pH as a proxy for estimating plant-available Si? A case study in rice fields in Karnataka (South India)

Background and aimsAlthough Si is recognized as a beneficial element for crops, the determination of plant-available silicon (PAS) in soils has become challenging. A correlation between pH and PAS indicators has been reported in the literature due to the higher degree of weathering of acidic soil. We tested this hypothesis in the cultivated rice fields of Karnataka State, India, which exhibit gradients of pH and climate.MethodTwo hundred surface soil samples were collected from rice fields representing nine of the ten agro-climatic zones (ACZs) defined for Karnataka. We analyzed the Si extracted by calcium chloride (Si-CC) and the Si extracted by acetate-acetic acid (Si-AA) as PAS indicators. The samples were analyzed for particle size distribution, pH, electrical conductivity and cation exchange capacity. Forty subsamples were selected for mineralogical and chemical analysis.ResultsPCA of the 200 samples showed that PAS indicators and pH were positively correlated. Si-AA was also positively correlated with electric conductivity (EC), CEC, and the silt fraction and negatively correlated with the sand fraction. A separation of the data arbitrarily made at pH7.5 showed that below 7.5, the correlation between PAS indicators and pH was better than considering the whole 200 samples. The distribution of Si-AA with pH matched the curve of adsorbed Si given in the literature. Soils characterized by low pH and high contents of sand, SiO2, Zr and Hf, showed a higher degree of weathering. They were preferentially located along the coast, where the rainfall is the most abundant. The depletion of PAS indicators was also in good agreement with the predominance of kaolinite instead of smectite, which characterizes a higher degree of desilication.ConclusionWe demonstrated that the correlation between pH and PAS indicators is explained by natural Si depletion (weathering intensity) and the effect of Si adsorption. We suggest that at pH values up to 7.5, pH can be used as a proxy for PAS in similar types of pedo-climatic conditions.