Soil pH

Soil pH levels adverse to crop growth can occur naturally in some regions; it can also be induced by acid rain or soil contamination from acids or bases. The role of soil pH is to control nutrient availability to vegetation. The principal macronutrients (calcium, phosphorus, nitrogen, potassium, magnesium, sulfur) prefer neutral to slightly alkaline soils. Calcium, magnesium and potassium are usually made available to plants via cation exchange surfaces of organic material and clay soil surface particles. While acidification increases the initial availability of these cations, the residual soil moisture concentrations of nutrient cations can fall to alarmingly low levels after initial nutrient uptake. Moreover, there is no simple relationship of pH to nutrient availability because of the complex combination of soil types, soil moisture regimes and meteorological factors. The important observation is that pH is the regulatory mechanism to plant nutrient uptake, and that the theoretical concentration of soil nutrients is meaningless until pH levels are in the optimum range for uptake. Soil pH can be raised by amendment by agricultural lime; the pH of an alkaline soil is lowered by adding sulfur, iron sulfate or aluminium sulfate, although these tend to provide costly short term benefits. Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulfate and monopotassium phosphate also reduce soil