Heavy Metals in Fertilisers: Their Effect on Soil and Plant Health

Keywords: Cadmium, long-term effects, phosphate fertilisers, phosphate rock, regulations.

Summary:

The term, soil health, is concerned with those conditions that relate to several functions of soil in the global environment. One of these is the production of crops that are essential for human and animal consumption. Another term that is used to describe these soil conditions is ‘soil quality’. This paper discusses one aspect of changing agricultural and horticultural practices on soil health as well as plant health — the long-term effects from applying some fertilisers containing heavy metals to the soil. While fertilisers, farmyard manure and crop residues contain varying amounts of heavy metals, their effects on crop uptake may be minimal so the transfer of heavy metals to the human food chain is low. Most of the interest in heavy metals in certain fertilisers has been concerned with cadmium (Cd), because Cd has been shown to have some adverse effects on human health. Some other metal contaminants in fertilisers of interest are arsenic, chromium, lead, mercury, nickel and vanadium. Certain management practices, such as increasing soil pH, growing low-Cd accumulating varieties, and applying fertilisers containing low Cd levels, can reduce Cd uptake by food crops. During the past 50 years, soil samples from numerous long-term (26-138 years) soil fertility experiments in Europe and the USA have been analysed for Cd. Results have shown that very little Cd had accumulated in soil fertilised with various P fertilisers. Analyses of crops grown in these experiments also showed few reports of increased Cd in cereal and vegetable crops. Because of concerns regarding the effects of Cd and other heavy metals in the human food chain, various governments have introduced some limits on heavy metal concentrations in phosphate fertilisers in an effort to decrease inputs of Cd and other heavy metals to soils. Allowable metal limits in P fertilisers differ with country because of differences in environmental conditions and agricultural practices as well as their concern of the effects of the resulting metal inputs on possible transfer to the human food chain. Because it is recognised that inputs of some of these heavy metals to soils also may be due to atmospheric deposition, further studies are needed to determine if other regulations concerning industrial emissions should be implemented in some regions. Maintaining soil and plant health is a necessary objective for ensuring an adequate supply of nutritious food for future generations.

John J Mortvedt, Department of Soil and Crop Sciences, Colorado State University, Fort Collins, Colorado, USA.

22 pages, 8 tables, 30 references.