organic material—the elemental results might
have been different if it had been removed. These
results suggest that rockwool and perlite could
be considered inert in their interaction with an
applied nutrient solution, while coir is not.
Based on the physiochemical nature of a rooting
media, the elements in an applied nutrient
solution can potentially interact with the rooting
media, either being physically adsorbed or
chemically bonded to form complexes, thereby
resulting in their accumulation. To determine
the degree of elemental accumulation that can
occur, samples from a rockwool slab and perlite
from BATO buckets were collected for elemental
analysis following the hydroponic growing of
greenhouse tomatoes using a drip irrigation
system for nutrient solution delivery. Since there
occurs an accumulation of applied elements
during the growing season, which is observed as
an increase in the electrical conductivity (EC)
of the residue solution, growers are advised to
monitor the EC of the retained solution and to
water leach when the retained solution reaches
a certain EC level. For both collected samples,
the growers were following is routine of periodic
water leaching.
The gathered rockwool slab and perlite samples
were first water leached, and then extracted using
the Mehlich No. 3 soil extraction method; the
results are expressed as pounds per acre (lbs/A)
so that the assay results can be interpreted using
established Mehlich No. 3 interpretation values.
The assay results are given in table four.
The water-soluble assay results confirm that
there is an accumulation of elements that remain
in the rooting media in solution, while the
Mehlich No. 3 extraction results indicate that
there exists another form of these elements,
probably as precipitates of calcium sulfate and
phosphate that either entrap other elements
or form chemical complexes. The elements in
these precipitates are probably available for root
absorption since the plant root surfaces are acidic
and when in physical contact with a precipitate
particle, some degree of dissolution could occur.
These results also suggest that there needs to be
an evaluation of the nutrient solution formulation
so that both water-soluble and precipitate
accumulation is minimized in order to avoid
essential element insufficiency from occurring in
the growing crop.
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