3. Soil pH

Introduction Soil pH is a measure of the acidity or alkalinity of a soil, which affects the availability of plant nutrients, activity of microorganisms, and the solubility of soil minerals. Major factors affecting soil pH are temperature and rainfall, which control the intensity of leaching and soil mineral weathering. Acidity is generally associated with leached soils; alkalinity generally occurs in drier regions. However, agricultural practices, such as liming or addition of ammonium fertilizers, can alter soil pH. The pH measurement is actually measuring the hydrogen ion activity [H+] in the soil solution.

Interpretations In general, pH values between 6 and 7.5 are optimum for general crop growth. Site specific interpretations for soil quality will depend on specific land use and crop tolerance. Site specific interpretations for soil quality will depend on specific land use and crop tolerance -> Table 7 shows the soil salinity class and general crop and microbial responses for each class.

Figure 1. Soil pH, ranges for pH classes, and associated soil conditions. Adapted from the National Soil Survey Manual (1993) and Troeh and Thompson (1993).

Figure 2. Relationship between pH and nutrients availability

Nutrient Availability Soil pH affects the availability of nutrients to plants or crops (Figure 2). Nutrient availability is affected by changes in the solubility of soil minerals. Most minerals are more soluble in acid soils than in neutral or slightly basic soils. The greatest availability for most nutrients is between pH 6 and 7 (Figure 2). Where nutrients are shown interlocking in Figure 2, those nutrients at that pH combine to form insoluble compounds, reducing their availability. Soil pH also affects the activity of beneficial microorganisms, which affects nutrient availability. In general, fungi function at a wide pH range, but bacteria and actinomycetes function better at intermediate and higher pH.

Table 7. Suitable soil pH ranges for selected crops (Whittaker et al

Comments The presence of salts affects soil pH by decreasing the reading by 0.2 to 0.3 pH units (Thomas, 1996). To mask the effects of salts, a 0.01 M CaCl2 solution has been commonly used instead of distilled water. Declining pH is a sign of inefficient N use where ammonia based fertilizers are used (see Smith and Doran, 1996).