.

When chemical agriculture and Masagana 99 were promoted in the 1970s, UPLB trained agricultural extension workers with plantilla positions to help ensure continuity.

The report proposes a similar approach for agroecology, with the PAO and MAO leading implementation of the provincial resolution adopting agroecology-based food systems and Food Nationalism as part of efforts to achieve food sufficiency and address malnutrition among children.

A path toward safer soils and food

Cadmium and arsenic contamination from phosphate-based chemical fertilizers poses a long-term food safety risk because the metals can gradually accumulate in Philippine paddy soils.

With rice consumption at 119 kilograms per capita per year, continued accumulation creates a potential pathway for chronic exposure if current practices continue.

The evidence presented in the report suggests that agroecology-based organic agriculture can help address the problem by preventing additional sources of contamination, reducing the availability of metals already present in the soil and diversifying food production.

For island provinces such as Catanduanes, the approach could also support efforts toward 100% rice self-sufficiency using 3,770 irrigable hectares at a production target of 10 tons per hectare per year, along with overall food sufficiency using only 1.7% of upland areas for root crops.

The challenge is not only to produce enough food, but to ensure that the soil supporting that production — and the food harvested from it — remains safe. /dm

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(Teodoro C. Mendoza is a retired professor and scientist at the Institute of Crop Science, College of Agriculture and Food Sciences, UP Los Baños, Laguna.)