Hydroponic Plants: Tips, Suggestions, and Techniques?

[maikeru]

Originally Posted by Getting A Life

Mycorrhizal fungi is a difficult field to learn about here as well. Many species have not had their fruiting bodies identified and growing them without hosts is proving nigh on impossible in many cases. Only some double as free living saprobes.

But, they are a crucial part of the picture. Big trees in the sun giving small trees in the shade carbs via a fungal bridge. A communication and transport system.

One thing I found extremely revealing was a statement made by Dr Elaine Ingham - paraphrased... "pH is determined by soil biota, not the other way round".

This statement lends the question, what does pH tell us about the inhabitants of our soil?

That's what I've been reading in my mycorrhizal book. Said a large number of mycorrhizae weren't identified, and it's likely the numbers of them will just keep growing as we keep investigating. I saw one of Dr. Elaine Ingham's books on Amazon...perhaps should think about getting it. I do agree with her statement to a large extent, in that the soil microbes and other biota mine minerals and chemicals and choose which ones are released or stored, soil microbes determine in part which plants they associate with and support in the soil, such as conifers or hardwoods for example, and that ecological succession among soil microbes also modifies pH. Plants too modify and maintain soil biota and their soil environment. I think it's a give and take.

I remember reading in a book about an Asian farmer who used sawdust to improve his soil, and other farmers warned him that it would not help, but the materials went through phases of fungal then bacterial decomposition, and the pH balanced itself out in a fairly short time. He said the end result was dark earth + humus. Even if one adds a high C/low N material like he did to the soil, I remember there are free-living microbes like Azotobacter who can fix N if they're given carbon-rich energy sources and should be able to alleviate such concerns. Though decomposition can slow down due to low N, it'll still happen regardless and improve the soil.

The only thing that is scary is truly dead soil, like sterilized potting soil or overworked, exhausted land. When the soil is dead, plants become like zombies and struggle just to live from day to day.

[Getting A Life]

Nice response, though we're heading off hydro into soil, if you don't mind, you can pull it back in to hydro anytime you wish. I do recommend Dr Inghams books she has a knack for explaining thee things to laymen and other scientists.

Have started reading about Mycorrhizal Helper Bacteria (MHB). These are also a give and take relationship. It is said they help set up the Hartig net - the bridge through which fungi and plants exchange nutrients, as well as detoxify metabolites that may inhibit fungal development. More information pending a well read man is doing a study as we speak.

I have started using wood waste on my section in conjunction with saprobic fungi. Paul Stamets talks about this in Mycelium Running. To assist the breakdown of wood waste is fairly simple.

1. Find wood waste already growing desirable saprobic species (like food species, or just fungi you like the look of).

2. Get squares of corrugated cardboard (6 inches is plenty big enough squares), soak it in water. Now, get a mushroom and pick it so the substrate comes up with it. Break off the stem butt and place this and any substrate attached inside the cardboard (peel it so it's like an envelope before adding fungi obviously).

3. Keep this moist and aired. The mycelium attaches to the cardboard using it as a new substrate. Get the cards 'growing' then move them to your plot.

4. Drop a layer of wood chip, add the cardboard cultures to this, cover in a generous amount of wood chip (4-8 inches).

5. Keep relatively moist over dry months.

6. To continue cultivating the same species in the same spot feed it more wood chips seasonally, or just let it spread out, the mycelium will grow outwards in a circle providing it has substrate to grow on and you'll have your own fairy ring.

VERY easy. And it works.

I've had dead soil. the mess the builders leave behind. Only adding biomass has helped. compost, leaving root balls in soil, worm castings, and now wood waste.

Bamboo is noted for a very healthy microbe population it it's rhizosphere. This may have helped accelerate the benefits to the farmer you mentioned.

[maikeru]

Originally Posted by Getting A Life

Nice response, though we're heading off hydro into soil, if you don't mind, you can pull it back in to hydro anytime you wish. I do recommend Dr Inghams books she has a knack for explaining thee things to laymen and other scientists.

Have started reading about Mycorrhizal Helper Bacteria (MHB). These are also a give and take relationship. It is said they help set up the Hartig net - the bridge through which fungi and plants exchange nutrients, as well as detoxify metabolites that may inhibit fungal development. More information pending a well read man is doing a study as we speak.

I have started using wood waste on my section in conjunction with saprobic fungi. Paul Stamets talks about this in Mycelium Running. To assist the breakdown of wood waste is fairly simple.

1. Find wood waste already growing desirable saprobic species (like food species, or just fungi you like the look of).

2. Get squares of corrugated cardboard (6 inches is plenty big enough squares), soak it in water. Now, get a mushroom and pick it so the substrate comes up with it. Break off the stem butt and place this and any substrate attached inside the cardboard (peel it so it's like an envelope before adding fungi obviously).

3. Keep this moist and aired. The mycelium attaches to the cardboard using it as a new substrate. Get the cards 'growing' then move them to your plot.

4. Drop a layer of wood chip, add the cardboard cultures to this, cover in a generous amount of wood chip (4-8 inches).

5. Keep relatively moist over dry months.

6. To continue cultivating the same species in the same spot feed it more wood chips seasonally, or just let it spread out, the mycelium will grow outwards in a circle providing it has substrate to grow on and you'll have your own fairy ring.

VERY easy. And it works.

I've had dead soil. the mess the builders leave behind. Only adding biomass has helped. compost, leaving root balls in soil, worm castings, and now wood waste.

Bamboo is noted for a very healthy microbe population it it's rhizosphere. This may have helped accelerate the benefits to the farmer you mentioned.

I will try this. I ended my hydroponic experiments. Became impatient after such a long time when comparing their growth rates to those in my biochar pots. Thus, I've converted my hydroponic plants to biochar ones as well. They seem to be gradually adapting. It was also interesting to note that compost tea turned a rough potting soil into very dark, rich, and fine soil over a month or two. Microbial action can be fast and powerful. I mixed this soil in with the biochar.

I am growing more indoor veggies now. I have carrots, turnips, beets, and even daikon growing in pots.

I'll try that sometime with oyster mushrooms or shiitake. My own fairy ring would amuse me, my girl, and the neighbors.

btw, I have some of Paul Stamets's books and Elaine Ingham's books on compost tea and a few others on order from Amazon.com. My birthday present and Christmas present all in one.

[Getting A Life]

Nice - you will love 'mycelium running'. Don't expect too much scientific detail - eg: this hosts that type information. There is masses of good information to be gleaned, but a mycorrhizal database it is not. You will pick up plenty of tips, and info on mycorrhizae.

'Mycomedicinals' is outstanding, wee little book that it is. I hope to assay some of our own fungi for medicinal properties at some stage (eg after degree). Fungi with activity against viruses is exciting stuff indeed!

And if you want to go into business 'The mushroom cultivator' and 'growing gourmet and medicinal mushrooms' are invaluable guides.

Then there's 'psilocybes of the world'. I just found a new psilocybe here! Need it under the microscope of a professional (have access) first. Distinguishing features like many psilocybes with the colouring, staining etc, but margin not striated and gills are sinuate. NEW!

A nice start to a career as mycologist/microbiologist.

One thing with Dr Ingham - her teachings are outstanding, and her publications worth their weight, but her opinions sometimes border on 'prejudiced'. Meaning, she is dismissive of other technologies proven to be effective - like char, and EM. The most productive soil in the world is anthropogenic - man made. Restoring a soils biota solves many problems for gardeners but it still will not compete with a char based soil made with good source materials.

All horticultural systems are man made. It doesn't matter how organic you are, fact is you are adding inputs and taking outputs. Adding char with your compost, to me, is a no brainer.

It's just sourcing the damn stuff.

I thought of the easiest food production system lately, just the bare bones of the idea for now.

Basically, an aquaculture facility beside a market garden. The aquaculture water is pumped onto the gardens providing free nitrates and biology. The fish get to stay in clean water, but the aquaculture farm no longer has a pollution problem, instead, it has fertiliser to provide. The market garden ustilises the free nitrates, and each year trims and chars it's shelter belts. Composting facilities provide source material for compost teas and char enrichment.

With char the available P levels will rise, with the free N and N fixing bacteria also available via irrigation.

Makes for a very cheap easy farm to run, and solves aquacultures biggest problem - pollution.

Of course, the monoculture culture hates diversity, but generally, the diverse culture hates them.

Could have agriculture - wastes (effluent and whey) feed black soldier flies - larvae feed fish in aquaculture - fish wastes irrigate and grow grass for agriculture....

So freaking easy to create systems with greatly reduced inputs. Maximising protein production with 'maggots' hehe.

Sidetracked as usual... Glad your char is performing well for you.

Got a bio exam, better go, gonna ace it.

[maikeru]

Originally Posted by Getting A Life

Nice - you will love 'mycelium running'. Don't expect too much scientific detail - eg: this hosts that type information. There is masses of good information to be gleaned, but a mycorrhizal database it is not. You will pick up plenty of tips, and info on mycorrhizae.

'Mycomedicinals' is outstanding, wee little book that it is. I hope to assay some of our own fungi for medicinal properties at some stage (eg after degree). Fungi with activity against viruses is exciting stuff indeed!

I got Mycelium Running and Growing Gourmet and Medicinal Mushrooms in the mail. I can't tell you how enthusiastic I am right now. They've been my weekend reading. He has many clever but sensible uses of fungi, such as the mycelial filters.

And if you want to go into business 'The mushroom cultivator' and 'growing gourmet and medicinal mushrooms' are invaluable guides.

I'll get The Mushroom Cultivator...when I don't break the bank. May need to wait a month or two. I'm leaving in a few weeks on a quick trip to visit my sweetheart.

Then there's 'psilocybes of the world'. I just found a new psilocybe here! Need it under the microscope of a professional (have access) first. Distinguishing features like many psilocybes with the colouring, staining etc, but margin not striated and gills are sinuate. NEW!

A nice start to a career as mycologist/microbiologist.

No kidding, I would figure there must be an untold amount of richness in your country! How jealous I am! :P Ahha, sometime, I hope to get back to using lab equipment and not just tinkering at home and the garden.

One thing with Dr Ingham - her teachings are outstanding, and her publications worth their weight, but her opinions sometimes border on 'prejudiced'. Meaning, she is dismissive of other technologies proven to be effective - like char, and EM. The most productive soil in the world is anthropogenic - man made. Restoring a soils biota solves many problems for gardeners but it still will not compete with a char based soil made with good source materials.

All horticultural systems are man made. It doesn't matter how organic you are, fact is you are adding inputs and taking outputs. Adding char with your compost, to me, is a no brainer.

It seems like that to me...I've noticed very little mention from her on biochar or terra preta from what I've been digging around on the net. Yes, soil biota helps out, but char raises the productivity and quality of growth by a few magnitudes. I have some pots growing herbs with simply compost tea additions, and they do well and continue growing productively in the kitchen. But they simply cannot compete with biochar pots I've been watching in my indoor garden room and bedroom.

I've done some reading on EM and bokashi. Pickled compost. It's a simple but innovative way to sterilise trash and I think more households, gardeners, and farmers might want to consider it. Even for those trying to save a few bucks, the ingredients (yoghurt bacteria, yeast, etc. + bran) can be found at the local supermarket or kitchen cupboard and fridge. More people need to consider composting and recycling, because we simply have too much garbage.

I thought of the easiest food production system lately, just the bare bones of the idea for now.

Basically, an aquaculture facility beside a market garden. The aquaculture water is pumped onto the gardens providing free nitrates and biology. The fish get to stay in clean water, but the aquaculture farm no longer has a pollution problem, instead, it has fertiliser to provide. The market garden ustilises the free nitrates, and each year trims and chars it's shelter belts. Composting facilities provide source material for compost teas and char enrichment.

With char the available P levels will rise, with the free N and N fixing bacteria also available via irrigation.

Makes for a very cheap easy farm to run, and solves aquacultures biggest problem - pollution.

Of course, the monoculture culture hates diversity, but generally, the diverse culture hates them.

Could have agriculture - wastes (effluent and whey) feed black soldier flies - larvae feed fish in aquaculture - fish wastes irrigate and grow grass for agriculture....

So freaking easy to create systems with greatly reduced inputs. Maximising protein production with 'maggots' hehe.

Sidetracked as usual... Glad your char is performing well for you.

Got a bio exam, better go, gonna ace it.

Interesting idea! I've read about black soldier flies before and their use as fish feed. I think aquaculture needs to stop using fish pellets to feed other fish. It's counterproductive and it's depleting the world's oceans. Buyers increasingly want healthy and nutritious foods at affordable prices, not the equivalent of the "fish feedlot," IMO.

[JMJones0424]

I have been thinking about how I would respond to this thread for some time, and have been continually putting off the response because I know it will end up being long winded. So I apologize in advance.

I have been playing off and on with hydroponics for nearly a decade. At one point I came close to setting up a commercial aquaponics food and cut flower farm, but at the last minute reason set in and I discarded the idea. Everything in this post is from personal experience, and while all of the conclusions are well thought out, some of them may very well be incorrect. There is very little USEFUL information about hydroponics available to the hobby gardener, outside of one specific black market crop that is so valuable that normal economic drives no longer apply to its production. I interned (at a now defunct) aquaponics operation and learned quite a bit there, as well as years of (mostly fruitless) late night internet study sessions and personal experiments. I HIGHLY recommend that anyone seriously looking into hydroponics, and most specifically aquaponics, take the time to attend the summer aquaponics presentations at the University of the Virgin Islands.

Maikeru- as has already been pointed out, your experiment was doomed to failure, as your design is simply an effective way to maintain a state of over-watering. I hope this post serves as an explanation as to why, and serves as a guide for you and others to have more successful experiments in the future.

First of all, most people have a basic understanding of how terrestrial plants grow. It is important to remember that plants take in nutrients through their roots in the form of ions. But even more important, they also take in oxygen dissolved in an aqueous solution for respiration through their roots. A lot of aquatic plants have developed special methods for obtaining oxygen in relatively anoxic environments. Terrestrial plants do not have this feature, and therefor dissolved oxygen in the water around the roots is very important. In the normal watering cycle of most terrestrial plants, the soil is first flooded with water that has very little dissolved oxygen, which is rapidly depleted by the plant. As water either evaporates, percolates down through the soil, or is drawn up by the plant, voids develop in the soil, and if the soil is not compacted and has adequate pore structure, oxygen slowly but steadily dissolves into the water trapped around the roots. Then finally, as the soil begins to dry, water itself becomes deficient, and the roots are once again oxygen starved. Plants have evolved to put up with this periodic lack of dissolved oxygen content, some do so better than others, but all plants in soil will experience this trend from low oxygen availability to high, and then back to low again.

Hydroponics is a way of reducing, and in some instances eliminating this periodic oxygen deficiency. There are many different approaches to do this, and many different variations on each approach. I will cover just the basics in order from most effective to least effective. I will not cover (in this post at least), the difference between organic and synthetic chemical approaches to hydroponics. When the plant has been removed from soil, there is no fundamental difference between the two approaches (at least from the plant's perspective), because the plant uses nutrients as ions disolved in an aqueous solution, the source of those ions is irrelevant TO THE PLANT. (It makes a HUGE difference to the soil biota, but we have eliminated soil).

Aeroponics
This method is extremely complicated, with many points of failure, but it provides maximum dissolved oxygen content at all times, and therefor produces the greatest increase in production. Nutrient solution is sprayed in an extremely fine mist directly onto the roots of suspended plants. Problems include massive capital costs, constantly plugging sprayers, and extreme monitoring because a failure in the pumping system will result in rapid plant die-off. I can think of no economical reason to use this method for any legally grown crop, so I will not expound further.

Deep Water Culture (DWC)
In this method, the plants are suspended in a nutrient solution with little to no medium used. This method requires constant temperature monitoring, because as the nutrient solution raises in temperature, biological activity increases, but oxygen availability decreases. Ideal temperatures are between 68 and 72 degrees F. Air is injected either through massive amounts of diffusers, or by continuously recycling the water and incorporating a "waterfall" drop to aerate the nutrient solution. Many hydroponic lettuce growers use this method with great success, but growing larger plants in this method becomes difficult due to the lack of any medium anchoring the plant. There are still a relatively high number of points of failure, and failure of one part may result in plant die-off within 4 to 12 hours depending on the system.

Flood and Drain or Ebb and Flow
In this method, the plants grow in a porous and bulky medium (I use lava rocks), and the nutrient solution is periodically flooded into the container of medium, allowed to rest for a short period, and then drained. This mimics the cycle taking place in soil, but as the planting medium is far more porous than soil, there is always a huge amount of surface area for oxygen to be dissolved into the water around the roots. Typical flood times range from every 30 minutes to every four hours during the day, and is largely plant and environmentally specific. While there are still many points of failure, there is no longer as much concern about nutrient solution temperature, and there is no longer any need to aerate the nutrient solution, so I recommend this method over DWC. However, because there is still somewhat of a cycle, growth rates will not be as fast as DWC or aeroponics.

Nutrient Film Technique (NFT)
This is another method used largely by commercial growers. In this method, plants are placed in long trays and grown in a porous medium (usually rockwool). Nutrient solution is slowly trickled down the trays so that it flows over the roots. Oxygenation takes place inside the tray and by waterfall back into the reservoir. This is the simplest "true" hydroponic method, as there is only one point of failure, the water pump, and there are no timers or any other monitoring devices needed.

Continuous Drip
This method is similar to Flood and Drain, except that the medium is finer and the nutrient solution is pumped into the top of the medium and allowed to percolate through. The nutrient solution can be collected at the bottom of the system and reused, or it can be discarded.

Wicking
This is barely hydroponics, but it has the least amount of possible failure points, and is relatively easy if done correctly. The plant is placed in a finer medium than in Ebb and Flow (perlite is perfect for this, or a 50/50 mixture of perlite and coco peat), and cotton or other fibrous "wicks" are strategically placed throughout the medium to draw nutrient solution up from the reservoir located directly below the plant. Because this method relies on evaporation and transpiration to draw fresh nutrient solution into the root zone, periodic flushing of the medium will be necessary to avoid a build up of salts. This method is great for plants that are not pushed to their biological limits through excess lighting.

It should be noted that in all of the above methods, where a growing medium is used, it should be something that is porous, does not compact, and is chemically inert. Hydroponics stores sell extremely overpriced hydroton balls which are essentially fired-clay balls, but I have found either well washed red lava rock or perlite to be a far more economical substitute.

If you use commercially prepared nutrient mixes, and closely monitor electrical conductivity (also measured as Total Dissolved Solids, but this is a misnomer, as the meter measures EC, and estimates TDS from that), pH, and temperature (if necessary) hydroponic nutrient solutions can be used for months at a time as long as you "top-off" with distilled or reverse osmosis water and the correct ratio of N/P/K/Ca/Mg/S and trace elements. It is finding that correct ratio that is tricky, however most plants respond well to a moderate range and only encounter problems under artificially high lighting conditions.

There is much debate over whether or not hydroponically grown plants "taste" better than organically grown soil-bound plants. I have found that usually this impression is made because people compare the relatively bland hothouse tomatoes found in supermarkets to home-grown organic tomatoes, and the difference in taste can be more accurately ascribed to plant variety than growing method. Remember, the plant has no clue were the ions come from, it only absorbs ionic nutrients. One important consideration though, is that there have been a few studies linking health problems to excess nitrate levels in leafy greens grown hydroponically. This is a reflection of over-fertilization, and the same could be said of soil farmers who apply excess synthetic nitrogen fertilizer on their crops. However, unlike in hydroponics, when a plant is grown in soil, a vast majority of the excess fertilizer leaches away from the root zone, so excess nitrate levels are not readily observed in soil-grown plants. Most commercial food-crops can be more accurately described as "field-hydroponics" as they grow in a similar method to continuous drip, but at a slower irrigation frequency due to the longer water-holding capability of the medium used (barren dirt).


I hope this helps to spur the discussion, and I will happily answer any questions to the best of my ability.

[maikeru]

Originally Posted by JMJones0424

I hope this helps to spur the discussion, and I will happily answer any questions to the best of my ability.

Excellent guide. I did try to increase aeration around the roots by using terra cotta and charcoal + perlite. Probably not enough. I also bet that the addition of compost tea with its soil microbes would have created oxygen debt through their metabolism, thus further starving the plants. Most hydroponic systems as I understand it are usually run and meant to be sterile and periodically cleaned or flushed with bleach. Mine was the furthest thing from it. Not with little white and green biofilms all over. Microbial gunk.

[JMJones0424]

Yes, that's the fundamental problem with using organic nutrients in hydroponics. For organic nutrients to break down into a form the plants can use, you need massive surface area. Since the soil has been removed from the system, there isn't enough effective surface area to provide nesting sights for the bacteria to deal with the organic matter. As the organic matter sits in the medium, it causes small localized pockets of anaerobic zones, which encourages root rot, but more importantly, causes pH to rapidly fall. I have noticed a nutrient solution with reasonable water hardness fall from 5.6 to 3 pH overnight, which of course obliterated the plants in the system.

The aquaponics farm I worked at used flood and drain beds that the plants grew in, and used twice the volume of water in the grow out tanks of fish as a baseline for the amount of growing medium necessary (they used small gravel that had been specially sourced to ensure that there was no limestone containing elements in it, I still prefer lava rock). However, this was horribly insufficient, and the beds had to be flushed on a monthly basis to prevent organic matter buildup. UVI uses separate tanks filled with plastic netting that the effluent flows through first in order to remove the organic matter and bacterial action from the plant's roots, and this seems to be a much better approach. I have found for the hobby gardener intent on using organically derived nutrients, the most space-efficient means to accomplish this is to use a fluidized-bed filter, such as those used in saltwater aquariums.

This brings up another important point, and I have a hypothesis but have never been able to either verify it or devise a way to appropriately test it.

When plants are grown in soil, appropriate pH for most plants seems to be around 6.5 to 7. However, as one decreases the water cycling time, the necessary pH drops, all the way down to just above 5 in DWC. I believe that the plants actually take up nutrients most effectively at a pH of 5.2 or so, but in soil, in order to maintain 5.2 directly around the roots at a time when the plant is most biologically active (once half the water has been removed from the voids around the roots), you must first start with a soil/water mixture at 6.5 to 7. As the water level decreases, but the ion content remains relatively stable, the actual properties of the microscopic water droplets around the rootzone changes. TDS increases, and pH decreases.

However, the bacteria that break down organic matter seem to respond best to a neutral pH. Yet another reason why it is best to separate the breakdown of nutrients from the plants in hydroponics.

[Getting A Life]

JM Jones - outstanding! You have a fantastic grasp of how these things work I relate to so much of what you say.

I enjoy tinkering with aquaponics and would love to go to Virgin Islands but geographically and financially this is not a reality (for now). I am getting a lot of schooling in now though, as my endless nights of internet grazing were fairly fruitless as well.

I have a specific aquaponics problem, perhaps you can help.

Pond - 1200 litres.
Stocking density low - approx 1 kilo per 100 litres.
Aquaponic ebb and flow beds - 6 square metres. (1200 litres substrate)
pH - 7.2. Total Hardness 500 ppm.

The pH is the result of too much limestone as substrate, I have removed this but the pH won't budge.

So, I am considering adding CaOH.

Q 1. Is this (chemical reaction in water) harmful to fish?

Q 2. Is this the right approach?

I think the sums to calculate CaOH required would go like this. Can you correct me if I'm wrong please.

-0.4 = desired pH shift.

+0.4 = required hydroxyl ions.

Current H3O+ = 10^-pH = 10^-7.2 = 6.309573445x10^-8 mol L-1

Current H30+ mol L-1 x 1200 L = 7.571488134x10^-5 mol

Desired H30+ = 10^-6.8 x 1200 = 1.901871831x10^-4 mol

Current H30+ minus desired H30+ = -0.0001144723018 mol

To subtract the H3O+ equal portions of OH- are added.

So, 0.0001144723018 mol CaOH is required. Approx 1 litre of 0.1 mol solution.

This sounds very low, what have I done wrong?

[JMJones0424]

Ack, it's been a few years since I did stochiometric problems. However-

Everything in this post is applicable to aquaponics, not hydroponics

With a pH of 7.2 in an aquaponic system, the last thing you want to do is add calcium hydroxide. This will raise the pH. Not only will a higher pH inhibit plant uptake of nutrients, but also higher pH causes ammonium ions to convert to ammonia, which is highly toxic to fish. If you really wanted to tinker with the pH (which I don't think you need to do at 7.2 pH), you need to add either nitric or phosphoric acid, depending on the needs of the plants. If you wanted to raise pH, unless you specifically needed the calcium, I would recommend potassium hydroxide instead.

This being said, there are a lot of other factors involved that I barely understand myself. How long has your system been running? How high of a protein content fish feed are you using? Is there any place where galvanized metal or concrete is in contact with the nutrient solution/effluent? Are the fish healthy? What are the other parameters of the nutrient solution (EC, ammonia, nitrite, nitrate)? By "total hardness" are you referring to GH or "carbonate hardness" (KH)? If you mean KH, then 500ppm sounds extremely high, are you sure you don't still have a carbonate source somewhere?

For those not familiar with aquariums -
GH is a measure of Calcium and Magnesium ions, measured in German degrees. Multiply by 17.9 to convert to ppm. Test for calcium and subtract to get magnesium ion ppm. High GH is often referred to as "hard water".

KH, or carbonate hardness, is a measure of carbonate and bi-carbonate ions. These ions provide the pH buffer for the nutrient solution. Also measured in German degrees, convert to ppm the same way. If you have a high KH, the addition of acids will cause insoluble compounds to form and drop out of solution, lowering the pH.

Generally speaking, you want about equal amounts of nitrogen and calcium ions, with significantly less magnesium (perhaps 1/3 of calcium levels, but dependent on plant needs). However, calcium especially, and to some degree magnesium, is not readily used up by the plants, and I rarely ever needed to add more than the fish food itself introduced into the system after the system stabilized.

I think the most important thing is to first determine whether it is KH or GH that you are measuring, and then if it is KH, try to determine where you are getting carbonate ions from. Did you fully replace all the nutrient solution after removing the limestone substrate? Are you using tap or well water that has a high KH?

Chasing pH is generally counter-productive. If you have no problems with the components of the system, and you are only adding distilled or RO water and quality fish feed to the system, then it should eventually stabilize just under 7 pH. What ever you do- don't add CaOH to 7.2 pH nutrient solution!