CF and EC PPM
‘CF’ or ‘EC’ is a commonly used measure to determine the strength of a hydroponic nutrient solution. As salts dissociate into ions in solution, they carry a positive or negative charge (eg KNO3 --> K+ + NO3-,) which can transmit electricity. Pure water will not transmit electricity, but as soon as salts are added, the ability of the solution to conduct electricity increases. This conductance increases with increasing solution strength. CF (Conductivity Factor) and EC (Electrical Conductivity) are a measure of this characteristic of nutrient salt solutions. While CF seems to be a very convenient measure, there are problems associated with relying only on CF to control hydroponic nutrient formula.
1) The CF will be roughly the same regardless of the element content of the solution. A nutrient solution with CF 20 can not be distinguished from a sodium chloride solution with CF 20.
2) Different nutrient salts show different capacities to conduct electricity when in solution, so that depending on the nutrient ratios and the individual salts used, the CF may give a very different indication of the true ionic strength of the solution. A solution of potassium nitrate at CF 20 will be approximately half the strength (in ppm) of a solution of magnesium sulfate at CF 20. This is because potassium nitrate conducts nearly twice as much electricity at the same ionic strength.
3) Even if the nutrient element content of the formula was known accurately at the start, once the solution has been recirculating through a growing crop for a few weeks, the element content changes - the CF may well stay the same.
CONDUCTIVITY OF SOME COMMON HYDROPONIC NUTRIENTS AT 2000 PPM
SALT mg/l, CF, EC,
Calcium Nitrate 2000, 20, 2,
Potassium Nitrate 2000, 25, 2.5
Magnesium Sulfate 2000, 12, 1.2
The CF of a nutrient formulation is a combination of the CF contributed by all the dissociated nutrient salts from the A
and B stock solutions as well as impurities from the water supply, and is not really any indication of the quality of the formula, just an estimate of its strength. In hydroponics the only way to determine the nutrient makeup of a formula is either to have a complete mineral analysis done, use a range of specific ion meters or to calculate the nutrients in advance and use these in drain to waste systems. Any solution in recirculating hydroponics will change over time and can often develop toxic levels of some ions in fairly short periods of times (a few weeks if your reservoir is small).
PPM
The other common indicator for hydroponic nutrient strength is PPM, or parts per million. 1 part per million is
1 equivalent to 1 mg per litre, or 1 g per m3. In theory, this is a measure of the actual strength of the nutrient elements in solution, and would seem to be an ideal measurement for hydroponics. However, measuring this in practice can be very difficult for a grower in hydroponics.
TDS METERS
An alternative to solve the problems with CF as a measurement may seem to be to use ‘TDS’ or total dissolved solids as a measure of nutrient solution strength, and if ‘TDS Meters’ in fact did this, it would solve the problems. However a ‘TDS’ meter is simply a ‘CF’ meter with different calibration and display — it still only measures electrical conductivity, and in fact is less accurate because of the assumptions made regarding the salt makeup of the solution - many assume potassium chloride and have a fixed conversion factor (eg 70ppm per CF unit) which can not be adjusted for different solution formulations. TDS meters which can be calibrated for different formulations are a better alternative, but still are only measuring CF in reality.
CF EFFECTS ON PLANT GROWTH
If we assume that in hydroponics, the CF is a measure of the strength of a nutrient solution, this has a significant affect on the growth of plants, regardless of the mineral content of the solution. Ions can enter root systems by various means. Simplest is the form of `passive’ transport using the symplastic pathway, where ions simply diffuse through spaces between root cells, into the xylem in response to a concentration gradient from the solution to the plant. The other two mechanisms involve transport across the semi-permeable membranes inside root cells, and occur either by osmosis or active transport using special protein ‘carriers’.
Osmosis describes the behaviour of ions in solution when separated by a semi-permeable membrane, as for example at the interface of root cells and nutrient solution. The concentration of ions on either sideof the membrane determines the net flow of ions through the membrane, as if ions are more concentrated in solution than in root cells and the membrane permits the transmission of ions, then ions will tend to flow into the roots. This process is known as `passive’ transport or diffusion, and is assisted by the flow of water in the transpiration stream of the plant. In fact, root cells tend to maintain quite high ‘osmotic potentiaIs’ but into the roots. Some ions, Ca2+ and K+, NO3-, for example, are able to be transported into root cells, even against a concentration gradient by the energy requiring process of active transport. Once water and ions are inside the roots they diffuse through into the xylem vessels and flow with the transpiration stream up into the stem. A natural reaction of some plants to increasing solution strength, is to accumulate assimilates in the leaves and fruit to equalise the osmotic potential with the root zone.
This explanation is the basis for the effects noticed by increasing or decreasing CF in hydroponics. CF influences the ‘osmotic potential' of the solution in the root zone, which influences the p|ant’s rate of water and nutrient uptake, and the adjustments made to osmotic potential inside the plant. Increasing CF will reduce water uptake by the crop, and it cause plants to concentrate organic compounds in foliage.
Increasing CF tends to slow vegetative growth, and ‘harden’ plants. Conversely, lowering CF will increase water uptake, and produce lush soft growth. Consequently, the CF of solutions is normally increased during winter and for budding, while vegeative growing is often run at a lower CF to maintain optimum plant vitality. This means in general it is better to run lower strength solutions (lower EC) and add additional small amounts of nutrients more often than to run high EC and add larger amounts of nutrients less often. Plants never need high EC nutrient solutions. They grow much betterwith medium range EC that is adjusted back to those levels constantly or often. It is often noted by experienced growers that peoples spalnts in sytems where they run high EC are putting on their best growth just before the growers adds more nutrients. when they add nutrient and raise the EC level back up the rateof plant growth slows until the EC lowers and then the rate starts incraesing, but then the grower raises it again. Go figure.
CF can be maintained at higher levels in solution culture than in media or drain to waste systems. In solution culture there is a constant supply of water and the CF does not fluctuate in the root zone, whereas in media systems evaporation from the surface of the media and plant water uptake can result in the CF becoming much higher in the root zone than in the ‘feed’ solution. The ratio of CF in the feed to rootzone and leachate solutions needs to be well regulated in drain-to waste systems, and CF ‘in’ (feed) and CF ‘out’ (drainage) should be standard daily measurements.
In short. IMHO Change the reservoir solutions as often as you can afford and clean the reservoir with chlorine or hydrogen peroxide (rinse well) before each refill. Personally I change reservoir solutions once per week on my aeroponic system grow rooms (every 2 weeks for remotelly located systems). I use auto top off for RO water that is always powered. I use both pH and conductivity analyzer/controllers with peristaltic pumps to maintain EC. The newer room are all going with air atomizers so they will be all drain to waste. That means within a few months I will have a lot of extra pH analyzer/controllers and conductivity analyzer/controllers and pumps, plus less electrical consumption.
This forum needs a thread for selling equipment.
