ABSTRACT
Most of what we know about ion uptake by plant roots is derived from studies using hydroponic methods of plant culture. Such methods generally provide high concentrations of the required nutrients in wellmixed, aerated, and buffered solutions maintained at moderate temperatures. If the plants suffer any deficiency, it is usually that of light, because most growth cabinets provide light at ~10-15% that of full sunlight. In the real world, plants experience very different conditions. Soils are notoriously heterogeneous, in terms of both their chemistry and physics. Extreme soil types, the socalled problem soils, are those that limit plant growth by deficiencies or excesses of various elements. In global terms, such problem soils may constitute up to 3 billion hectares (Dudal, 1976). For example, Serpentine soils contain excessive levels of magnesium (Mg+), combined with relatively low levels of calcium (Ca2+), as well as toxic levels of nonessential ions such as cobalt and nickel. But even when we set aside such extreme soils, available data for agricultural soils indicate that the concentrations of nitrogen, phosphorus, and potassium, elements that typically limit crop productivity, may vary across orders of magnitude. For example, soil solution K+, nitrate ammonium
and sulfate concentrations from 35 different U.S., Australian, and N.Z. soils ranged across 2-4 orders of magnitude for each nutrient, and standard deviations were larger than the mean values (Wolt, 1994). In addition, variations in pH may exert potent indirect effects on the availabilities of the essential nutrients, as well as impacting directly upon root growth and the concentrations of potentially toxic ions such as aluminum or manganese.
