Soil pH
Soil pH is a measure of the acidity or basicity of a soil. Soil pH can be used as an important indicator to make analysis of both quantitative and quantitively regarding soil characteristics. In soils the pH is commonly measured as a slurry of soil mixed with water or a salt solution and normally falls between 3 and 10 with 7 being neutral. Alkaline soils are basic soils with a pH above 7, soils with a pH below 7 are acidic soils. Ultra-acidic soils with a pH < 3.5 and very strongly alkaline soils pH > 9 are rare but can occur. The soil pH can affect may chemical processes and it specifically affects plant nutrient availability and influencing the chemical reactions they undergo. The best pH range for most plants is between 5.5 and 7.5. Many plants have adapted to survive at pH values outside this range.
Soil pH ranges and classification
| Denomination | pH range |
|---|---|
| Ultra acidic | < 3.5 |
| Extremely acidic | 3.5–4.4 |
| Very strongly acidic | 4.5–5.0 |
| Strongly acidic | 5.1–5.5 |
| Moderately acidic | 5.6–6.0 |
| Slightly acidic | 6.1–6.5 |
| Neutral | 6.6–7.3 |
| Slightly alkaline | 7.4–7.8 |
| Moderately alkaline | 7.9–8.4 |
| Strongly alkaline | 8.5–9.0 |
| Very strongly alkaline | > 9.0 |
Methods of determining pH in soil
Observing a soil profile can reveal profile characteristics that can be possible indicators of acidic, alkaline or neutral soils.
- Poor incorporation of an organic surface layer with underlying mineral layer can indicate strongly acidic soils
- Columnar structure can be an indicator of sodic condition
- Presence of a caliche layer or a mineral deposit indicates the presence of calcium carbonates, which are present in alkaline conditions.
Observation of predominant flora. Calcifuge plants prefer acidic soils and include species from nearly all Ericaceae species, many birch, foxglove, gorse, and scots pine.
Use of an inexpensive pH testing kit where a small sample of soil can be mixed with an indicator solution which changes color according to the pH of the soil.
Use of a commercially available electronic pH meter in which a glass or solid-state electrode is inserted into moistened soil or a mixture of soil and water. The pH usually appears directly on the digital display screen.
Factors affecting soil pH
Sources of acidity
- Rainfall: average rainfall has a pH of 5.6. When the water enters the soil it results in the leaching of basic cations from the soil.
- Root respiration and decomposition of organic matter by microorganisms releases carbon dioxide and increases carbonic acid concentration and subsequent leaching which acidifies the soil.
- Fertilizer use, ammonium fertilizers react in the soil by the process of nitrification to form nitrate and in the process releases of Hydrogen ions which acidify the soil.
- Acid rain will increase the acidity in soils where the rain falls and flows into the soil.
Sources of alkalinity
- Weathering of silicate, aluminosilicate, and carbonate materials will cause the soil basicity to rise.
- Addition of water containing dissolved bicarbonates, occurs when irrigating with high-bicarbonate waters.
The accumulation of basic elements such as carbonates and bicarbonates occurs when insufficient water flows through the soils to leach the soluble salts.
Soil pH effects on plant growth
Acidic soils Plants grown in acidic soils can experience many stresses including aluminum, hydrogen, and/or manganese toxicity, as well as nutrient deficiencies of calcium and magnesium. Aluminum toxicity is the most widespread problem in acid soils. Aluminum is present in all soils to different degrees, dissolved aluminum is toxic to plants and aluminum is the most soluble at low pH, above a pH of 5 there is little aluminum soluble in most soils. Plants do not use aluminum as a nutrient and instead takes it up through osmosis through the plant roots. This uptake of aluminum has several effects on the growth of plants. The aluminum inhibits root growth, lateral roots and root tips become thickened and roots lack fine branching.
Alkaline soils Alkaline soils have low infiltration capacity so rain water stagnates on the soil easily and it is harder for plants to get established. Alkaline soils require irrigated water and good drainage. This soil is only used in agriculture for crops tolerant to surface waterlogging such as grass and rice. The productivity of this soil is lower.