Aluminium and Manganese Toxicity

Many plants grow poorly in acid soils. Early workers supposed that this was a consequence either of hydrogen ion toxicity or of Ca and Mg deficiencies. The soil acidity must be greater than about pH 3, however, before the H+ concentration itself is toxic to most plant species. Although the components of acidity are emphasized in acid soils, the major exchangeable cations are Ca, Mg, and to a lesser extent K in soils of pH 4,5 to 5. 

The effects of low pH on plant growth are generally caused by increases of toxic ions, or decreases of essential ions, in the soil solution. Such effects can also arise from nutritional imbalances because the concentrations can increase or decrease as soil acidity changes. 

Another important facet of nitrogen availability in acid soils is the pH dependence of ammonium-ion fixation between the lattices of expanding layer-silicate minerals. Such fixation generally decreases with increasing soil pH. Although the mechanism for this pH effect is incompletely understood, the decrease may be due to islands of hydroxy aluminium and hydroxy iron polymers, which prevent the complete collapse of mineral lattices and hence decrease NH4 fixation. 

The availability of soil phosphate is highly pH dependent and, as with nitrogen, Is only partially understood. The main mechanism for phosphate fixation (decreased availability) under acid conditions appears to be the precipitation of highly insoluble iron and aluminium phosphates. Phosphate availability also tends to decrease at high 

The change in ion-exchange specificity with pH (Chapter 8) can also cause opposing trends of K availability in limed soils. Increased availability can be attributed to greater quantities of K in the soil solution, because of Ca replacement of K in the DDL of the soil s colloids. Decreased K availability after liming can be due to greater quantities of K leaching from limed soils. 

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Foy, C.D. (1984). Physiological effects of hydrogen, aluminium and manganese toxicities in acid soil, pp 57-97 in F. Adams (ed.) Soil Acidity and Liming, 2nd edn. American Society of Agronomy, Madison. 

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