Toxicity and Deficiency

The boundaries between toxicity, sufficiency, and deficiency are vague. The amounts vary with the species of plant and animal, vary within the species s growth cycle, and vary with the organism s general health and the supply of the other essential elements. The elements in the crosshatched areas of Fig. 2.1 are the toxic elements of primary concern to government regulatory agencies—Be, Cd, Hg, Pb, and all 

In the good old days, lead water pipes were common and lead was in pewter tableware and in ceramic glazes. Lead arsenate was a pesticide spray on apples and 

Plants must grow in the environment within reach of their roots and leaves. To survive, plants have had to evolve a broader tolerance to imbalances of essential and toxic elements than have animals. Deficiencies and toxicities in plants are usually evident by reduced yield or reproductivity, abnormal coloration, and plant and fruit deformities. Animals can supply their more stringent nutritional requirements by ingesting food grown on a variety of soils. 

Deficiencies of soils to supply the essential elements are more obvious in plants than in animals. The nitrogen content of most soils is low enough that plants benefit from added N. In many soils, plants, especially food crops, also grow better with added P, K, and S. Because the response differs among plant species, fertilization can change the distribution of plant species. Since we usually define the untouched plant community as the ideal state, species changes are considered detrimental. 

The ability of soils and soil microbes to neutralize excess fertilizers has limits. Nitrogen and phosphate draining from excessive fertilization of sugar cane fields favors cattail plants over native plants in the Florida Everglades. Nitrogen and phosphate from fertilizers and municipal wastewater are causing water problems worldwide. 

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Acid sulfate soils are an especially difficult class of acid soil formed in former marine sediments that have been drained. The acidity is generated from the oxidation of pyrite in the soil resulting in acute aluminium toxicity, iron toxicity, and deficiencies of most nutrients, especially phosphate which becomes immobilized in ferric oxide. The development and management of acid sulfate soils are discussed in detail in Dost and van Breemen (1983) and Dent (1986). 

Gupta, U.C., YW. lame, C.A. Campbell, A.J. Leyshon, and W. Nicholaichuk. 1985. Boron toxicity and deficiency a review. Canad. Jour. Soil Sci. 65 381-409. 

The kinetics of Mn with regard to tissue uptake and metabolism under conditions of Mn toxicity and deficiency were investigated. With Mn toxicity there is a marked difference in Mn uptake among tissues. 

Figure 10.7. Experiments with different chelators and a wide range of trace metal concentrations demonstrate that trace metal toxicity and deficiency are determined by metal-ion activities and not total concentrations. Motility data of the dinoflagellate Gonyaulax tamarensis as a function of total copper [Cul and cupric ion activity [Cu ] for two chelators, Tris and EDTA. (Adapted from Anderson and Morel, 1978).

In livestock, concern for selenium toxicity and deficiency is high. In areas of the country with selenium-poor soils, dietary selenium supplementation for livestock has been necessary to prevent chronic selenium deficiency diseases. Dietary supplementation programs have resulted in cases of accidental poisonings from misuse of the selenium supplements (Hopper et al. 1985). 

Molybdenum (Mo) is an essential plant nutrient. It acts as a metallic cofactor in plant and animal enzymes. At high concentrations in forages, it can be toxic to ruminants by interfering with assimilation of copper (Cu). The range between toxicity and deficiency in animals is narrow, and therefore careful control of Mo in animal diets is essential. 

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