A thread on nutrient recipes, and some reference links
Growing Greenhouse tomatoes
http://www.ces.ncsu.edu/depts/hort/g...gGHTomates.pdf
"The cation exchange capacity of the soil
When small quantities of inorganic salts, such as the soluble mineral
matter of soil and commercial fertilizers, are added to water they dissociate
into electrically charged units called ions. The positively charged ions
(cations) such as hydrogen (H+l, potassium (K+l, calcium (Ca2+l,
magnesium (Mg2+), ammonium (NH

, iron (Fez+), manganese (Mn2+1,
and zinc (Zn2+1 are absorbed mostly on the negatively charged surfaces of
the soil colloids (microscopic clay and humus particles) and exist only in
small quantities in the soil solution. Thus, the humus-clay colloids serve as
a storehouse for certain essential ions (cations). The negatively charged
ions (anions), such as nitrates (NO; 1, phosphates (HPO:?, sulfates WI-I,
and chlorides (Cl-), are found almost exclusively in the soil solution and can
therefore be leached away easily with overwatering. The roots and root
hairs are in intimate contact with the soil colloidal surfaces, which are
bathed in the soil solution, and therefore nutrient uptake can take place
either from the soil solution or directly from the colloidal surfaces (cation
exchange).
As plants absorb nutrients (ions) they exchange them for other ions.
For example, for the uptake of one potassium (K+) ion or one ammonium
(NH

ion, one hydrogen (H+) ion is released into the soil solution or
directly into the soil colloids by the process of cation exchange. Similarly,
for the uptake of one calcium (Ca2+) or one magnesium (Mg2+) ion, two
hydrogen (H+) ions are released by the root. Thus, as the plant absorbs
these essential cations, the soil solution and the colloidal particles contain
more and more hydrogen (H+) ions, which explains why the removal of
cations (ammonium (NH ‘;I nitrogen is a good example) by crops tends to
make soils acidic, i.e., having a low pH. Also, as the plant absorbs essential
anions such as nitrates (NO ;1 and phosphates (HP0 4 ), the soil solution is
enriched with more and more hydroxyl groups (OH-1 and bicarbonates
(HCO; 1, which explains why the removal ofanions (nitrate (NO; 1 nitrogen
is a good example1 by crops tends to make soils alkaline, i.e., having a high
PH. "