pH Meter Data Book-Soil and pH

Soil and pH

There are two types of pH meters for measuring soil, acidity meters that are inserted into the ground and pH meters that measure water and soil mixed into a supernatant liquid. Japan's Soil Nutrient Analysis Act indicates that "when water is added to soil and shaken, the activity of the separated hydrogen ions is measured with a glass electrode pH meter."
In its natural state, Japan's soil is around pH 4.5 to 5.6, tending to the acidic side.
Plants have a suitable soil acidity (pH) for growth, and they need to be grown in appropriate soil. Soil with a pH level of 4 to 6 is referred to as acid soil, and soil with a pH of 8 to 9 is called alkali soil. Most vegetables grow well in soil with a pH of 6 to 6.5. In acid soil, the vegetable's roots are damaged, preventing them from absorbing enough nutrients.
Slaked lime, magnesium lime, and organic lime (oyster shells) are added to the soil to neutralize acid soil (raise the pH).

Amount of Lime Needed to Adjust pH (g/㎡)

Alkali soil is not common, but it can occur when alkaline materials such as lime are used in excess or soil is used in a greenhouse for an extended period of time.
In alkali soil, required nutrients such as phosphoric acid and iron become insoluble and cannot be absorbed by the plant. Malnourished plants can decay or wither from wind or salt damage.
To neutralize alkali soil, either apply acid fertilizer or take the time to cultivate corn or spinach, which absorbs the salt and lime in the ground.

Fig. 4 Ideal Soil Acidity for Major Vegetables
The following are targets for improvement set out in Japan's Basic Guidelines for Increasing Fertility.
  • ・Rice paddies pH 6.0–6.5 (6.0–8.0 for calcareous soil)
  • ・Crop fields pH 6.0–6.5 (6.0–8.0 for calcareous soil)
  • ・Orchards pH 5.5–6.5 (4.0–5.5 for tea fields)

Electrical Conductivity of Soil

Electrical conductivity (EC) is an index used to indicate the salinity of soil that is often discussed in connection to pH.
EC and nitrate nitrogen content have a high correlation and are used to estimate fertilization. If the EC is too high, it becomes difficult for crops to absorb nutrients and moisture, inhibiting growth. As an estimate of fertilization, EC of 0.3 mS/cm or less is insufficient fertilization. EC of 0.8 mS/cm or higher is excessive fertilization. EC before fertilization for vegetable and crop fields is usually between 0.1 and 0.3 mS/cm, and EC of pastures is usually 0.1 mS/cm or less.

Salt Tolerance and EC by Crop Type
Soil diagnosis using pH and EC.