Selenium in minute quantities has been shown to be a dietary essential for animal life, and soil-plant-animal relations have been identified in the distribution of the element. In some cases, soils are frankly deficient in selenium—most particularly those derived from igneous rocks, and the deficiency in surface layers may be aggravated by intensive irrigation. Alternatively, soil selenium may exist in a form that is either unavailable to plants or absorbed by them with difficulty. Representative of such a form is the highly insoluble ferric oxide-selenite complex which frequently occurs in high-moisture, acid soils. Uptake of selenium by plants may also be inhibited by presence of interfering substances in the soil, such as sulphur, or it may be enhanced by liming. Analytical surveys have revealed also that considerable variation exists among plant species in their abilities to take up and retain selenium from the soil. Legumes have been consistently implicated as forages conducive to white muscle disease, a selenium-responsive myopathy, and New Zealand observations have shown white clover (Trifolium repens L.) to contain significantly lower levels of selenium than grasses, and particularly a native grass, browntop (Agrostis tenuis Sibth.). In addition to the differences in absolute selenium uptake, it has been suggested that some plants, and again legumes are suspect, may contain organic inhibitors of selenium utilization by livestock. Some experiments have investigated the effectiveness of additions of selenium to the soil in overcoming selenium deficiency among farm animals. Protection for 2 yrs has been achieved by this technique; however, the various factors influencing the soil-plant-animal relations of selenium direct caution in its application.

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