What effect does liming have on silicon availability in agricultural soils?

Controversy surrounds the effects of pH on Si availability in agricultural soils with some workers reporting a negative and others a positive relationship. The factors contributing to this contradiction are examined and discussed. Because soil development typically involves both desilication and decreasing pH, when a group of soils from a region with different ages and weathering rates are compared, there is typically a positive relationship between pH and Si extractability. Raising pH by liming can effect Si availability through a number of mechanisms including weathering. Over the normal range of liming (i.e. pH 4.5 to 6.5) increased pH decreases the solubility of crystalline and amorphous aluminosilicates but above about pH 7 their solubility increases. Specific adsorption of monomeric monosilicic acid (the major form found in soil solution) is at a maximum between pH 9 and 10 (pK1 for silicic acid is 9.8). Thus, increasing pH increases the proportion of Si adsorbed and decreases that present in soil solution leading to a negative relationship between pH and soil solution Si. With time, polymerization of monomeric Si can occur at the surface of metal oxide surfaces and this may hinder desorption reactions. Nonetheless, Si is generally highly mobile in soils and easily leached so that in the longer term, it is conceivable that increased adsorption may be a retention mechanism which effectively increases Si availability. Soils contain a biogenic pool of amorphous silica (mainly of plant phytolith origin) and its solubility has been shown to increase greatly with increasing pH from 2 up to pH 12. Thus, liming would increase Si availability in the short-term by increasing dissolution of biogenic Si but in the longer term it would tend to decrease Si availability through diminution of the biogenic pool of Si. The most common Si fertilizers are slag materials which are both sources of Si and liming materials. Thus when soils with different histories of slag application are compared, there is often a positive relationship between pH and Si extractability. The relationship between pH and Si solubility/availability can therefore be positive or negative depending on the situation and which pool(s) of potentially available Si (lithogenic/pedogenic, biogenic, adsorbed or residual fertilizer Si) is the most important source of soluble and available Si.