Mn and Fe are essential to both plants and animals and are transferred successfully from the soil in all situations in which life persists. The transfer of each is subject to numerous influences and these are likely to be more important than Fe and Mn content of parent materials. At the soil:plant interface pH and Eh are dominant factors and are frequently regulated by agronomic technology.
Of the plant macronutrients used in fertilizer, P is most likely to reduce directly the availability of Fe and Mn. Because of the cost of P, however, excess applications are likely to be infrequent.
Availability of Fe and Mn of plants to animals is characteristically described as low, in the order of 10−1. However, availability increases during deficiency and, in the case of Fe, following hemorrhage. The availability data reflect homeostatic mechanisms which minimize the chances of accumulation of excess Fe. Mn homeostasis appears to be largely achieved through excretory paths.
Fe deficiency is considered one of the most common trace-element deficiencies of man. Intensive agronomic practices will have little effect, however, on the Fe or Mn status of grazing animals; and, to the extent that he is a tertiary consumer in the food chain, effects on man of regional geochemical differences and changes induced by intensive agricultural technology are likely to be damped.