PULLMAN, Wash. — Washington State University scientists in Puyallup, Prosser and Pullman are gearing up to start a comprehensive two-year study of plant uptake of metals for the Washington State Department of Agriculture.
The $400,000 study was authorized in a fertilizer reform package passed by the state legislature this spring. The package also included provisions for labeling fertilizer products containing industrial waste byproducts, funding for two studies to be conducted by the state Department of Ecology and, on an interim basis, adopted Canadian standards for maximum annual metal additions to the soil.
“In a nutshell, we are trying to find out whether the Canadian standards are adequate or overly protective in regulating the input of heavy metals into soils,” said Shiou Kuo, soil scientist at the WSU Puyallup Research and Extension Center. “We will also try to find out what the biotransfer factors are.”
Biotransfer is the transfer of a substance from a growing medium to living organisms or from one living organism to another. This study will look at the degree to which plants take up heavy metals found in various types of fertilizers.
“That’s the key to assessing human exposure risk,” Kuo said, who is leading the WSU research project. Fertilizers contain different amounts of heavy metals. They have different chemistry and react differently with soils. The magnitude and rate at which these heavy metals are released from fertilizers and their subsequent uptake by plants may vary considerably, depending on the type of fertilizers, soil, crop, and environmental conditions.”
In field studies at Prosser, scientists will try to determine how much metal in fertilizer can be transferred to winter wheat and potatoes following normal and above normal applications of phosphate fertilizers, including rock phosphate and triple superphosphate, and granular zinc.
These fertilizers are known to contain elevated concentrations of lead and cadmium.
In Puyallup, using rock phosphate, triple superphosphate and ironite, a mine tailing known to contain high levels of arsenic, scientists will perform similar tests with lettuce and cucumber. They also will determine how soil acidity and alkalinity affect metal availability. Metal transformation and availability will be measured in the laboratory after the crops are harvested.
The crops were selected for testing because of their economic importance to the state or their propensity to take up metals. The field test sites represent diverse soil types and climatic conditions.
The first crop winter wheat will be planted this fall. The other crops will be planted next spring. Preliminary test results are expected next fall. The tests will be repeated the following year.
“You need valid biotransfer factors to assess human exposure risk,” Kuo said. What has been used in the human exposure risk assessment for heavy metals in biosolids, or sludge as it is sometimes called, may not apply to fertilizer products because of fundamental differences in composition.”
Laboratory studies in Pullman will focus on the effects of adding metals in different forms to the soil. “By adding different fertilizers to the same soils used in the field study, we can monitor how different fertilizers affect the solubility of the metals over time,” said Jim Harsh, WSU soil chemist.
“Over time, the metals tend to reach a state of equilibrium where their concentration in the solution will not change. At this point we will try to identify the forms of the metal in the soil and assess their long-term availability.
“In the laboratory, we cannot know exactly what things are going to look like 40 years from now, especially if people keep adding more metals every year. We can compress things a bit, add things in different increments at different times, and age the soil in between to get some idea how the soil and metal will respond.”
Results of all the research will be passed on to the Washington State Department of Health to assess potential effects of metal accumulation on human health and to the state departments of ecology and agriculture who want to know the extent of the transfer of metals to crops and if application of metals up to the Canadian standards will affect crop productivity.
The legislature allocated $258,000 for the WSU project during the last session. The balance will be requested next year.
The other WSU investigators involved in the project are William Pan, professor of soils, in Pullman, and Robert Stevens, Cooperative Extension soil scientist at the WSU Prosser Irrigated Agriculture Research and Extension Center.
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