Reader Inquiries

Answers by Rick Donnan

What is NFT?

I have been reading hydroponic books and articles lately and have noticed that there is quite a lot of jargon used. One I particularly notice is NFT, which is occasionally defined, but much more often not. I recognise that it stands for Nutrient Film Technique, but what does that mean?

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What is ‘false drain’?

Higher ECs within the medium than in the runoff

I have been growing tomatoes in rockwool slabs for over three years. Recently, my plants have been looking stressed and I have had some blossom end rot.

Friends have suggested that it might be high EC in the slabs, but my run-off EC (electrical conductivity) is usually below 3 (mS/cm).

At their suggestion I recently bought a WET (Water, EC, Temperature) meter to measure properties within the slab. I now find that I am sometimes getting much higher EC readings within the slab than in the drain. Is this normal, and what should I do?

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What is outgoing longwave radiation and what is its influence on plants?

I notice that there was a paper titled, ‘The underestimated effects of Longwave Outgoing Radiation on your greenhouse crop” at the recent Protected Cropping Australia Conference. Would you please explain what this means. 

Answer RICK DONNAN

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Should I set up a commercial aquaponic venture?

I have been a hobby grower using aquaponics for several years. I have had reasonable success although also some difficulties. I am considering setting up a commercial operation and have been reading your answers and articles on planning for commercial hydroponic growing. Would you have any extra advice relating to aquaponics, possibly organic?

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What is hydronic greenhouse heating?

I am a commercial grower of hydroponic tomatoes in basic unheated plastic tunnel houses. My yields are reasonable, but far lower than I am told growers achieve in good heated greenhouses.

I am considering upgrading to a better heated greenhouse. I often see references to hydronic heating as a common method for heating greenhouses. Would you please explain what hydronic heating is?

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What are good management practices for media-based systems?

Thanks for your answer highlighting the importance of measuring the run-off solution from media-based systems (Reader Inquiries April 2017) and your detailed description of how to do this. Are there other management issues for media-based systems that hydroponic growers need to be aware of?

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How do you collect a sample of the root zone solution?

In my answers I frequently refer to the root zone solution. That is, the solution around the plant roots from which the plant takes up water and nutrients, as explained in my previous two columns. I stress that it is this root zone solution, which is the basis for good hydroponic management. Many growers obsess about their feed solution, but this is only a means to managing the root zone. I am often asked how to collect a root zone solution sample from a dripper-fed container of medium, so this is my practical answer to that question.

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How do plants take up nutrients?

From an indian grower:

Please refer your answer to the reader’s question in the February 2017 PH&G issue.

You have mentioned that in osmosis only water moves from lower concentration to higher concentration and salts do not get through the membrane. If this is so, then how do nutrients reach plants? I am a grower and a regular reader of your magazine. Thanks a lot. See More

Why are my tomatoes and capsicum turning black?

I have a small greenhouse in which I grow 12 tomato and six capsicum (sweet pepper) plants. They are growing in bags of dripper fed coco peat. I use a standard “bloom” liquid fertiliser at the recommended strength (4 ml each of A and B per litre water). The older fruit of both the tomato and capsicum have turned black on the bottom. The tomato plants generally look OK, but the capsicum plants are looking poorly.

Can you suggest what is wrong and what I can do to fix the problem?

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How do I visually assess nutrient deficiency symptoms?

I am a hobby hydroponic tomato grower. My plants are producing well, but recently I noticed what appears to be a nutrient deficiency. The youngest leaves are turning yellow between the veins. Can you suggest what it is?

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