Control of nutrient strength levels in hydroponics

Growing in hydroponics generally involves the use of a reservoir of water into which plant food has been added. The water is then fed periodically or constantly to the plants and then returned to the reservoir to be reused or sent to waste. The main exception to this is DWC (Deep Water Culture) or “Bubblers”. In DWC, the plant roots actually grow into the reservoir containing the nutrient solution.

Plants use their roots to absorb nutrients from the solution. However, it is very important that the solution strength is correct for the stage of growth that the plant is at and also that the pH is monitored and adjusted regularly. pH will be discussed soon in a separate article. For this article I will just stick to the importance of getting your nutrient strength correct, and how to do it.

Why it's so important to get the nutrient solution strength right:

One of the great things about hydroponics is that it allows us to easily and continually optimise the nutrient solution strength that the plant is being fed. If a plant is fed too weak a solution, it will not be grow as fast as possible. If the nutrient solution strength is too strong then the plant will become overfed and become sick or, in the extreme case, they will look as though they have been “burned” and then begin to die, unless corrective measures are taken very quickly (i.e. flushing out the grow-medium with water and feeding the plants just water with no food in for a few days).

A properly fed plant will grow like it's on steroids. Getting the nutrient solution strength right is very important indeed!

So getting the nutrient solution strength right for the type of plant you are growing, and the stage of growth that the plant is at, is critical to getting the maximum growth speed that is possible with hydroponics. Here is a general guide to nutrient strengths as it relates to plant growth stages:

Getting the nutrient strength correct

This requires 3 things:

1)      A chart to tell you how strong the nutrient strength should be for the variety of plant that you are growing and the stage of growth that the plant is at. (Use the chart  below for this)

2)      A nutrient strength meter so that you can measure the present strength of the nutrient solution. We recommend the excellent Bluelab Truncheon for this.

3)      Some means of adjusting the strength of the nutrient solution. (Nutrients and water)

One Stop Grow Shop can provide excellent feeding charts for most of the brands of nutrient that we stock. These feeding charts have been thoroughly tested and are known to produce excellent results. These feeding charts will work well with a wide range of plants, but One Stop Grow Shop can also advise on nutrient strength requirements for specific plant types so that you can optimise your grow for a particular plant type/species. For example, delicate plants such as lettuces can only withstand a very low nutrient strength, while broccoli which is a very heavy feeding plant, requires a nutrient solution almost 3 times as strong.

Here is a chart giving the preferred nutrient strengths for a variety of different plants:

Nutrient Strength Meters

A quality nutrient strength meter really is the hydroponic grower’s best friend. It reliably and accurately gives a reading of the strength of the nutrient solution. There are 3 main measurement units: CF (Conductivity Factor), EC (Electrical Conductivity) and PPM (Parts Per Million). EC, CF and PPM are simply different scales for measuring the same thing, a little like Fahrenheit and Centigrade are just different scales for measuring the same thing i.e. temperature. Some meters will read in only one of the units of measurement. More versatile meters (such as the Bluelab Truncheon) display their readings in all 3 scales. The following conversion chart shows the relationship between the three units of measurement:

Nutrient strength meters work by holding a small voltage between 2 electrodes which are dipped into the nutrient solution. A small current then passes between the electrodes. The amount of current that flows between the electrodes is directly proportional to the strength of the nutrient solution. The meter measures this current and then displays the result in terms of EC, CF and/or PPM.

In the UK we mostly use CF and EC. Unfortunately (and confusingly), PPM actually comes in 2 scales depending on which nutrient/meter manufacturer or country the advice is coming from. The EC and CF scales are much easier to use as they only come in one flavour. Also, it is easy to interchange between the CF and EC scales because CF is simply the EC multiplied by 10. For example a CF of 12 is the same as an EC of 1.2.

At the One Stop Grow Shop we have always said that it is very well worthwhile investing in a quality Nutrient Strength Meter which will give reliable, accurate readings. If we had our way, every customer would invest in a Bluelab Truncheon! A decent nutrient strength meter takes the guesswork out of making sure you are feeding your plants the correct strength of nutrient. In other words, they are completely invaluable.

The Nutrient Strength Meter in use

So, you have your reservoir full of fresh water, your nutrients to hand and a copy of the feeding schedule for the brand of nutrient that you are using (with any plant species adjustments already made). Where do you go from here?

First of all, take a reading of the water that you have in your reservoir. Tap water will have a reading just on it’s own. This is because it naturally contains some Calcium, Magnesium and other mineral elements and compounds. The only types of water that will have a reading of zero are distilled water and reverse osmosis water which has had everything other than the water taken out. It is important to know the baseline reading of your water so that this can be taken into account in your calculations. This value varies from area to area, and even from day to day in the same area. Therefore, it is important to follow this first step, each and every time you make up a fresh nutrient solution.

Most base nutrients are supplied in 1, 2 or 3 bottles. Single bottle nutrients are very easy to use but are not always of the highest quality. 2 bottle nutrients have a part A and a part B which usually need to be added to the reservoir in equal measure. 3 part nutrients typically have a Grow part, a Bloom part and a Micro part. The ratios of these 3 parts often change over the course of the grow.

The following charts make it easy to find your target CF or EC for many plants. For example, if your Tap Water has a CF of 5 (EC=0.5) and your schedule calls for a three-quarter strength nutrient for that week then the chart tells you that you should be aiming for a CF of about 17 (EC=1.7)

Add the amount of nutrient necessary to reach your target CF/EC. For 2 or 3 part nutrients, add the first part to the reservoir and mix it in thoroughly. Then, add the next part and mix it in thoroughly. For a 3-part nutrient add the last part in and mix thoroughly. Never mix the different parts together straight out of the bottle. It is well worth mentioning that our favourite meter, the Bluelab Truncheon, is just the right shape for stirring the nutrient solution. Once you have added all the parts, take a reading of the CF or EC. If you have not quite reached your target CF/EC then add more nutrients until you reach the correct strength. If you have gone over the target CF/EC, replace some of the nutrient solution with straight water to get the strength back down to the correct level.

Once the CF/EC has been set correctly, the pH of the nutrient solution should be checked and adjusted which is the subject of our next blog article: Hydroponics and pH of nutrient solutions.

On-going checking over the course of the week until the next reservoir change

The nutrient solution in your hydroponics reservoir should be changed once a week. However, it is a good idea to get into the habit of checking the nutrient strength (and the pH) every day and topping up / adjusting when necessary. The following is how to find the perfect feed strength for your plants in a recirculating hydroponics system:

Each day, test the nutrient strength in your reservoir before topping it up. Then you can compare today's reading to the reading from yesterday.

The nutrient strength will do one of 3 things:

  1. Stay the same, indicating that the plant is using as much water as food. In this case just keep the reservoir topped up with solution of the same nutrient strength. You have hit the perfect nutrient strength when the readings stay the same from one day to the next. However, keep checking the nutrient strength every day so that you can react immediately when your plant's requirements change.
  2. Go down. This means that the plant is using more food than water. The nutrient strength should be topped back up to the original strength that it was at yesterday plus a little more. The nutrient strength meter's readings are indicating that the plant is requiring a stronger nutrient solution. Increase the nutrient strength by a little (1 or 2 CF points) and observe for a few days. The aim is to reach scenario 1) where the readings are the same on consecutive days. If the nutrient is too low for too long then the plants may begin to show signs of yellowing, slowed growth or other signs of deficiencies.
  3. Go up. This means that the plant is using more water than nutrient. The nutrient strength should be diluted back down and a little further. The nutrient strength meter readings are indicating that the original nutrient strength was too strong. Lower the strength of the nutrient solution by 1 or 2 CF points than it was the day before and observe for a few days. The aim is to reach scenario 1) where the readings are the same on consecutive days. If the nutrient strength is too strong for too long then the plant may begin to show signs of over-feeding such as burned (brown) leaf-tips, distorted-shaped leaves, slowed growth and an unusual over-deep green colour. In extreme cases of over-feeding, plain pH adjusted water can be fed for a few days until the plant gets back to normal again.

Looking after your meter

After using your meter, always give it a rinse off with clean water before putting it away for storage.

To maintain the accuracy of your meter, most need to be calibrated every now and then (once a month or so). Refer to the instructions that came with it for details on how to do this. Usually you will need calibration fluid. The Bluelab Truncheon does not need calibrating but the end does need to be kept scrupulously clean to maintain its accuracy. 

A little of the fluid is poured into a small container. The meter is then dipped into it and a calibration screw is turned or a button is pressed to perform the calibration automatically. Never return used fluid back to the bottle. Dispose of the used fluid and use fresh every time.

A special word about the Bluelab Truncheon: This meter is does not need calibration. It only requires careful cleaning of the electrodes once every week or 2. Full instructions on how to do this come with the special Bluelab cleaning & care kit that we recommend every Truncheon owner buys:

As long as the electrodes are kept clean, the Bluelab Truncheon will give years of reliable service, helping you grow healthy plants at the fastest rate possible.

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