Leaching and Weathering

The high rate of decomposition plus the continuous warm temperatures and high rainfall in the wet tropics result in optimum year-round conditions for leaching of soil nutrients. Positively charged nutrient ions such as calcium and potassium are held on the surface of clay particles, but when the soil solution is acidic, the nutrients are replaced by hydrogen ions. Hydrogen is formed in the soil when carbon dioxide respired by soil organisms reacts with soil water to form carbonic acid, which dissociates into bicarbonate and a positively charged hydrogen ion (Johnson et al. 1977):

Negatively charged anions such as bicarbonate replace negatively charged nutrient ions such as nitrate and sulfate. The nutrients that have been ex changed into the soil solution can then be adsorbed by soil colloids, used by plants, or leached into streams or groundwater.

The reactions in Eqs. (2.3) and (2.4) also result in soil weathering. Weathering occurs when hydrogen in the soil solution reacts with minerals in the soil or bedrock, resulting in removal of nutrient elements. For example, feldspar is an aluminosilicate (aluminum and silica compound) containing nutrients such as sodium, potassium, and calcium. When it is hydrolyzed, the nutrients are removed from the aluminosilicate.

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