Low pH means there is a higher concentration of hydrogen ions (H+)
than at higher pH. Calcium is present in neutral to acid soils mainly
as calcium ion (Ca++). At soil pH 8.3 and above, soils also contain
solid calcium carbonate. In neutral to acid soils, most Ca++ is held
by negative charges on soil minerals and organic particles. The
negative charges that hold positively charged ions is termed the CEC
or cation exchange capacity. Potassium ions (K+) and magnesium ions
(Mg++) are also held mainly on the CEC. At soil pH declines, hydrogen
ions (H+) displace Ca++, Mg++ and K+ from the negative sites into the
soil solution. This is termed cation exchange. The Ca++, Mg++ and K+
are then usually leached out of the rootzone.

Soil pH is usually raised by adding limestone or calcium carbonate
(CaCO3). The Ca++ displaces H+ from the cation exchange sites. Two H+
combine with CO3-- to form H2CO3 or carbonic acid. The carbonic acid
breaks apart to form CO2 and H2O so the H+ are neutralized. Liming is
also done using dolomitic limestone which contains MgCO3 and CaCO3, so
supplies magnesium and calcium.

Using acidic tapwater may take a long time to lower the pH of a
neutral soil (pH of 7). Clay soils especially are well-buffered
against pH changes.

The availability of all mineral nutrients are affected by soil pH, but
they are not all affected the same way. For example, iron, zinc,
copper and manganese availability are usually greater in acid soils
than in neutral and basic soils. For these mineral nutrients, pH is
mainly affecting solubility. The solubility of iron (Fe+++) increases
1,000 times for each decrease of one pH unit.

For nitrogen, pH mainly affects bacterial growth. Most soil nitrogen
is in organic matter. Soil bacteria are adversely affected by low pH.
At low pH, bacteria decompose less organic matter, which means less
nitrogen is made available to plants.


Reference

Graph of soil pH and nutrient availablity:
http://www.extension.umn.edu/distrib...s/1731-03.html




Tanya wrote in message ...
hello,
1. i know that calcium is *unAvailable* in acidic environments and
wonder whether or what other plant nutrients are affected by pH
2. if i was using acidic tap water (with vinegar) in fairly neutral pH
soil, are the nutrients permanently bound by the acidity or would they
become available once hitting the neutral soil
(the water WAS ~ pH 5 perhaps less)
thanks in advance!