|
Soil pH is an indication of the alkalinity or acidity of soil. It is
based on the measurement of pH, which is based in turn on the concentration
of hydrogen ions (H+) in a water or salt solution.
When in balance (pH 7) the soil is said to be neutral. The pH scale
covers a continuum ranging from 0 (very acidic) to 14 (very alkaline
or basic). It is however uncommon to find soils at either extreme of
this range. Under many conditions soils tend to become more acid or
alkaline over time if steps are not taken to maintain a balance.
pH is important for the organic gardener for several reasons, including
the fact that many plants and soil life forms prefer either acid or
alkaline conditions, that some diseases tend to thrive when the soil
is alkaline or acidic, and that the pH can affect the availability of
nutrients in the soil.
Nutrient availability in relation to soil pH
The majority of food crops prefer a neutral or slightly acidic soil,
because the solubility of most nutrients necessary for healthy plant
growth is highest at pH 6.3-6.8. Some plants however prefer more acidic
(e.g., potatos, strawberries) or alkaline (brassicas) conditions.
When the pH falls below 5.5, most major plant-nutrient minerals (those
needed in substantial quantities to promote healthy plant growth include
nitrogen (N), phosphorus (P), potassium (K), sulfur (S), magnesium (Mg),
and calcium (Ca)) and some micronutrients (elements important to plant
growth in very small amounts) become insoluble and hence unavailable
for uptake by plant roots.
Many cationic (positively charged) nutrients such as zinc (Zn2+), aluminium
(Al3+), iron (Fe2+), copper (Cu2+), cobalt (Co2+), and manganese (Mn2+)
are soluble and available for uptake by plants below pH 5.0, although
their availability can be excessive and thus toxic in more acidic conditions.
In more alkaline conditions they are less available, and symptoms of
nutrient defficiency may result, including thin plant stems, yellowing
(chlorosis) or mottling of leaves, and slow or stunted growth.
>> Testing and Adjusting Soil pH
|
