Fertilizer Measuring TDS EC Osmolality Osmolarity Water Quality
Plants need to take up certain chemicals in their water. We use fertilizer to supply these things.
The water can't be too salty, though, or it harms the plant.
There are several ways we measure things related to fertilizing plants, and it might be helpful to have an explanation here.
You don't have to read the whole message. If you want to read just one or a few things, scroll down.
The best advice for orchid growers is to use a little bit of fertilizer on a regular basis. More is not usually better. Until you read and learn more, less is better. At the start, use much less than you would use for garden vegetables and flowers.
We hear about concepts like water quality, total dissolved solids or TDS, electrical conductivity or EC, osmolarity, osmolality. This can be confusing to gardeners who haven't studied chemistry or biology.
EC
Truly pure water doesn't conduct electricity. (Even lakes and rivers do not contain truly pure water, so they do conduct electricity.) When particles with no electrical charge are dissolved in water, it doesn't conduct electricity. When electrically charged particles are dissolved in water, it does conduct electricity. The more charged particles dissolved in the water, the more current can flow.
Measuring electrical conductivity, EC, measures how many charged particles are dissolved in the water. It does not measure how many non-charged particles are dissolved in the water. It does not measure how much mass of anything is in the water. Examples:
Salt is sodium chloride. Its chemical formula has one sodium atom with one chlorine atom, NaCl. The number of identical basic units of a chemical compound in a pile of chemical is measured with a unit called a mole. One mole of salt, one mole of NaCl particles, has a mass of about 53.5 grams. When one mole of salt dissolves in a liter of water, that liter of water has 53.5 grams of salt dissolved in it. The salt crystals come apart, and the sodium and chlorine separate from each other in the water. This yields one mole of sodium particles and one mole of chloride particles in the water. The sodium and chloride atoms each bear one unit of electrical charge, resulting in two moles of electrical charge in the liter of water. Salt dissolved in water yields water that conducts electricity. You can measure the EC of this solution.
One mole of potassium chloride has a mass of about 74.5 grams. When one mole of potassium chloride dissolves in a liter water, that liter of water has 74.5 grams of potassium chloride dissolved in it. The potassium and chloride atoms each bear one unit of electrical charge, similarly to the sodium chloride, resulting in two moles of electrical charge in the water. One mole of potassium chloride dissolved in one liter of water yields water that conducts electricity. You can measure the EC of this solution, and it will be essentially the same as the EC of the sodium chloride solution discussed above, because it has the same number of charged particles per volume of water.
If you use one liter of water for each, the EC of the potassium chloride solution will be the same as the sodium chloride solution, because there will be the same number of electrical charges in each solution. But there will be 74.5 grams of potassium chloride in the potassium chloride solution, as opposed to 53.5 grams of sodium chloride in the salt solution. EC varies with the number of charged particles in solution, not with the mass of chemicals in solution.
Urea is not a salt. One mole of urea has a mass of approximately 60 grams. When one mole of urea dissolves in one liter of water that water has 60 grams of urea dissolved in it. When dissolved in water, urea does not form electrically charged particles. Water with only urea dissolved in it will not conduct electricity. The EC of urea in water is zero. But the water will still have 60 grams of urea dissolved in it. EC measures electrical charge in the solution, not mass in the solution.
If you have a mixture of compounds, and you know exactly what compounds, in exactly what proportions, are present in your mixture, you can calculate what the EC should be, for any quantity of that mixture you add to your water. So, knowing exactly what is in your fertilizer, you can calculate what the EC should be for different concentrations. Likewise, if you know the EC for a given concentration of your known fertilizer, you can use an EC measurement to calculate multiples or fractions of this. But EC cannot tell you what chemicals are in the water, nor how many grams of stuff are dissolved.
TDS
Total dissolved solids represents the total mass of chemicals dissolved in the water. It is measured in mass per volume of water, something like milligrams per liter. It does not refer to the number of particles dissolved in solution. Think of it as the mass of all minerals remaining when you evaporate the water. Water with a high amount of TDS leaves a lot of water-spotting on glass as it dries.
TDS is not related to electrical conductivity. Example:
Prepare two solutions of water: one containing 53.5 grams (1 mole) of sodium chloride, and one containing 60 grams (1 mole) of urea. Splash the water on two windows, and let it dry. Both windows will be water-spotted, because both solutions contain chemicals that do not evaporate. Both solutions have a TDS that can be measured. But the sodium chloride solution conducts electricity, and the urea solution does not.
TDS is not related to the number of particles in the solution, but to the mass of chemicals in solution. Example:
Prepare two solutions of water: one containing 53.5 grams (1 mole) of sodium chloride, and one containing 53.5 grams (0.89 mole) of urea. When it dissolves, each molecule of sodium chloride results in one sodium and one chlorine in the water. So there are two moles of particles in the sodium chloride solution. The urea does not break down when dissolved. So there is only one mole of urea in the water. The TDS of both solutions will be the same, because the mass of chemical in each solution will be the same. But there are more sodium chloride particles in the first solution than there are urea particles in the second solution.
TDS can't tell you what compounds are in your water. It can't tell you how many particles are dissolved in your water.
Molar Concentration and Osmolar Concentration
These aren't phrases most people use in daily conversation.
Molar concentration refers to how many moles of a given compound are dissolved in one liter of water. One mole of sodium chloride has a mass of about 53.5 grams. So dissolving 53.5 grams of sodium chloride in 1 liter of water yields water a molar concentration of 1. We also say this solution is 1 molar sodium chloride.
One mole of calcium chloride has a mass of about 110 grams. So dissolving 110 grams of calcium chloride in 1 liter of water yields water a molar concentration of 1. We also say this solution is 1 molar calcium chloride. Recall, one molar sodium chloride solution has 53.5 grams per liter. So molar concentration refers only to the number of moles of compound we put into solution, not the mass of chemical in solution.
Osmolar concentration refers to the total number of PARTICLES dissolved in water solution, no matter how they got there. When sodium chloride dissolves in water, each molecule of sodium chloride dissociates into one sodium and one chlorine. So one mole of sodium chloride dissolved in water yields two moles of particles. We say a solution of one mole / 53.5 grams of sodium chloride dissolved in water yields a solution of two osmoles per liter, or we say it is a two osmolal solution.
In order to figure out molar concentration, you need to know how many moles you have. In order to figure out osmolal concentration, you need to know two things: how many moles of your compound you have, and how many particles your compound breaks into when dissolved in water.
In the sodium chloride situation, sodium chloride breaks down into two particles in water. But not all salts have only two particles. Calcium chloride has one atom of calcium and TWO atoms of chlorine per molecule. So 110 grams or one mole of calcium chloride, dissolved in one liter of water, yields a solution that is one molar calcium chloride, but THREE osmolal.
Of note, adding one mole of calcium chloride to a liter of water yields a higher EC than adding one mole of sodium chloride to a liter of water. This is because one mole of calcium chloride breaks down into three moles of electrically charged particles, whereas sodium chloride breaks down into only two moles of electrically charged particles. The water with calcium chloride can carry more electricity than the water with salt.
Living things
Living things require very narrow concentrations of chemicals in their internal juices to stay alive. Living things spend a lot of energy controlling these concentrations. Both osmolarity and osmolality matter to living things, but for different reasons.
Remember osmolarity is the concentration of a compound in solution.
Many chemicals are used by living systems to make chemical reactions occur. For example, animal muscles rely on calcium concentration to make muscles contract. At too low a calcium concentration, or osmolarity, muscles cannot contract properly. At too high a calcium osmolarity, muscles stay contracted, unable to relax. This is called tetany. The disease tetanus is caused by a bacterial toxin that prevents muscle cells from controlling calcium properly. This is an example of the concentration, or osmolarity of something, mattering to the organism.
Remember osmolality is the total number of particles dissolved in solution. It doesn't matter in what form these particles entered solution - the organism cares about total number of particles in solution.
Living things have membranes around their cells. Most of the time water flows freely across these membranes. But dissolved particles may not flow freely; the cell controls access. For example, animal red blood cells allow water to pass freely, but sodium does not pass freely. Proteins in the blood do not pass freely. In living organisms, the osmolality (number of dissolved particles) is maintained almost the same on both sides of the membrane.
An observed principle of chemistry is that, when two solutions of differing concentration of anything are allowed to mix, concentrations of everything dissolved in the two solutions will equilibrate, and wind up the same everywhere in the final product.
If the osmolality (number of particles in solution) on one side of a biological membrane is different than on the other, and the particles themselves cannot cross the membrane, water will move across the membrane, to equalize osmolality on both sides of the membrane. If a red blood cell, with relatively high osmolality, is put into tap water, with very low osmolality, so much water enters the red cell it swells, then bursts. This is why tap water is so good at getting out fresh blood stains. If a red cell is put into a concentrated salt solution, the water leaves the red cell in an attempt to equalize osmolality across the membrane, and the cell shrinks into a spiky mess. The concentration of things that don't cross the membrane rises inside the cell, and too-high concentrations of things like calcium may damage the inside of the cell, killing it.
Plant cells are similar. If water with too high an osmolality is applied, water flows out of plants cells, and they may die. We see this when putting salt water on terrestrial plants. We also see this using too much fertilizer. We say it "burns" the roots, but it is not a burn: it is an osmolal problem.
Similarly, ocean plants are generally accustomed to water with high osmolality. If you put them in water with low osmolality, they may absorb so much water their cells swell and burst.
Measuring fertilizer
Plants care about getting certain chemicals from their water. They need those particular chemicals. Plants care about the chemical assay of the fertilizer. This is why reading the label is important.
Plants care about the osmolality of water around their roots. If it's too high, it might harm the plant. Plants care about the osmolality of fertilizer solutions. This is why calculating how much to add is important.
Plants don't care about TDS per se, nor EC per se. EC and TDS are derived numbers that help us think about fertilizer. But they are much easier to measure than actual chemicals in solution, or osmolality.
Unless you're using rain or distilled water, there will probably already be some minerals in the water you use for mixing fertilizer solutions. This has to be taken into account. If your water already has a lot of dissolved salts, fertilizer makes it even more likely to damage plant roots.
Some people have water with so many dissolved minerals it will damage orchid roots straight from the tap. This is the case in metro Phoenix, Arizona. New York City tap water, however, has so few dissolved minerals, it can be used for orchids. You can read about what is in your water in the water quality report found on your water utility's Web site.
TDS is a loose approximation for osmolality. Solutions with higher TDS usually have higher osmolality. So we try to keep TDS down for orchids.
EC measures charged particles in solution. It can be used to compare how much of the same salt mixture has been added to two different solutions. EC is only useful to hobbyists knowing what the measured EC is of a defined concentration of a defined fertilizer solution. It is not useful without this information.
The best advice for orchid growers is to use a little bit of fertilizer on a regular basis. More is not usually better. Until you read and learn more, less is better. At the start, use much less than you would use for garden vegetables and flowers.
Last edited by estación seca; 08-19-2017 at 05:05 PM..
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