Using Conductivity (EC) and pH Measurements to Control Hydroponic Solutions

Hydroponics can be a great way to grow plants but it is important to make sure the nutrient solution contains enough food for the whole growth cycle. If either pH or EC (electrical conductivity) is out of whack, plants grow poorly or stop growing all together. It is therefore important to measure and control the nutrient solution. This post will look at what the numbers mean and what you should do about them to keep plants growing well.
 

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How do Hydroponic Solutions Work?

Hydroponic solutions are mixtures of water and salts that the plant needs, including nutrients such as nitrate, calcium, phosphate, potassium etc. The amount of each nutrient at the start of the growing process is formulated to match the amounts used by plants.

In a perfect world, the plant would absorb the correct amount of nutrients and water and over time the level of hydroponics solution would decrease without any change in the concentration of nutrients. Life is not that perfect. In the real world the species of plant, its growth stage as well as the growing condition influences the amount of nutrients the plant absorbs. Over time the level can become unbalanced resulting in poor plant growth.

Why is this a bigger problem in hydroponics than soil? Soil has a natural ability to buffer both pH and EC, which means that soil keeps these values in a narrow range compared to the water in hydroponics. Soil takes care of many changes automatically to keep values in the range that plants prefer. Hydroponics does not have this buffering capability and the gardener has to more closely monitor and adjust the values.

It is the gardeners job to keep the solution balanced so the level of nutrients never changes much from the original formulation. This is done by monitoring both pH and EC.

What is EC?

EC stands for electrical conductivity which is an indirect measurement of the total amount of dissolved ions in a solution. All of the nutrients in fertilizer are ions in water. Tap water and a very dilute fertilizer solutions have a very low EC value while a concentrated fertilizer solution has a high EC value.

EC is measured with a meter, which can be as simple as a probe, or it can consist of an actual metering device with an attached probe. The common units for EC are millisiemens per centimeter (mS/cm).

It is common to have people state an EC value without including units which can cause confusion. The EC of hydroponic solutions are normally between 0.5 and 4.0 mS/cm. If someone talks about EC values that are over 500 then they are using µS/cm (microsiemens per centimeter).

1 mS/cm = 1,000 µS/cm

The Difference Between EC and TDS?

To complicate things some people like to use TDS instead of EC for measuring fertilizer. EC measures only particles that allow the conductance of electricity and these include all of the nutrients in a fertilizer solution.

TDS is short Total Dissolved Solids and includes all particles, not just the ones that conduct electricity. In the case of water and fertilizer solutions, almost all the particles conduct electricity. This means EC and TDS are essentially the same thing but there are two important facts that complicate things.

• TDS meters do not measure total dissolved solids. They measure conductivity (EC) and convert that number to a TDS number (mg/l = ppm).
• The conversion factor from EC to TDS is not a standard value because it is non-linear and depends on the actual particles being measured.

TDS meters may be referred to as TDS testers or PPM (parts per million) testers, but these are just different names for the same thing.

Most TDS meters use one of two conversion factors, either 0.7 or 0.5 to convert EC to TDS. If the EC measurement is 1,000 µS/cm, the meter either reports 500 or 700. Some meters allow you to change this conversion factor.

The 0.5 factor is based on sodium chloride and the 0.7 factor is based on the 442 Natural Water Conversion system. If someone online reports a ppm value (TDS) without stating the conversion factor used, you don’t know which factor is used and the number is not very useful.

What does this all mean? If you are using a TDS meter, find out the conversion factor for the meter and convert the reading to EC. If your meter does both TDS and EC, ignore the TDS completely and only use EC. If you are buying a new meter, buy one that reports the EC value.

Work in EC units – not TDS units.

Some TDS Myths

You will find quite a bit of misinformation about TDS online.

• “EC is the preferred method for measuring the nutrient concentration in hydroponics because it is more accurate and reliable”. This is not true. Since only EC is measured with meters and the number is then converted to TDS by a specific factor, both values are equally accurate and reliable.
• “µS/cm and ppm (TDS) should be used because they are more accurate than mS/cm.” Not true. Just because you see more digits in a number does not mean it is more accurate. The accuracy depends on how clean the meter is, how well it has been calibrated and the accuracy of the electronics. Both µS/cm and mS/cm are equally accurate.
• “Total dissolved solids (TDS) measures both the EC-generating particles AND the particles that are unable to conduct electricity in the water.” This can be true, but in the gardening world it is not true. If TDS is measured properly in a laboratory, using gravimetric methods, it does measure all particles. However, if TDS is measured using a meter, it is really measuring conductivity (i.e. EC) and so both TDS and EC are measuring only the ionic particles (i.e. nutrients).

The Right EC for Hydroponics

Most plants do best with EC between 0.5 and 2.5 mS/cm. Values below 0.5 or above 2.5 are almost always detrimental to plant growth. However, each type of plant has a preferred range and the closer you are to its requirements, the better it will grow.

You would normally start with an EC that is close to the middle of the plants referred range. If the plant absorbs water and nutrients at the same rate, the EC will not change, but this is usually not the case. Water and nutrients can be absorbed at different rates.

If the plant absorbs water faster than the nutrients, the EC will go up since the nutrient solution is becoming more concentrated. If the plant absorbs nutrients faster than the water, the solution gets more diluted and the EC value drops.

It is important that the EC is checked on a regular basis and that any large changes in EC are corrected. You want to ensure that the plant is always growing in its preferred range.

How Often Should You Check EC?

Professional growers check daily or even more frequently. Checking daily for homeowners is overkill.

I suggest you check it daily if you are new to measuring EC. See how much it is changing. If the change is very small, check twice a week. If it is still not changing much go to once a week.

Keep in mind that when you start new plants they are small and use very little water or nutrients. So the EC change will be small. As the plant gets bigger it uses more which results in faster EC changes. Ideally you check EC often enough so that you catch it before the value is outside of the preferred range.

What to Do if the EC Level Is Too High or Too Low?

If your nutrient solution EC is too high you can add some water to bring it back into range.

If the EC is too low you can add a more concentrated nutrient solution. This could be a fertilizer solution that is made with twice the amount of nutrients as usual.

In both cases add some of the adjusting solution and retest the mixture. If it is still out of range, add some more and retest.

Preferred EC Ranges for Different Plants

The following table of plants is from Oklahoma State University.

Plant	EC (mS/cm)	pH
Asparagus	1.4 to 1.8	6.0 to 6.8
African Violet	1.2 to 1.5	6.0 to 7.0
Basil	1.0 to 1.6	5.5 to 6.0
Bean	2.0 to 4.0	6
Banana	1.8 to 2.2	5.5 to 6.5
Broccoli	2.8 to 3.5	6.0 to 6.8
Cabbage	2.5 to 3.0	6.5 to 7.0
Celery	1.8 to 2.4	6.5
Carnation	2.0 to 3.5	6
Courgettes	1.8 to 2.4	6
Cucumber	1.7 to 2.0	5.0 to 5.5
Eggplant	2.5 to 3.5	6
Ficus	1.6 to 2.4	5.5 to 6.0
Leek	1.4 to 1.8	6.5 to 7.0
Lettuce	1.2 to 1.8	6.0 to 7.0
Marrow	1.8 to 2.4	6
Okra	2.0 to 2.4	6.5
Pak Choi	1.5 to 2.0	7
Peppers	0.8 to 1.8	5.5 to 6.0
Parsley	1.8 to 2.2	6.0 to 6.5
Rhubarb	1.6 to 2.0	5.5 to 6.0
Rose	1.5 to 2.5	5.5 to 6.0
Spinach	1.8 to 2.3	6.0 to 7.0
Strawberry	1.8 to 2.2	6
Sage	1.0 to 1.6	5.5 to 6.5
Tomato	2.0 to 4.0	6.0 to 6.5

The above requirements also change for different growth phases. They are lower during the growth stage and higher by about 0.5 mS/cm during flowering and fruiting stage. Each brand of fertilizer will also have a different starting EC depending on both the form and amount of each nutrient.

The idea that a higher phosphorus level is needed to increase blooming is a myth.

What is Nutrient Lockout?

The term Nutrient Lockout is popular in the cannabis growing world and in hydroponics. It is rarely used by gardeners growing in soil and is almost never used in the scientific community. As such, it does not have a good definition and many online definitions are incomplete or only partially correct.

Nutrient lockout does not really exist because nutrients are not locked out of the plant. There are however conditions that make it more difficult for roots to absorb nutrients. Here are some situations that can cause this issue.

• Insoluble nutrients. When nutrients are mixed incorrectly or get too concentrated, they start to form insoluble compounds and precipitate out of solution. Plants can’t use these nutrients.
• Incorrect pH. Nutrients are less available to plants at extreme pH.
• High osmotic pressure. When nutrients become too concentrated, the osmotic pressure of the solution is higher than inside the plant which causes water to leave the roots. In effect they become hydrated. When this happens, plants can’t absorb water or most nutrients.

Statements such as “your plants are starving although they’re surrounded by food” are not really correct since nutrients are only food for plants when they are in the correct chemical form.

How do you prevent nutrient lockout? Check the EC and pH regularly and keep the values in the normal range for the plant.

The Right pH for Hydroponics

pH is a measure of the acidity or alkalinity of a substance and provides a number between 0 to 14. The availability of nutrients is affected by pH and most plants grow well in a range of 5.6 to 6.5. When pH drifts outside of this range some nutrients become less available and others can become toxic. Keeping the value between 6.0 and 6.5 will ensure that pH does not create a nutrient issue.

Here is an example of what happens when the pH gets too high. A higher pH causes iron to be converted from the Fe2+ form which plants can use to the Fe3+ form which is harder for plants to absorb. Iron also combines with carbonate and phosphate to form precipitates that leave the solution. You will see this as white powder in the bottom of the hydroponic unit. Without adequate iron, plants show interveinal chlorosis and don’t grow very well. This is not really an iron deficiency but a pH imbalance.

How to adjust pH

If pH is too low, an alkaline solution is added to raise the pH and an acid is added if the pH is too high. You can use commercial “pH up” and “pH down” products (Amazon affiliate link) for this. The pH up products usually contain potassium hydroxide or potassium carbonate, while pH down products contain phosphoric acid. Mild organic acids such as household vinegar or citric acid can also be used but their effect is not long lasting.

Be careful using commercial adjusting products since a small amount can result in a large pH change.

Hard Water in Hydroponics

Hard water tends to have high levels of calcium, magnesium, sodium and/or carbonate. These can cause several issues in hydroponics.

• Too much sodium is toxic to plants.
• Hard water commonly has a high pH.
• The EC is naturally high.
• Most hydroponic fertilizer adds extra calcium and magnesium, which is not needed for hard water.

Most hydroponic nutrients are formulated based on “pure water”. For this reason many people use softer water (RO, distilled, rain water) to make up the hydroponic fertilizer solution. Alternatively, look for a fertilizer that does not include calcium and magnesium. Keep in mind that nitrogen should be provided in a nitrate form, not a ammonium form, which is more common in non-hydroponic fertilizers.

Alkalinity vs Harness

It is also important to understand the difference between alkalinity and hardness.

Alkalinity is a measure of the total carbonates (CO3), bicarbonates (HCO3) and hydroxyl ions (OH) and is usually expressed as the equivalent of CaCO3, e.g. 100 ppm CaCO3.

Water hardness is the amount of dissolved calcium and magnesium in the water. High harness is anything over 150 ppm, but values up to 200 ppm usually don’t cause growth problems for plants because neither calcium nor magnesium are not very toxic to plants.

High levels of calcium, magnesium and carbonate can all combine with nutrients and precipitate out of solution forming a white powder at the bottom of the tank. This can reduce micronutrients to levels that are too low for proper plant growth.

The Limitation of EC Measurements

Using EC to monitor hydroponic solutions has a serious limitation that few people talk about. Remember that EC is the total electrical conductivity of a solution. Consider these three solutions:

• A complete Masterblend fertilizer solution with an EC = 2.0 mS/cm.
• A solution of potassium nitrate with an EC = 2.0 mS/cm.
• A solution of sodium chloride (table salt) with an EC = 2.0 mS/cm.

Each solution has the same EC and is a good EC value for growing strawberries. The first one will grow good strawberries because it contains all the needed nutrients. The second solution will grow very poor plants because it has no phosphorus and the third will kill the plants because sodium is toxic. All three solutions have the same EC value.

EC is only valuable if the ratio of nutrients in the solution are correct. It tells you nothing about the actual ratio of these nutrients. This is important because the ratio changes over time.

You might start with the perfect ratio of nutrients, but plants will take what they want from the mixture. Some nutrients are absorbed by active transport and others are absorbed at the same rate as water. In any mixture, the ratio of nutrients changes over time and EC gives you no insight into what they are.

What can a gardener do?

Short of doing a lot of lab tests, all a gardener can do is assume the ratio is acceptable for a while and after that point, throw out the mixture and start with a fresh one. How often do you need to do that? I really don’t know because I have not found any information about how fast the solution changes. If you have some data please add a link to it in the comments.

The rate of replacement does depend on the number and size of plants in the nutrient solution, but I think it would be a good idea to replace the solution every 6-8 weeks. It should certainly be replaced if you suspect nutrient issues with the leaves. The old solution can be poured in the garden.