Maximum Yield June 2015
By Mark Wittman
What are these terms and why do we use them?
Maximum Yield – https://www.joomag.com/magazine/mag/0306142001431620100/p136
When talking with growers we often hear the terms EC, TDS and ppm used in relation to feeding regimes. With Coco Coir, growers & retailers often ask us what the EC is? Almost like, if the EC is low, they think the product will be great.
We now explore what these terms mean and what their relationship to each other is. We will attempt to make this a little clearer. Let’s start by talking about each term.
EC or electro conductivity – EC is a measure of the ability of water to conduct an electrical charge. Perfectly pure water, pure meaning that it is only H2O with nothing dissolved in it, does not conduct electricity. It is a good insulator. Water is called the universal solvent because it will dissolve most things. So although pure water is a good insulator you should never mix water and electricity because water is seldom pure.
Water quits being a good insulator once it starts dissolving substances around it. Many of the molecules dissolved in water are salts. In chemical terms, salts are ionic compounds composed of cations (positively charged ions) and anions (negatively charged ions). Salts can be easily identified, since they usually consist of cations from a metal with anions from a non-metal. Salt, such as common table salt (sodium chloride, NaCl), is the most common one we know or think of. In solution, these ions essentially cancel each other out so that the solution is electrically neutral (without a net charge). Even a small amount of ions in a water solution make it capable of conducting electricity. The higher the salt concentration the better it conducts electricity.
EC meters read in either mS/cm (milliSiemens/cm) or μS/cm (microSiemens/cm). The conversion of μS/cm (microSiemens/cm) to mS/cm (milliSiemens/cm) is 1000 μS/cm = 1 mS/cm.
While the electrical conductivity is a good indicator of the total salinity, it still does not provide any information about the ion composition in the water. For instance we don’t know if there is a lot of potassium or table salt in the solution. To further complicate the EC measurement, for a grower, not all elements that are considered fertilizer will add to the EC of a solution. Urea is probably the most important nutrient that does not affect the EC.
TDS – total dissolved solids – is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular suspended form. To be considered dissolved generally the solids must be small enough to go through a 2 micrometer filter.
The two main methods of measuring total dissolved solids are gravimetry and conductivity. Gravimetric methods are the most accurate and involve evaporating the liquid solvent and measuring the weight of the residue remaining. If most of the TDS comes from inorganic salts, gravimetric methods give an accurate result, but if organic material is in the makeup of the TDS, then much of it may be destroyed during the heating process. In reality, the way to measure TDS of water in a laboratory setting would be to boil the water until there is none remaining, and then to weigh the remaining material. This of course is totally impractical for the average grower, as accurate scales and special equipment would be required to achieve such measurements. The TDS meter is therefore the next best device to achieving an estimated TDS reading.
The connection between EC and TDS – TDS meters are really EC meters in disguise. A TDS meter reads the EC of the dissolved salts in the solution and uses a conversion factor to give you a ppm reading. Let’s say the EC is 2200 microsiemens. Some dissolved salt or mixture of dissolved salts with an EC of 2200 microsiemens will have a ppm of 1540 (I used a conversions factor of 0.7). It does not mean that the solution that you measured has that 1540 ppm. The conversion factor all depends on what the salt solution is, it could be the exact correct conversions factor but that is unlikely. This is why I personally question the use of a TDS meter and while an EC meter is not more accurate as such, it does tell you what you have.
The relationship of TDS and conductivity of water can be approximated by the following equation:
TDS = keEC
where TDS is expressed in mg/L and EC is the electrical conductivity in micro-siemens per centimetre at 25 °C. The correction factor ke varies between 0.55 and 0.8. The correction factor is actually very specific to each salt. When you have a mixture of salts (like in a feeding solution) a correction factor that will emulate that mixture should be used. EC meters that read as TDS (ppm) often use a correction factor of 0.5 or 0.7 depending on the meter used. When comparing ppm reading with someone you need to know that you are both using meters with the same conversion factor.
PPM – parts per million.
TDS meters give readings as ppm. In the metric system, ppm is easy to figure because it is 1 gram in 1000 litres (1 cubic meter, 1000 litres of water is 1 million grams) of water or 1 milligram in 1 litre of water (1 litre of water is 1 million milligrams). Also in soil it is 1 milligram in 1 kilogram of soil. It is a weight per weight measurement which is not always apparent when water is involved because 1 litre of water is equal to 1 kilogram of water. To put ppm in perspective, a 1% solution is the same as a solution containing 10,000 ppm.
So you have taken an EC reading (or a TDS reading) of your nutrient mix. What do you now know from the reading? Not much really. An EC meter only tells you what the electro conductivity is and not anything more. If you know you want your feed to have and EC of 2.5 mS/cm, you can achieve this with mixing fertilizers together or you could mix sea water with tap water to the right proportions and it will give an EC of 2.5 mS/cm. If you want to know what is actually contributing to the EC reading and how much it is contributing, you need to have a chemical lab analysis done. It will tell you how much Nitrogen is in the solution, how much of the nitrogen is nitrate nitrogen, how much is ammonia, and any other element that you ask to have tested for but if you don’t ask for the analysis to include copper it won’t tell you. The lab usually has a standard nutrient solutions test that will include the most common fertilizer elements so you don’t have to list everything for them. If you are feeding something unusual you will have to ask for that element.
The EC meter is a good tool for checking up on yourself. If you test your feed solution and it has the EC that you expected then you have used the right proportions, if it is to high you have used too much or and if it is to low you have not used enough. If you test your drain water and it is too high, you are not watering enough and if it is too low you are watering too much.
I always have a difficulty understanding why growers use a TDS meter and read in ppm because it is really EC meter using a corrections factor. But then I have to remember that both meters are only giving very basic information, which is how much salt is in the solution. The meter is only a tool that helps you repeat a procedure mixing up your feed solution as close to previous times as you can. The meter lets you check if the end result is close to previous results. Calibrate your meter periodically or even that information may be suspect.
I hope this hasn’t confused you. Continuing to monitor your TDS or EC reading is important. It helps you to understand if your feeding too much or too little & assists you in getting the results you want. But there are many other factors involved and you should keep this in mind. Using a well balanced feed is probably the most important. Well balance nutrients is like eating a well balance diet, the body is healthier and performs better. When and how much water is also very critical to a healthy plant and a healthy plant gives a higher production.
Salt – In chemistry, a salt is an ionic compound which is made up of two oppositely charged ions called a cation (positive) and an anion (negative). The number of each ion in the salt is important, since overall the compound must be electrically neutral (in other words has no charge). Salts can be easily identified, since they usually consist of positive ions from a metal with negative ions from a non metal. The salt we put on our fries is actually sodium chloride and is made up of metal Na1+ and non metal Cl1- ions. Often you will see this written as Na+ and Cl- (the 1 is dropped) or simply NaCl.
Ionic compound – In chemistry, an ionic compound is a chemical compound in which ions are held together in a structure by ionic bonds. The positively charged ions are called cations and the negatively charged ions are called anions.
Ion – An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.
Ionic bond – is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions. These ions are known as cations (positively charged atoms that have lost one or more electrons) and as anions (negatively charged atoms that have gained one or more electrons).
Metal – In chemistry, a metal is an element that readily forms positive ions (cations) and has metallic bonds. They tend to be lustrous, ductile, malleable, and good conductors of electricity. This is a list of metals, not complete but the most common. Sodium, Potassium, Rubidium, Cesium, Beryllium, Magnesium, Calcium, Strontium, Barium, Radium, Aluminum, Tin, Lead, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Zirconium, Molybdenum, Palladium, Silver, Cadmium, Tungsten, Iridium, Platinum, Gold, Mercury.
Metallic bond is the bonding between molecules within metals called alkali reactive force. It is the sharing of a large group of delocalised electrons amongst a lattice of positive ions, where the electrons act as a “magnetic force” giving the substance a definite structure.
Dissolve – A solution is made when one substance called the solute “dissolves” into another substance called the solvent. Dissolving is when the solute breaks much smaller groups or individual molecules up from a ‘larger’ crystal of molecules. This break up is caused by the crystal coming into contact with the solvent.
In the case of salt water, the water molecules break off salt molecules from the larger crystal lattice. They do this by pulling away the ions and then surrounding the salt molecules. Each salt molecule still exists. It is just now surrounded by water molecules instead of fixed to a crystal of salt.
Solutions – A solution is a homogeneous mixture of two or more components. The dissolving agent is the solvent. The substance which is dissolved is the solute. The components of a solution are atoms, ions, or molecules. Examples are sugar in water and salt in water.
Homogeneous mixture – where the components that make up the mixture are uniformly distributed throughout the mixture.