Greenhouse Canada

Features Fertilizer Inputs
Growing Points: Fertilizers’ fear factor

April 25, 2012  By Dr. Mohyuddin Mirza

I have known that many growers are afraid of calculations about fertilizer rates in parts per million.

I have known that many growers are afraid of calculations about fertilizer rates in parts per million. After a recent Alberta Greenhouse Growers Association (AGGA) bedding plant workshop, many growers commented that they want to learn more about calculations and are worried about underfeeding or overdosing.

Look at these mature cucumber crops from the bottom to the top. The lower leaf is showing damage due to an EC of over 5.0 millimhos, and the top growth shows symptoms of calcium deficiency. 



(These fears probably originate with the math in high school as not being a favourite subject for many students!)

So, I would like to take the fear factor out of fertilizer calculations. Within this feature are pictures sent by a grower where a fertilizer miscalculation caused problems.

It is obvious that it was a miscalculation and electrical conductivity (EC) was high enough to cause this type of damage. The grower said the petunia seedlings in the photo did not receive the “rooting fertilizer,” but it is apparent that damage is due to “salt burn.” With further checking, the grower was able to supply me with the information that EC was 5.71 millimhos and pH was 6.0 from a 10-30-20 fertilizer being used for rooting purposes.
■ Understand the basics: Plants need 17 elements for growth and they are non-selective. It means that the presence of an element in plant tissue does not mean that they are essential.

The results of a fertilizer miscalculation.


Out of these 17 elements, carbon, hydrogen and oxygen are supplied through carbon dioxide from the air and water through the roots. So we have to supply nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, copper, zinc, boron, molybdenum and chloride. Silicon is being recognized more and more as an element found in plants, and it is provided as part of a fertilizer program or is available from water.

Understanding fertilizers: The above mentioned elements don’t exist on their own, they exist as salts, a combination of two or more elements. So, when you buy a fertilizer you will see names like potassium nitrate, calcium nitrate, mono potassium phosphate, etc.

Ready-made fertilizers may contain additional elements, such as iron, manganese and other trace elements. Growers are familiar with 20-10-20, 20-8-20, 12-2-14, and many other combinations. These numbers indicate per cent nitrogen, per cent phosphoric acid, and per cent potash. Many experienced growers don’t realize that per cent phosphorus will be 43 per cent, and per cent potassium will be about 80 per cent. Thus, 20-10-20 will look like 20-4.3-16 on the N-P-K basis.

Most fertilizers don’t have calcium. There are some exceptions, such as 12-2-14 that has six per cent calcium in it.

■ In order to use fertilizers wisely, one must understand the role of each element in the growth of the plant.

Out of all these elements, the type of nitrogen must be understood. Nitrate nitrogen is most commonly utilized by the plants and results in the least stretching of the plant. Ammonium nitrogen and urea nitrogen are more readily utilized compared to nitrate nitrogen and the ratios are listed on the label.

Similarly, we know phosphorus is involved in rooting. A helpful publication has been prepared by the AGGA in which all these aspects have been described. A PDF copy or hard copy can be obtained by e-mailing me at

The title of this publication is Plant Nutrition and Fertilizer Management in Greenhouse Grown Crops. The cost is $30 and the funds raised will go to the AGGA’s Dr. Mirza Educational and Scholarship Foundation.

Preparing a fertilizer program: That is probably the fear factor. How to prepare the program specific to your crops?

Parts per million (ppm) is the commonly used term for preparing the fertilizer solution.

Simply stated, one part of a chemical in one million parts of water will be considered one ppm. It is a measure of concentration.

I will give you a simple example that can be easily remembered.

Just memorize one gram and one litre. Take one gram of 20-10-20 fertilizer and dissolve it in one litre of water (1,000 ml) – that will be 200-100-200 ppm. So I just multiplied the 20-10-20 by 10. That is all to remember.

Take any fertilizer, dissolve one gram/litre of water and multiply by 10 and you get the numbers in ppm.

If you have a 100-litre tank, then you will dissolve 100 grams of 20-10-20 and if the tank is 1,000 litres, then you will dissolve 1,000 grams (1 kg).

Now take it a step further. You have an injector at 1:100 ratio, meaning that one litre of stock solution will be taken up and diluted 100 times. That simply means you will dissolve your 100 grams of 20-10-20 in one litre of water and set the dilution at 1:100. If your stock tank is 10 litres, you will add 10 times more and that is 1 kg.

Now that is the simple calculation that you can expand to your situation by logical extension. There are different types of injectors and dosing pumps available.

Stock solutions for calcium-based fertilizers are always kept separate from other fertilizers. The mixing occurs after dilution has occurred. Don’t try to experiment with mixing calcium with fertilizers containing phosphates and sulphates. The laws of nature are eternal – you will get a precipitate and lines will be plugged up.

Adjust pH and EC as needed. You should have a basic knowledge of these factors. The booklet I mentioned earlier has very good information on their management.

Bedding plant growers generally need three-stage fertilizers. Included are early growth and rooting stage with higher phosphate; main growth stage with a nitrogen-to-potash ratio of 1:1; and the hardening stage with higher potash than nitrogen and lots of calcium and magnesium.

Also remember that elements such as calcium are “stubborn.” Its movement within the plant depends on a “calcium uptake climate” that features active air movement, a relative humidity below 70 per cent, and good root health.

Familiarize yourself with the mobility or non-mobility of elements. It is a popular topic of discussion for me!
Take water quality tests at least twice a year and make adjustments accordingly. One of your biggest problems could be the presence of high levels of sodium in your water.

Keep yourself updated with new fertilizers. A lot of exciting things are happening with the use of new manufacturing technologies.

Print this page


Stories continue below