what is soil pH ?

Soil pH: What it Means-

Soil pH  is an indication of the acidity or alkalinity of soil and is measured in pH units. Soil pH is defined as the negative  of the hydrogen ion concentration. The pH scale goes from 0 to 14 with pH 7 as the neutral point.the amount of hydrogen ions in the soil increases the soil pH decreases thus becoming more acidic. From pH 7 to 0 the soil is increasingly more acidic and from pH 7 to 14 the soil is increasingly  alkaline or basic.

• Extremely acid: more than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2–4.
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• Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5.
• Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3.
• Moderately acid: 5.6–6.0; potatoes=5.6.
• Slightly acid: 6.1–6.5; salmon=6.2; cow's milk=6.5
• Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0
• Slightly alkaline: 7.4–7.8; eggs=7.6–7.8
• Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4
• Strongly alkaline: 8.5–9.0; borax=9.0
• Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=12

Measuring  of  Soil pH-

Soil pH provides various clues about soil properties and is easily determined. The most accurate method of determining soil pH is by a pH meter. A second method which is simple and easy but less accurate then using a pH meter, consists of using certain indicators or dyes.

 How  pH Affects Nutrients, Minerals and Growth--

The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.
Phosphorus is never readily soluble in the soil but is most available in soil with a pH range centered around 6.5. Extremely and strongly acid soils (pH 4.0-5.0) can have high concentrations of soluble aluminum, iron and manganese which may be toxic to the growth of some plants. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients.
But some plants, such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees, tolerate strong acid soils and grow well. Also, some plants do well only in slightly acid to moderately alkaline soils. The soil pH can also effect on a plant growth by its effect on activity of beneficial microorganisms Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.