Also see Soil pH- Its not a real substance and Nutrient Management on high pH soils


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pH is a scale that chemists use to measure acidity. Values below 7 are considered acidic, values above 7 are alkaline ( the opposite of acidic) and 7 is neutral.

Most plants can tolerate a wide pH range in solution culture, but they cannot tolerate a wide range of acidity in the soil.

When soil acidity changes, the solubility of a number of metal ions also change. Plant growth is really affected by the varying concentration of these metals in solution rather than by the acidity itself.

Under acidic conditions, many soil minerals dissolve and increase the concentration of metal ions to toxic levels. The primary toxic metal is aluminum, but high levels of manganese and iron can also inhibit plant growth under these conditions. The nutrients phosphorus and molybdenum are less available in acidic soils and calcium and/or magnesium may also be deficient.

Under alkaline conditions, the solubility of minerals decrease to the point that nutrient deficiencies occur. Plant growth is therefore limited by deficiencies in iron, manganese, zinc, copper and boron. Phosphorus is also less available in alkaline soils and high levels of calcium may inhibit the uptake of potassium and magnesium.

The aim in managing soil pH is not to achieve a particular pH value, but to adjust the acidity to the point where there are no toxic metals in solution and the availability of nutrients is at its maximum. This condition is usually achieved when the soil pH is between 5.8 and 6.5, however some plants have special acidity requirements.

Limestone is used to treat acidic soils, but the soil pH value alone does not indicate the amount needed. An exchangeable acidity analysis must also be done to determine the amount of limestone required, and the soil calcium and magnesium levels must be analyzed to determine which type of limestone (dolomitic or calcitic) is required.

Some alkaline soils can be acidified using sulfur or acid forming fertilizers, but soils with free calcium carbonate cannot be easily acidified. It is often easier to manage the nutrient deficiencies that occur on alkaline soils than to acidify the soil.