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Documents: Supplied By GrowthTech: PH and Conductivity

pH and Conductivity are the two most important variables in nutrient Solution management. A basic understanding of them is essential for successful hydroponic gardening.


pH is a measurement of the acidity or alkalinity of a solution. On a scale of 1-14 neutrality is represented as 7. The ideal pH for almost all hydroponic applications is 6. pH for RockWool cultivation should be slightly lower, ideally about 5.8.

Different Plant Species

Plants growing in soil have widely differing preferences for pH. This is NOT the case in hydroponics and all plants in hydroponic systems will function best at a nutrient pH of 6.

Measuring PH

This is best done with a meter but there are also various kits using liquids and papers available. Kits are adequate for applications such as pot culture where the nutrient is supplied to the plant once and is used up before being replenished. Growers using re-circulating systems such as NFT or Ebb and Flow will certainly need a meter to monitor their solution accurately. Meters will require regular calibration and instructions for this are included below.

PH In Passive Hydroponics

Passive hydroponics refers to growing in pots full of a medium such as Perlite or GreenMix or to hand-watering RockWool slabs. Normally for this type of cultivation we would recommend the grower to make up a large container of nutrient at a time. A 200 Litre plastic drum is an ideal container. Once this is made up to the correct “strength” (Conductivity), the pH can be checked and adjusted to the ideal level of 6. A careful note should be made of the exact amount of pH UP or DOWN that is added at this time. In future this amount can be added as routine when making up solutions and pH should remain pretty constant from batch to batch although spot checks are recommended from time to time. A good pH Kit, such as the Future Garden pH Kit can be used for this because although the pH measurement takes longer than with a meter, it only needs doing occasionally. The nutrient solution in the drum should remain quite pH stable and can be applied to the plants as needed.

PH In Active Systems

Active systems are ones in which the nutrient solution is supplied to the plants by pumping, such as Ebb and Flow tables or NFT systems. In most domestic systems the solution is re-circulated to the roots continuously over a period of time. In an active system the pH will need checking and adjusting in the main tank on a regular basis. In most systems fresh water is added to the tank to replace that used by the plants. The incoming water is usually of a higher pH than the nutrient solution so there tends to be an upward drift in pH. This can be corrected by the regular addition of small amounts of pH DOWN. This process of pH control can be carried out with a pH Kit but as it needs doing often the busy grower will be better off using a meter. If the water supply is classified as “Hard” the grower must be very careful with re-circulating systems. (See note on Hard water.)

Hard Water

Hard water is characterised by high levels of Bicarbonates and it makes itself known by depositing lime scale in kettles and by reducing the lathering ability of soap. Hard water will usually have a high pH but not necessarily. What will distinguish hard water is that it will take much more acid to reduce the pH than with an equivalent sample of softer water. This is because the Bicarbonates have to be neutralised and this uses up quite a lot of acid. The obvious problem for the grower is that he will be adding quite large amounts of acid on a regular basis. If he is using Phosphoric acid this may lead to a build up of Phosphate in the nutrient tank over time. High levels of P in the solution can inhibit the uptake of other salts, Zinc for instance, and cause general nutrient imbalance.

Solutions to Hard Water Problems
  1. The first and most obvious solution is to change the nutrient tank regularly. This will reduce the chances of Phosphate accumulation and ensure maintenance of a good nutrient profile. Frequency of changes will depend on tank size and number of plants. In very Hard water areas however a large amount of Phosphoric acid will be needed to correct pH when nutrient is first made up.
  2. It is possible to use Nitric Acid to control pH. This will overcome the problem of phosphate accumulation but Nitric Acid is a VERY unpleasant and dangerous liquid. It should NEVER be handled by anyone who is not experienced with aggressive liquids and all good laboratory procedures should be observed.
  3. If water is very Hard the grower may need to use a specific formulation which is usually based on more acidic components. Hard Water OPTIMUM was formulated in response to demand from growers in various areas of the United Kingdom such as London, Thames Valley and other areas with very hard water. It was specifically formulated to correct the pH of alkaline water and minimise the amounts of Phosphoric Acid that are required to maintain it at correct levels.


The conductivity of a solution is an expression of the capacity of that solution to conduct an electric current. Distilled or de-ionised water will conduct virtually no electricity at all and will therefore have a conductivity reading of zero. As salts are dissolved in the water so the conductivity of the solution increases. The conductivity of a nutrient solution is therefore a measurement of its “strength” as indicated by the actual amount of salts dissolved. Conductivity is usually expressed in terms of CF. CF stands for Conductivity Factor and the units of measurement are usually milliSiemens, microSiemens or Siemens. The difference between these is purely in the placing of the decimal point. There are a thousand microSiemens in a milliSiemen A CF unit is equal to a hundred microSiemens so there are 10 CF units to a milliSiemen.

CF Meters

CF meters usually read in milliSiemens or CF units but for the grower it is simply a matter of getting the decimal point in the right place. As a rule of thumb it can be said that a good conductivity level for most purposes would be 2.0 milliSiemens ( expressed as 2.0 mS/cm2 ). We can call this a standard working solution.

Therefore Standard working solution = 2.0mS = 2000uS = 20 CF units.

Parts per Million

Conductivity is sometimes expressed in terms of parts per million (PPM). This is somewhat misleading and the grower will find it better to think and work in milliSiemens. However if you already have a meter that reads in PPM you will wish to use it. One gram of any salt, dissolved in a Litre of water will make a solution containing 1,000 PPM of that salt. However different salts have a different capacity to conduct electricity so two solutions made with different salts could both be 1,000 PPM. but have quite different readings on a CF meter. It is therefore not possible to make a exact conversion from parts per million to CF units, however as most nutrient solutions have approximately the same ratio of constituent salts we can make an approximate conversion;

1 mS = 10 CF units @ 850 PPM.

Please be aware that this figure is not universally accepted and you will hear different, much lower conversion values being suggested. We believe this figure to be valid and we stand by it.

Therefore Standard Working Solution = 2.0 mS = 20 CF @ 1700 PPM.

RockWool Systems

Plants grown in RockWool slabs need slightly different nutrient regimes than in other systems. pH should be somewhat lower, at 5.8 rather than 6. Conductivity should also be slightly lower than in other applications. It is very important to remember that the readings in the tank may not reflect the reality within the slab. Nutrient should be extracted from the slab regularly for measurement. In a well-maintained system there should not be too much variation between tank readings and those obtained from within slabs. If Conductivity begins to rise in the slab it will be an indication that irrigation is either too infrequent or too short. It is necessary to trickle nutrient into slabs for just long enough to get a 10-15% run off. The timer should be adjusted to provide this amount of nutrient at least four times during the daylight period. More frequent irrigation may be beneficial. Remember that as plants grow they will take up much more nutrient and watering cycle will need to be extended regularly throughout life of the crop. The regular check of nutrient from within the slab itself is the most important routine for the RockWool grower. If the Conductivity begins to rise then the grower will increase irrigation to ensure run off at each watering cycle. If Conductivity remains high then it is a simple matter to flush the slabs.

Growth Technology Products For The Control Of PH

PH Down

GT pH Down contains Orthophoshoric acid (H3PO4). The concentration is 81%, which is very strong so handle with care. Some other products on the market are considerably weaker so check the concentration before you buy. This is a very dense liquid and should feel much heavier than water.


GT pH Up contains Potassium Hydroxide (KOH). This is a very caustic liquid and should be handled with extreme care. Once again our product, at 50%, is stronger than most competing products. Always check the label before you buy for best value.

Health & Safety

PH control chemicals are highly toxic and aggressive liquids. Please follow these simple guidelines for safety and peace of mind.

  1. ALWAYS store these materials in a safe place, preferably in a locked cupboard. Always keep them out of the reach of children.
  2. ALWAYS wear rubber gloves and protective goggles when handling any aggressive chemicals. This may seem extreme but it will all be worth it the first time that you make a mistake.

Nitric Acid

Many commercial growers use Nitric Acid (HNO3) for pH control. This would be a good option if you have Hard water but it is not recommended for small-scale growers. Nitric Acid is a very unpleasant and dangerous liquid; it is truly aggressive and should only be handled by experienced operators.


Never add water to nitric acid. All Growth Technology nutrients are supplied with comprehensive instructions. Please follow them carefully until you are familiar with the process.

Growth Technology products are designed by professionals and fine-tuned by experience. Available at a hydroponic store near you.

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