Procedure for Dumping the System..
1. Regular solution dumping is not necessary for properly managed solutions running on good quality water. Occasional dumps may be necessary where poor quality water, and especially water high in sodium, has to be used, or when solution analyses show that the nutrient balance in the NFT solution is so unbalanced that it is most easily corrected by dumping and starting a new solution. The only other reason why solutions might need dumping is when accumulation of some organic toxin or other toxin not measured by solution analysis is suspected of accumulating in the solution. There is some data which supports the contention that organic root exudates do not accumulate to harmful concentrations in NFT solutions.
2. When it is necessary to dump, the dump can only be done when the rate of water uptake by the crop is very low, i.e. usually at night, with most growers preferring to dump after dark during the evening. However it is necessary to remember that in summer and after bright summer days crops may continue to take up water for upto about 4 hours after sunset, and when these conditions occur it is better to wait until water uptake is very slow before dumping.
3. The stop cock in the fresh water supply line to the sump must be closed before starting to dump.
4. The dump is made by shutting off the flow to the flow mains and header pipes and by opening the dump valve, usually with the circulating pumps running as usual. The dumping process needs to be watched as solution is usually dumped faster the solution flows back from the gullies, with the result that pumps might run dry. Dry running will damage most pumps, and so the pumps will need to be stopped before they start sucking air. Start the pumps again as soon as sufficient solution has run back to provide a reasonable depth in the sump. The stop start process may need repeating several times. Continue dumping until the flow back through the return pipes has reduced to the merest trickle. This may take more than an hour for some systems.
5. The pumps will start sucking air before the sump is completely empty and the last volume of solution should be diluted by running a hose into the sump while pumping out, and continuing to run in fresh water until the volume left in the bottom of the sump is nearly all fresh water. There is an opportunity to clean the sump walls by wiping down during this time if it is necessary.
6. Read the water meter and note the reading in your log book or on your record sheet.
7. Close the dump valve and open the fresh water stop cock and allow the sump to fill with fresh water until the Jobe valve shuts off the water flow. Then start one or more circulating pumps, making sure that valves to the flow mains are still closed, and that the valve to the sparge pipe is open so that the solution is circulating and mixing quickly. Circulating fresh water through the gullies (or flushing) may be harmful to the roots (by bursting root hairs or leaching nutrients out of the roots), and to fruit (by causing splitting). Add A & B solution until the solution in the sump is brought up to the usual working CF.
8. When the solution in the sump is at the proper CF and thoroughly mixed open the valves to the flow headers.
9 The level of solution in the sump will drop and more fresh water will be added through the Jobe valve. The CF will fall and more A & B solution will need to be added to keep the CF correct.
10 Watch for flow of solution back into the sump from the return pipes. The system is full when the solution is flowing back strongly and when the Jobe valve closes. Read the water meter as soon as this is so. Note the reading. The difference between this reading and the reading before starting the filling operation is the system volume.
Adjustments to the nutrient solution.
11. When the system is full make a final check of the solution CF and pH and add A & B solution and or acid or alkali if necessary. Add acid if the pH is over 6 (for tomatoes, peppers eggplants or lettuce) or over pH 5.3 for cucumbers. Adding alkali is only necessary if the pH (for any crop except cucumbers) is below pH 5, and then only add sufficient alkali to increase the pH to about pH 5.3.
12 When dumping during the season, the new solution might not be on target for phosphorus, iron or other trace elements. This is because recipes are adjusted to meet crop uptake demands and are not necessarily correct for mixing large volumes of nutrient solutions from fresh water. The recipes supplied by us (Feed Calculation sheets) show the concentrations of nutrients in new solution for the CF range 10 to 60. Read the concentrations of phosphorus, iron, manganese, zinc, copper and boron on the line for the CF you are using and compare these concentration with the target concentrations listed below:
NFT solution target concentrations for tomatoes.
|Winter - ppm
||10 - 15
||5 - 10
If the concentration of any nutrient in the newly made solution is less than the target then a direct addition of that nutrient is needed to bring the solution upto target. If for example the iron concentration in the solution is 4 ppm and the target is 10 ppm then the sufficient iron has to be added raise the concentration by 6 ppm. Remember that 1 ppm is the same as 1 gram per m3. The system volume is known from the water meter readings, and if the system volume is 10 m3 then 60 g of iron has to be added. To calculate the amount of iron chelate need to provide this, divide by the percentage iron in iron chelate and by 100. The amounts of nutrients in each fertiliser is shown on the top part of each recipe sheet. In this example iron chelate is 13% Fe, so 60 g Fe divided by 13 x100 =461 g iron chelate to be added.
Data for NFT sample advice notes..
13 Make sure that the dump is recorded on your record sheet or in your log book, with the date, the water meter readings before and after filling, the system volume, the recipe number used to make up the new solution and the volumes of A & B added, the volumes of acid or alkali added and the amounts of any direct additions.
©:R A J White November 1999