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Plant Hygiene
 

 

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.

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Last modified: May 27, 2013