(Excerpt from Integral Hydroponics Evolution by G.Low. Coming soon!)
EC – Electrical Conductivity and Ions in Solution
Maximum crop production is primarily a function of environmental conditions and genetic potential. The extent to which this limit can be reached relies directly on the degree and effectiveness of practices which serve to optimise the plant’s environment. Fulfilling the crop’s water and nutrient requirements by providing optimal levels of plant nutrients through, among other things, closely monitoring EC and by ensuring high bioavailabity of nutrients through maintaining optimal pH levels are among the most important factors to consider when striving for maximum yields.
About EC, ppm and TDS
EC stands for electrical conductivity, which is the potential of any material to conduct electricity. Although most growers are used to measuring the amount of feed they give to their plants in ounces per gallon, millilitres per litre (ml/L), millilitres per gallon, or some other unit of measurement, EC goes a little further and gives us a reasonably precise measurement of how much plant nutrient we have in the hydroponic ‘working solution’ (i.e. the solution that is being fed to the plant).
When I say “reasonably precise measurement”, hydroponic nutrients consist of nutrient salts that have positive electrical charges (cations) or negative electrical charges (anions). For example, some important elements with a positive electrical charge (cations) in their plant-available form include potassium (K+), ammonium (NH4+), magnesium ( Mg++), calcium (Ca++), zinc (Zn+), manganese (Mn++), iron (Fe++), and copper (Cu+). Some of the nutrients that have a negative electrical charge in their plant-available form include nitrate (NO3–), phosphate (H2PO4– and HPO4—), sulfate (SO4–), borate (BO3–), and molybdate (MoO4—). When these elements (ions) are in solution they produce an electrical charge and conduct electricity from one to the other. Plants are able to acquire the essential mineral elements via the root system utilising the chemical properties of the ions, particularly the negatively charged anions because the plants roots have sites that are positively charged (opposites attract). The plant is also able to attract positively charged cations to negatively charged sites on the roots. The more nutritional salts (ions) added to the nutrient tank/reservoir, the more the electric conductivity (EC). EC, therefore, tells us how many nutrient salts we have in solution and, in turn, this tells us how much nutrition we are providing to the plant.
However, this said, no EC meter has the ability to distinguish between different types of ions. This means the use of EC measurement is only helpful in checking total salt concentrations in the solution, but the concentrations of individual nutrients may vary considerably from the desired concentration. This is because EC only tells us the relative amount of total salts (ions) and nothing about each specific nutrient concentration in the solution. So, for instance, the true concentration of N, P and K may be lacking even though the EC is ideal.
This is something that you need to be aware of. As we progress through this chapter you’ll come to see that some nutrients are removed from the nutrient solution very quickly by plants while other nutrients are used at far lower levels and can accumulate in the solution and substrate. Given this, incorrect maintenance practices of your nutrient tank/reservoir can lead to deficiencies of key elements even when your EC is running at optimum.
Plants require a wide range of nutrient ions to support growth. Each of these specific nutrient ions has an ionic sufficiency range in which growth is optimized. The uptake and utilization of nutrients depends not only on the quantities, but also on the ratios among nutrient types. If a particular nutrient is deficient, yields can be negatively affected. A similar reduction in plant growth can arise when a particular nutrient is present at a concentration that is too high. All the nutrient ion types need to be within their respective ranges if plant productivity is to be optimized. Departure from these optimal levels in any of the nutrient ion types will have an influence on all the others as well.
Given that EC only measures total salts (ions) and not specific ion types this measurement alone does not provide sufficient information to allow growers to realize optimum yields from a hydroponic solution fertility perspective. This is extremely important information to understand, and certainly it has implications for recycling system growers where the preferential uptake of certain nutrients by plants can quickly deplete nutrients that are in high demand (i.e. N, P, K and Mn) and leave high degrees of other ions which are in lesser demand such as Ca, Mg and S in solution. The outcome, when nutrient maintenance best practice is not implemented, is an imbalance of the nutrient ions in solution, even though the EC might be ideal.
Actually, this one warrants more attention….
Preferential Nutrient Uptake by Plants – Its Influence on Nutrient Ions in Solution and EC
It is important to note that plants can quickly remove their daily ration of some nutrients while other nutrients accumulate in solution. For example, high levels of nitrogen, potassium, phosphorous and manganese are taken from solution by plants while lower levels of calcium, sulfur and magnesium are taken. This means that the concentrations of nitrogen, phosphorous, potassium and manganese can be quickly depleted from the solution (to a few ppm) because these nutrients are in the plant where we want them. On the other hand, maintaining high concentrations of nutrients in the solution to compensate for high uptake needs can result in excessive uptake (overfeeding) that can lead to nutrient imbalances in the plant.
For example, according to Bugbee (1996), the water removed from a recycling hydroponic system through plant uptake and transpiration must be replaced and it is necessary to have about 15.4ppm of phosphorous in the refill solution. If the refill solution was added once each day, the phosphorous would be absorbed by the plant in a few hours and the solution phosphorous concentration would be close to zero. This does not indicate a deficiency; rather it indicates a healthy plant with high nutrient uptake. However, if phosphorous was maintained at 15.4ppm in the solution, the phosphorous concentration in the plant could increase to 1% of the dry mass, which is 3 times higher than the optimum in most plants. This high phosphorous level can induce microelement (e.g. Fe, Cu and Zn) deficiencies.
See following table that demonstrates the uptake elevation of essential plant nutrients (Bugbee 1996)
Looking at our ‘uptake elevation of essential plant nutrients’ table, the essential nutrients can be placed into three distinct groups based on how quickly they are removed from solution.
Group one elements (N, P, K, Mn) are actively absorbed by roots and can be removed from solution in a few hours.
Group two elements (Mg, S, Fe, Zn, Cu, Mo, and Cl) have intermediate uptake rates and are removed from solution at lower rates than the group one elements, which means that the ratios/balance between the group one and two elements is altered by the preferential uptake by plants of group one (higher uptake rates) over group two elements (lower uptake rates).
Group three elements (Ca and B) are passively absorbed from solution and often accumulate in solution. This information brings us back to my earlier point about EC not distinguishing between different types of ions and that given N, P, K and Mn are taken up at higher rates than other elements, N, P, K and Mn may be deficient even though the EC is ideal. It’s something that you need to be very aware of, particularly in recycling hydroponic systems where the various nutrients are taken at different volumes and as a result nutrient imbalances and deficiencies can occur.
This is why, where growers are using mains/tap water, I recommend to dump the nutrient in recycling systems regularly (dependent on nutrient tank/reservoir volume, plant numbers and size) to ensure a well-balanced nutrient is supplied to the plants at all times. I have heard some say that dumping is unnecessary. In my mind, given the science and the demographic in the retail hydroponics sector, this is flat out bad advice that will lead to nutrient imbalances and yield losses.
Later in this chapter I go into detail on adding Cal Mag to solution through the use of agricultural base fertilizers. In this material I show the theoretical ppm of nutrient salts after fertilizers have been added to solution versus a measured EC of the solution. These values being:
Nutrient ppm in solution
Cl = 60ppm
Ca = 53.36ppm
NO3 N = 43.74ppm
Total ppm = 183.41
EC Measurement of Solution
EC = 570 micro Siemens (microS/cm)
The EC measurement is taken from a solution that consists of only RO water and fertilizers that add these elements to solution. No other nutrients have been added to solution.
Therefore, in this example there is a total of 183.41ppm of plant nutrients in solution (no other nutrients have been added) but when measured with scientific testing equipment the EC of the solution is 570 micro Siemens (microS/cm) or EC 0.57 (mS/cm1).
However, based on the EC testing standard mS/cm1 which is commonly referred to by hydroponic growers as EC, 183.41 ppm should be, at least in theory, approximately 0.287 (mS/cm1) EC – or about half of that which an EC meter is showing.
Other than this, 570 micro Siemens theoretically gives us a TDS (total dissolved solids) ppm of 365. So the measurement a scientific EC meter has given us is showing higher ppm of elements/ions/salts than the nutrients that are actually in solution. Quite simply, this is because EC meters not only measure electrically charged nutrient elements/ions but they also measure electrically charged non-nutrient elements/ions in solution. Basically, an EC meter measures the total electric conductivity of the electrically charged ions in solution and does not distinguish between ions.
Think about things this way. Hydroponic liquid nutrients and additives are produced using dry agricultural base fertilizers. In production these agricultural dry base fertilizers are added to water and this (water + fertilizers) becomes a hydroponic nutrient or additive product. However, the agricultural base fertilizers used in producing the nutrient or additive only contain a percentage of nutrient elements with the bulk of the base fertilizer often consisting of non-nutrient elements. For example, when looking at just one calcium nitrate fertilizer that might be used in producing a hydroponic nutrient or additive, the fertilizer contains16.7% Ca and 11.6% NO3 N. This means that only 28.3% of the fertilizer constituents are plant nutrients, leaving us with over 70% of other non-nutrient elements that are added to solution in the production of a hydroponic nutrient or additive. The next thing to understand is that at least some of these non-nutrient elements have electrical charges which is what an EC (Electric Conductivity) meter measures for. Therefore, not only is our EC meter telling us how many ppm/EC/TDS of calcium and nitrate is in solution but it is also reading the ppm/EC/TDS of non-plant nutrients in solution. The end result is that EC measurements are inaccurate for precisely measuring the amounts of nutrient elements/salts/ions in solution because they simply give us an overall reading of the electrically charged elements/salts/ions (nutrient salts + other salts) that are in solution.
EC measurements are also complicated by the fact that not all salts conduct an electric current equally. For instance, ammonium sulfate conducts about twice as much electricity as calcium nitrate and more than three times that of magnesium sulfate. Also, nitrate ions do not produce as close a relationship with conductivity as do potassium ions. Consequently, the higher the nitrogen to potassium ratio in a nutrient solution, the lower the conductivity value will be. Since every element in a multi-element solution has a different conductivity factor, EC measurements, albeit reasonably accurate, are only approximate. Following is a table that demonstrates the various ECs of several different chemicals when added to water at varying percentages.
Electrical Conductivity (mS/cm) of Aqueous Solutions Indicated Concentration in Mass Percent
All values refer to 200C (680F)
References. CRC Handbook of Chemistry and Physics 70th Edition (1989): Wolf, A. V, Aqueous Solutions and Body Fluids (1966)
Electric conductivity (EC) requires mobile ions in solution; when the mobility rises because of increases in temperature the conductivity measured also rises. For every 1OC temperature change, the conductivity of a nutrient solution will increase by approximately 2%. This temperature coefficient varies with the type of salts in the nutrient solution, the concentration of those salts and the temperature itself. When calibrating EC meters the calibration solution temperature should be as close as possible to the nutrient solution to be tested to minimise temperature induced errors.
Electrical conductivity can be expressed using a number of different units, but the typical unit is siemens per meter2 per mole (S/m2/mole) or millisiemens per centimetre (mS/cm). The mS/cm unit is generally used in Europe and elsewhere as a guide to the concentration of nutrients in water. In North America, electrical conductivity is typically converted into a count of ions in the water using parts per million (ppm). Parts per million represent literally how many salts/ions you have to 1 million parts of distilled water. Parts per million can also be converted directly into milligrams per litre (mg/L). That is, 1 ppm is 1mg/l, 100ppm is 100 mg/l and so on. Basically, to dumb things down somewhat, ppm and mg/l are exactly the same thing with different units of expression.
The ideal EC is specific for each crop and somewhat dependent on environmental conditions; however, the EC values for hydroponic systems are typically stated to range from 1.5 to 3.0 mS/cm. Higher EC may hinder nutrient uptake by increasing osmotic pressure and reducing water uptake, whereas lower EC may severely affect plant health and yield due to there being too low nutrient levels in solution. In simple terms, if the EC is too high, specific nutrient absorption will cease/slow and the plant will be subjected to osmotic stress. If the EC is too low not enough nutrients will be available to the plant and yields will suffer as a result. Either way, where EC is too high or too low yields will be decreased, so striving for and maintaining optimum EC is a critical factor in achieving optimum yields.
Following is a table that outlines the recommended ECs of several specific crops.
|Salinity Group||Threshold EC, mS/cm||Example of crops|
|Sensitive||1.4||lettuce, carrot, strawberry|
|Moderately sensitive||3.0||Tomato, cucumber, pepper, chili|
|Moderately Tolerant||6.0||Soybean, ryegrass|
Author’s note #1: You can perhaps see that EC is a somewhat flawed way of measuring what actual nutrients there are in solution. EC measurements act only as a guide to tell us our nutrient strength is within an ideal/acceptable range for a given crop. However, a far more accurate way of understanding things is through analysing the ppm of individual nutrients we have in solution through dissecting a nutrient labels guaranteed analysis.
Optimized nutrient management programs should begin with an understanding of the nutrient solution concentrations in parts per million (ppm) for the various nutrients required by the crop of choice. By managing the concentrations of individual nutrients, growers can maintain optimal nutrition in solution. We’ll be going into detail on this later in this chapter.
Author’s note #2: There is scientific testing equipment that can measure specific nutrient ions in solution. However, while there are ion specific meters available that are capable of measuring the ppm (mg/L) of elements such as Nitrate N and P in solution, these meters tend to be costly (cost prohibitive), often require a high degree of technical expertise to operate correctly and to date meters don’t exist for measuring some of the nutrient ions found in hydroponic solutions.
Another important factor to consider where nutrient strength (EC) is concerned is ‘osmotic pressure.’ Put simply, osmotic pressure reduces water potential, which is the tendency of water to move across a semi-permeable membrane from one area to another. Where nutrient levels in the solution/substrate/soil are too high osmotic pressure becomes highly negative and reduces the potential for the plant to uptake water. Where osmotic pressure becomes highly negative and the plants can’t uptake adequate amounts of water, they are subjected to ‘osmotic stress’. The end result is a plant that can’t uptake adequate water and nutrients which results in a reduction in growth.
Osmosis, Osmotic Pressure and Osmotic Potential
In order to understand osmotic pressure and osmotic potential it is necessary to understand the principle of osmosis in plants.
Osmosis is a vital function to the growth and stability of plant life. Without osmosis, photosynthesis couldn’t occur and plants would wilt and die.
Osmosis is the transfer of water through a semi-permeable membrane driven by a difference in concentration of the solutions on either side of the membrane in the direction that tends to equalize the solute concentrations on the two sides.
Where plants are concerned, the cell walls of the plant roots act as the semi-permeable membrane in osmosis.
Therefore, osmosis uses the difference in concentrations of nutrients between the nutrient/ substrate/soil and the root to move water into the plant. More nutrient ions are in the center of the root, which is an area called the stele or vascular cylinder (higher concentration of nutrients), than are in the outside of the root (lower concentration of nutrients).
With normal hydroponic root zone solutions, their strength will typically always be lower than the solution within the roots. As a result, the water flow will be from the root zone solution into the plant roots.
With hydroponic solutions, the higher the strength (EC) of the root zone solution the smaller the difference to the internal root solution and, hence, the slower the rate of water uptake. Conversely, the weaker the root zone solution (the lower the EC) the greater the concentration difference across the root cell membrane and the faster the rate of water uptake. See following illustration which demonstrates how solution strength alters osmotic potential.
Based on the principles of osmosis, when a solute (e.g. nutrient salt) is dissolved in water, water molecules are less likely to diffuse away into the plant roots via osmosis than where there is no solute. The more solute that is dissolved in the water the more pronounced this situation becomes. A solution (salts + water) will have a lower and therefore more negative water (osmotic) potential than that of pure water. Additionally, the more solute molecules added to a solution the more negative the osmotic potential becomes. Therefore, the higher the EC of a nutrient solution, the more solutes in solution and the more negative the osmotic potential.
Osmotic potential has important implications for many living organisms including plants. If a living cell is surrounded by a more concentrated solution, the cell will typically lose water to the more negative water potential of the surrounding environment.
As an example of highly negative osmotic pressure, if you were to put a carrot in salty water, the salt water would draw the pure water from inside the carrot and within a few hours the carrot would be limp, its cells shriveled.
According to Guzman and Olave (2006) optimal yields are achieved up to a given threshold of nutrient salt concentration specific to each crop, determined by EC. Beyond this threshold there is a percentage of reduction in yield for each unit increase in EC. It is well known that high EC reduces yield. This is a result of reduced uptake of water into fruits/flowers caused by high osmotic pressure and as a result the fruit/flower size is smaller.
Additionally, osmosis has extremely important implications for nutrient uptake and translocation because water movement into and throughout the plant is interrelated with the movement of nutrients into and throughout the plant. Therefore, a reduction in osmotic pressure (negative osmotic potential) will not only impair water uptake but also reduce nutrient uptake and translocation.
High EC and Salt Buildup in Substrates
A very important issue to be aware of when discussing how much nutrient is provided to the plant (measured through EC/CF/ppm) is that excess nutrient salts can buildup in hydroponic substrates.
A salt is simply an inorganic mineral that can be dissolved in water. When raw ingredients used to make inorganic and synthetic fertilizers are added to water they become soluble salt, often termed a fertilizer salt. Plants can readily use mineral nutrients that are in the form of soluble mineral salt ions. The roots of a plant naturally contain different levels of mineral ions called root salt that help create a stable, natural flow of water and nutrients into the plant’s vascular system. However, where nutrient concentration is higher than that required by the crop, the plants will not absorb all the nutrients, resulting in unused salts building up in the substrate. As the salt accumulates the EC in the substrate begins to rise and the salt buildup can start to disrupt the flow of water and elemental nutrients into the root. If salt levels reach a point of extreme excess they can actually begin to draw water out of the plant and back into the substrate.
Following is a table that demonstrates yield losses as a result of excessive salt levels in the ‘rhizosphere’ (the region of the media in contact with the roots).
Crop salinity sensitivity, threshold and yield decrease (Maas and Hoffman, 1993).
|Crop||Salinity threshold expressed in ds/m||Percentage of yield decrease above the salinity threshold
% per every ds/m
Although some salts are absorbed by the plant, there can be a sharp increase in the concentration and a build-up of some undesirable salts. When growing plants in soil outdoors, root volume and soil space are large enough that salt accumulation does not interfere with plant growth as quickly as in hydroponics where pot size limits substrate volume, leaving little space to buffer this salt build-up.
When plants are supplied with mineral fertilisers, although some are consumed and some are lost by leaching, the medium solution electrical conductivity is increasing compared to the nutrient that is being fed to the plants. The accumulation is mainly of nitrate and chloride; however other salts (e.g. calcium, magnesium and sulphur) can also accumulate in the substrate.
Other than this, salt buildup and pH are interrelated. Substrates that have high soluble salt content will also have a high pH. As the pH of a substrate rises, the result will be a change in the overall availability of certain nutrients, and sometimes it can even cause an alteration in the form of some nutrients, changing them into plant unavailable forms. In these cases, the plant might show visual signs of a nutrient deficiency but this can be misleading. Although the apparent deficiency might be real (salt buildup is causing lockout and thus a deficiency), adding more fertilizer would create more of a problem, leading to further plant injury.
Therefore, providing plants excessive nutrients – a practice that is common amongst many indoor hydroponic growers – presents with some serious issues.
I cover monitoring and dealing with salt buildup on pages…
EC and PPM Standards
You’ll note that in Integral Hydroponics I discuss nutrient and additive strengths in terms of EC (mS/cm). This is because EC is a universal measurement/standard while ppm meters do not apply a universal standard and therefore ppm varies on a country-to-country basis.
As such, EC is a universal standard that can then be converted into ppm based on several standards that are used worldwide. Unfortunately, the same cannot be said for ppm where different countries apply different EC to ppm conversion rates. This means that there is no single universal ppm standard, so when talking in terms of ppm things can become somewhat confusing (i.e. which standard are we talking about and how do we convert this to EC?)
It is important to note that all ppm meters first measure in EC (electric conductivity) and then run a conversion program to display the reading in ppm. There are three different conversion factors (standards) that various manufacturers use for converting from EC to ppm. These can be simply stated as:
USA 1 mS/cm (EC 1.0 or CF 10) = 500 ppm
European 1 mS/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 mS/cm (EC 1.0 or CF 10) = 700 ppm
Hanna, Milwaukee 1 mS/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 mS/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 mS/cm (EC 1.0 or CF 10) = 700 ppm
Understanding this becomes important when interpreting data and advice given through books and other media such as magazines, blogs and forums. For instance, a grower in the U.S. may say that he/she is running their nutrient at 1000ppm (2 EC mS/cm) to get optimal results. However, if a grower in Australia uses their ppm meter to achieve the same nutrient levels in solution that the U.S. grower has recommended (1000ppm) they would need 1400ppm. Other than this, understanding how to convert ppm to EC becomes important when talking to other growers who are using different units of measurement than your own (i.e. ppm versus EC and vice versa).
Always read the manufacturer literature supplied with your ppm meter to establish what EC to ppm conversion factor is being used.
Fortunately, there is a fixed calculation for the relationship between all these units, which is given in the following table. Use this table to convert between the recommended ECs found in Integral Hydroponics and ppm.
Converting EC to ppm based on the different standards
Symptoms of Overfeeding
An early symptom of overfeeding is leaf and stem wilting and ‘tip burn’, where the ends of the leaves begin to look burnt and go brittle and yellow to brown. If your plants show symptoms of this you’re possibly overfeeding somewhat; although, as a word of caution, tip burn and wilting can also be caused by other factors such as overly warm ambient air temperatures or a shortage of calcium. i.e. calcium strengthens plant cell walls and tip burn results from the plants inability to supply sufficient calcium to developing leaves during periods of rapid growth. Since calcium is needed to strengthen cell walls and to maintain membrane integrity, calcium deficiencies lead to the collapse of cells, resulting in tissue enzymatic browning. This said, if you’re running your environment to optimums (temp, humidity, regular nutrient changes etc) the likelihood is that you may be overfeeding. Drop back on nutrient strength and see if this corrects the problem.
Symptoms due to excessive EC
– Wilting of leaves and stems
– Reduced growth.
– Leaves show signs of burning at tips and edges and may wither or die.
– Leaves may drop off and shoots die back
It is important to note that many of these symptoms are also indicative of other problems such as a lack of water, disease, nutrient deficiencies and excessive light or heat.
Optimum EC is Influenced by Genetic and Other Factors
It is important to note that various plant nutrition factors come into play when discussing optimum EC. For example, the nutrient itself (its NPK ratios etc) will have some bearing over optimum EC. As a simple example, in flower ammonium nitrogen, magnesium, sulfur, phosphorous and particularly potassium are in higher demand than when compared to the vegetative stage of growth. Therefore, if using a high N, low K containing fertilizer (i.e. a grow formula in flower) a higher EC would be required than compared to a high K, lower N formula to provide the required amounts of potassium for optimal flowerset (although this would also mean risking too much N in solution which would result in a high leaf to cola ratio and reduced yield). Other than this, environmental factors such as ambient air temperatures play an important role in determining optimum EC. For example, where ambient air temperatures are too high (above 300C/860F) EC needs to be reduced somewhat to compensate for a reduction in the plants rates of photosynthesis (photosynthetic potential). Other than this, genetics will play some role in determining optimum EC. That is, some varieties are heavy feeders while others are easy to over-fertilize. For this reason some cautious experimentation, to establish optimum EC for your crop/genetics, is advised.
 GUZMAN, M. & OLAVE, J. 2006. Both Electrical Conductivity and Sodium Absorption Ratio of the Fertigation Solution Affect Yield and Quality of Soilless Melon Crops. Acta Horticulturae, 718, 485-490.
 CHARTZOULAKIS, K. & KLAPAKI, G. 2000. Response of two greenhouse pepper hybrids to NaCl salinity during different growth stages Scientia Horticulturae, 86, 247-260.
 SONNEVELD, C. 1988. The salt tolerance of greenhouse crops. Netherlands Journal of Agricultural Science, 36, 63-73.