Plant Hygiene
See the UV and Ozone
sections in the Pool and Spa information area for
more on sterilization. Some special sanitization methods for plants
are available.
Sterilizing recirculated nutrient solution before re-use is an
insurance against crop losses. Total crop failure due to not sterilizing
is very real. The cost of crop loss would be much greater in a long-term
crop than a short-term crop, such as lettuce, which is harvested only
once. Most growers think that it is essential to sterilize as insurance
when growing long-term crops such as tomatoes, cucumbers and roses.
The quantity of solution recirculating through an NFT system for 1
hectare of tomatoes is about 2 million litres per day. The run-off from 1
hectare of Rockwool tomatoes is about 20,000 litres per day (1% of that of
NFT). Media-based systems like Rockwool are being modified to collect the
run-off and return it for sterilizing and re-use.
There are a number of types of sterilizing techniques. Some are slow
sand filtration, heat treatment, ozonisation, UV (ultra violet) treatment
and 'ultrafiltration'.
Heat treatment involves heating the solution to over 95 OC (203
OF) for 30 seconds. Heating is done in a counter current
heat exchanger where the hot treated nutrient solution is used to heat the
incoming untreated water cooling the treated solution back to normal
temperatures. Hence operating costs are fairly low. Most temperate zone
glass houses already have boilers for heating, the only capital cost is
for the heat exchanger. A difficulty is the fouling of the internal
surfaces of the exchanger which can be reduced by lowering the pH of the
solution.
In ozonisation, the solution is recycled through an ozone
producing machine.
Ultra filtration is similar to advanced reverse osmosis. It uses
a machine which has a membrane that permits the passing of nutrient
molecules while filtering out pathogens. There are technical problems
keeping the membranes clean.
UV sterilisation has problems of reduced efficiency due to dirty
tubes and shielding in dirty solutions. It knocks iron out of solution and
is relatively ineffective against viruses. The dirt problem is reduced by
filtration and automatic mechanical cleaning of the tubes.
Slow sand filtration is slow with flow rates in the order of
100 to 300 Litres/hour per square meter of surface area. Their
reliability, and simplicity, is a major recommendation in eliminating Phytophthora
and Pythium from recirculating nutrient solutions or drainage
water. High efficiency was observed against Cylindrocladium, Verticillium
dahliae, Thielaviopsis and Xanthomonas bacteria. There
is also a report of high efficacy against a virus, pelargonium flower
break (Berkelmann et al, 1993).
Pathology work with Fusarium spp has demonstrated a 99.9%
reduction rate of microconidia (small resting spores) which were poorly
filtered by early designs of sand filters. It is assumed by researchers
that this level of efficacy is sufficient to prevent serious problems with
distribution of Fusarium through recirculating filtered water. Fusarium
microconidia are more resistant to heat and UV treatment than other
pathogens and are most likely to be most poorly controlled by any
disinfestation method.
It is important to avoid disturbing a skin that forms on the surface of
the sand filter soon after it begins operating. This skin consists of
organic and inorganic material and a range of biologically active
micro-organisms which break down organic matter.
The filter appears to have biological activity in the top 40cm of sand,
thus it is recommended that the filter thickness should be a minimum of
50-60 cm. Over time, some cleaning of the filter bed may be necessary and
it is recommended that an initial thickness of 80-120 cm is more
appropriate, to allow for scraping off a few centimetres during the
cleaning process. Beneath the sand filter are three layers of gravel,
which prevent sand from getting into the growing system. Slow sand filters
can be housed in such containers as concrete or corrugated iron tanks, or
large plastic bins. The pH and conductivity of the water are not affected
by the process.
Chlorination or bromination can be quite effective in
disinfecting water of serious nursery crop pathogens such as Phytophthora
and Pythium.
Hydrogen peroxide dosing can also sanitize nutrient
solution while increasing the available oxygen at the plant root.
Copyright © 1996, 1997, 2001 TPS Pty Ltd
