Ask the Professor – Prev. Questions 3

Professor –

We are growing lettuce, we start with well water that is 7.2 pH and 300 ppm. we fill a 500 gal. tank and pump thru a 2″ then 3/4″ pvc pipes into 36 – 25′ gutters covered with black rigid plastic with 2′ holes for the net pots, we use a commercial nutrient and phosphoric acid and the pH just keeps climbing back up . we are in an area of high sunlight and the greenhouse temp does get to 90°. any suggestions on controlling the pH?



There is a normal rise in pH as the nutrients are being used up. You didn’t mention how radical the pH shift is so I can’t really say if your pH rise is normal or there may be a problem. I believe that part (if not all) of the reason for the pH fluctuation may be the size of your reservoir, 500 gallons is not all that much when you are running 900 feet of channel, that is a lot of lettuce plants (I am guessing about 1200 – 1500 plants?). Assuming 1500 plants and 500 gallons that is only a third of a gallon per plant. I believe that you may be experiencing pH shift due to the rapid uptake of nutrients caused by an undersized reservoir.

What happens to the temperature of the nutrient solution? The black plastic and high sunlight combination should produce lots of heat at the root zone. I live in Florida and that combination would probably boil the roots. I don’t believe that this would cause the pH swing but if the nutrient solution gets too warm the plants can be injured.



I would like to convert one bay of my greenhouse tomato production to NFT lettuce. I don’t want to spend a lot of $ to do it. The bay is about 2300 sq.ft. Any solutions on an inexpensive system, either home built or commercial? thanks



I assume that you are referring to a loose leaf type of lettuce and not a “head” type. Probably the easiest way to grow lettuce is with a water culture type system. You can build a large shallow trough out of plywood.

Waterproof the trough with fiberglass or line it with plastic film (such as “Panda” film). make “rafts” out of Styrofoam panels which are available at a good home center. Cut holes in the Styrofoam sheet to hold “net” cups which in turn hold the plant. keep the level of the nutrient between 3 to 6 inches deep, and float the Styrofoam rafts on top. Recycle the water slowly (as in a NFT system), and aerate, aerate,aerate.


Professor –

How to wire a 400 watt halide to ballast?


Darrell –

I may be a lot of things but an electrician isn’t one of them…..So I copied a diagram from a book entitled “How to Supercharge Your Garden” by Marseene Mainly. The drawing might be of help, If you can’t figure it out from there I suggest that you contact a qualified electrician or the manufacturer of the ballast for advice. I don’t like to play around with electricity (where I wire, there will be fire) 🙂 CLICK HERE to view drawings.


Dear Professor,

What would be the recommended depth of expanded clay in a Flood and Drain table for me to be able to grow tomatoes? Is 5 cm (2 inches) enough?


Brad –

I seriously doubt that 2″ of clay pellets would work real well for tomato plants. Tomatoes have a pretty large root system, and I think the plant would suffer from such a shallow growing medium. I believe that you would be better off to plant the tomatoes in pots full of clay pellets, then just set the pots in the Ebb & Flow tray. The pots need to be pretty large for tomatoes ( probably 3 gallon or better). If you don’t want to use pots then I would keep the clay pellets at least 4 inches deep to leave adequate room for the root systems. Another idea might be to use smaller pots (1 or 2 gallons) AND 2″ of pellets covering the tray between the pots.


Professor –

I am a doc of traditional oriental medicine. I have a patient who was using rockwool and has been suffering from itching and even red bump and ulceration’s…they start out small and become bigger with redness. I am wondering if he used rockwool indoors with fan ventilation if the fibers or due to the rapid growth cycle of the botanical growth that perhaps crystals from plant could cause Central or Peripheral Nervous system hypersensitive and irritation. If this is a common problem how is it remedied..some type of nervous system sedative or is there something that can dissolve the fibers…should it come off the skin when washed? what if you scratched or rubbed and it got under the skin as an irritant? Or do you think I am off base on this?

Doc CJ

Doc CJ –

I must admit that my knowledge of medicine is severely lacking. However, I will tell you what I know and I have included a link at the bottom that takes you to the Grodan company’s website. Grodan is the manufacturer of rockwool and they should be able to give you more advice.

I have never heard of this problem before, but rockwool is an irritant so maybe your patient has severe sensitivity to the fibers. From what I understand that if rockwool is wet (as in gardening conditions) the fibers do not get airborne, and I don’t think that the plant would absorb anything from the rockwool that they wouldn’t get from soil, so I doubt that the rapid growth of the plant has anything to do with your patients health problems. I wish I could be of more help, but this is out of my area of expertise and I don’t want to mislead you (especially where someone’s health is concerned). If you are pretty sure that rockwool is the source of the problem than I would suggest that your patient switch to a different growing medium and see if the problem goes away. Please use the link below and check with the manufacturer, I’m sure they will provide you with additional information.



Regarding an earlier question about chillers…….are chillers needed for Florida climate? Does the nutrient liquid temperature affect how the plant absorbs nutrients? What should the temperature be and why?



The need for chillers in Florida depends on your system and how warm the nutrient solution is getting. Ideally, the temperature of the nutrient solution should be about 72°F. I usually bury my reservoirs which helps to insulate them from the brutal Florida sun. The temperature of the nutrient solution doesn’t affect the uptake of nutrients but when the temperature of the solution rises the ability to hold oxygen decreases, which can cause all sorts of problems for the root system. Root rot and pythium can run rampant in overly warm nutrient solutions.



Need info on submersible pumps for drip irrigation. I am trying to use a Little Giant, model 2E pump which specifies that it will pump 70 gal per hour at a height of 10 ft. I am only raising the nutrient level about 6 feet, yet getting no flow at this height. The system is pumping from a 16 gallon reservoir into seven 5 gallon buckets. Any advice? Thanks


Bill –

The 2e little giant pump should have no problem pumping to six foot height (called the pump’s “head” in pump lingo) . I believe The maximum “head” of the 2E pump is 11 feet, so 6 foot is just slightly more than half capacity. I have a couple of things that you can check. Are you using the pump submerged in the nutrient solution or out of the solution (model 2e works either way)? If you are using the pump external of the reservoir, the pump still needs to be below the level of the nutrient. These pumps do not create enough suction on the inlet side to raise the water out of the reservoir. If this isn’t the problem then make sure that the lines are clear and that the pump is clear of obstructions. Make sure that the pump is pumping at all by turning the pump on and lowering it into the reservoir (or other water source) if it’s working you’ll know it. If the pump isn’t pumping at all look and see if the motor is turning the impeller, if not make sure that the impeller isn’t obstructed and turns freely (motor unplugged). If you still have no luck then the pump is defective.