Copyright © 1997 The Seattle Times Company
Thursday, July 3, 1997

Here's what's known, and not known, about toxics, plants and soil

by Duff Wilson
Seattle Times staff reporter

So far, no study has documented harm to human or animal health in the United States from the recycling of hazardous wastes into fertilizers.

In Japan, however, studies showed that subsistence rice farmers had been sickened by ingesting cadmium that had passed from fertilizers through the rice crop.

And there is consensus among scientists that toxic chemicals from fertilizers can go into the plants growing in the soil.

The disagreement concerns whether those substances move along the food chain at levels that pose any danger.

For fertilizers made from biosolids, or sewage waste, levels of heavy metals are strictly regulated by the federal government. But for those made from recycled industrial wastes, there are no federal controls.

There's not even a requirement that toxic materials be included in the list of fertilizer ingredients.

When consumers buy fertilizer, they usually don't know exactly what they're getting. A fertilizer labeled "20-20-20" has 20 percent each of the beneficial ingredients nitrogen, potassium and phosphate. That adds up to 60 percent. The manufacturer doesn't have to say what's in the other 40 percent, which often includes trace metals.

In its 1992 rules regulating the use of biosolids applied to land, the Environmental Protection Agency named nine metals of potential health concern. Three - lead, cadmium and arsenic - have been identified as possible concerns by health, environmental and fertilizer experts studying recycled hazardous wastes.

Scientists say there has been much study of the food-chain effect of biosolids but not enough of other fertilizers. Here is some of what is known about heavy metals, soil, plants and health:

-- A 1980 study showed lead and cadmium linger in the top layers of soil for long periods of time and are absorbed efficiently for many seasons of crops.

-- Scientists disagree about whether there is any safe level of lead, the most studied toxic metal. Infants are particularly vulnerable to lead poisoning.

-- Two studies published in April 1997 say lead may be even more toxic than previously believed, causing high blood pressure and kidney damage at unexpectedly low levels. Lead also causes neurological disorders, reproductive problems, diminished intelligence and a host of other ills.

-- The major exposure of lead to the general population is through fruits and grains, according to the Agency for Toxic Substances and Disease Registry, part of the U.S. Public Health Service.

Lead in the food chain comes mostly from direct deposit from the air to plants and from livestock eating soil laced with lead as they eat the plants. The bans on leaded gasoline and paint have reduced exposure.

-- The cadmium in the edible portion of many plants increases in direct proportion to the cadmium concentration in the soil.

-- Food products account for more than 90 percent of human exposure to cadmium, except in the vicinity of cadmium-emitting industries, the Agency for Toxic Substances and Disease Registry says. Cadmium has fast uptake through the roots to edible leaves, fruits and seeds.

-- Cadmium builds up in animal milk and fatty tissues. A 1990 study showed that acute cadmium toxicity from food is rare, but chronic exposure at lower levels increases cadmium in certain body organs.

-- The uptake rate of arsenic through soil to plants is lower than the other metals, but arsenic is highly toxic to plants and animals. Root plants are most at risk. Threshold levels are extremely difficult to set.

Some fertilizers, though, actually have higher levels of arsenic than the Asarco Superfund site in Tacoma. Some are higher in lead than banned paint. Their manufacturers say the products are safe for plants.

A 1995 World Health Organization report on biosolids says there is still very little information on pollutant transfer from soil to plants.

"In land application, the human-health-related issues involving toxic chemicals must be addressed," three University of California scientists wrote in the report.

Until the safety or danger can be established, Canada, Australia and many European countries have set strict limits on allowable heavy metals and other toxins in fertilizers.

In the United States, meanwhile, a hands-off approach persists as the scientific debate heats up.

Some experts, such as Rufus Chaney of the U.S. Department of Agriculture, say consumers have little, if any, reason for concern. Chaney's studies, since 1976, indicate that dangerous substances are highly unlikely to move through the food chain to humans, he said.

Others, such as Bill Liebhardt of the University of California-Davis, say we just don't know enough yet.

"How much lead and cadmium is going to get in a particular crop - all you can say is it depends on a lot of factors," he said. "There is no clean, easy answer. Some crops may not take up hardly any of it, and other crops may take up quite a bit and not be affected in terms of their external appearance. This has the potential to move up the food chain.

"When these inert ingredients have the potential for moving up the food chain, then it's not just the farmer that ought to be concerned, it's the consumer, because we all consume these products."

Let us know what you think. E-mail Duff Wilson at

Background & related info.

  • Fear in the Fields, Part 1 & 2
  • Examples throughout the country
  • Tag-along toxics
  • From factories to fields
  • Two approaches to toxins in fertilizer
  • Chart: heavy metals in fertilizers
  • Experts: How to reduce risk
  • Resources on the World Wide Web
  • Here are some officials to call or write