Reader Inquiries

Answers by Rick Donnan

How do I grow hydroponic mint?

I have read the article on mint in your Issue 132. I am also trying to grow mint in hydroponics in India. Would you please guide me in a few things:

  • What is water pH and TDS levels? 
  • What is the temperature to be maintained in the greenhouse?
  • What is the pest control method used? 
  • What nutrients are used?


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Should I use copper fittings in hydroponics?

I plan to set up a small hydroponic system in my village allotment. I have some copper fittings left over from a kitchen renovation. I am concerned about using these because, although we have very little pilfering of produce from the allotments, the lure of stealing copper may be higher. Is there something cheaper and less attractive I can use instead?

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Can I sterilise my water before planting?

I will soon be going to transplant lettuce seedlings into my Deep Water Culture tank—20 cubic metres (m3) for the first time. I would like to ask you if it will be OK to add some pool chlorine into the tank at this stage, before I put any plants in, just to sterilise the tank before I commence. If it is safe to do, what amount of chlorine would you suggest I add per 1000 litres?

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What are the most important greenhouse properties?

I am considering upgrading to a more advanced greenhouse, either glass or plastic. I intend looking at buying and moving into an existing facility. I can’t afford the time or cost to build a new set-up. What do you believe are the most important aspects of the greenhouse properties to consider, apart from glass versus plastic? 

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Setting up a trial NFT system

I am in the process of setting up a small NFT (Nutrient Film Technique) system to start a trial commercial basil farm. I am planning on using a header tank system to supply the nutrients to the plants. I’m choosing this system because I feel at this time it provides me with more safeguards. The flow rate of the nutrient solution to the plants from the header tank would be controlled by a manual inline valve. I will have a primary pump to transfer solution from the catchment tank to the header tank. I also plan on using a secondary pump as a backup in the event of primary pump failure. When I change the solution I will swap the pumps so the primary becomes the back up and vice versa. Both pumps would be activated either by their own float or perhaps one float would control both for simplicity, but I need to figure out which would be more practical. Answer by RICK DONNAN
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Can I stop my Deep Flow Technique recirculating pump overnight?

Thank you for all the answers you gave me. I have just one short question if I may, in reference to the system in my previous question (4m x 20m DFT tank).

If I shut the pump down at night and only run it during the day time, do you think it will be ok in terms of oxygenation? I am planning to construct an additional three grow tanks each with its own pump, and would like to save a bit on energy costs by turning the pumps off at night.

(The previous question was answered in my August column. It related to the set up and management of a 20,000 litre Deep Flow Technique (DFT) system, which held an extremely high 13 litres of nutrient solution per plant.)

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Setting up a DFT (Deep Flow Technique) system

I am setting up a DFT grow tank for lettuce in South America, first one, and then more in time. The size is 4m wide x 20m long and 25cm deep with a 1000L control tank. I have had five similar sized sub-irrigation tanks with gravel before, but this is my first DFT system. I want to use a swimming pool pump with a venturi to circulate and aerate the nutrient solution, and aim to do a total replacement of the grow tank volume every 2 hours. Where in the system would be best to place the pump? Between the control tank and the grow tank, pumping from the control tank into the grow tank with an overflow back to the control tank. Or the other way around? Answer by RICK DONNAN See More

How can I change a fertiliser formulation?

I have recently gotten into hydroponics and came across your blog containing much detailed information. I am trying to move away from buying expensive pre-mixed solutions to making my own. I have a query about DIY soluble fertilisers and hope you could help.

I might buy the following fertiliser: N = 6%; P2O5 = 12%; K2O = 36%; MgO = 2.5%; SO3 = 10%; Ca = 0%. It also includes trace elements B, Cu, Fe, Mo, Mn, Zn.

I want to upgrade the N content to 20 and P to 18. What do I need to buy to do that? Also I have seen all commercial soluble fertilisers do not have Calcium, so how do I add this to my mix and in what dosage? Will adding Calcium affect the NPK?   Answer by RICK DONNAN
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How is a blend valve used in recirculation?

I attended one of the recirculation workshops in which you were a co-presenter. I am finally thinking of taking the plunge with my cocopeat system, but I am confused about the use of a blend valve in controlling the recirculation volumes.

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What is the future for hydroponic fertilisers?

How are hydroponic fertilisers made and what reserves are there for their future availability?

Answer by Rick Donnan

Population predictions indicate that there will be a corresponding increase in the demand for food. In turn, because of a shrinking in good arable land, there will be an increasing demand for fertilisers to meet the food requirements. See More