Fertilization part 2: pH


For many a difficult item, for plants the difference between to be or not to be.The following advices are general guidelines and plant behavior is always subject to local circumstances. Growers should always follow up on local legislation and consult a professional agronomist when in doubt. Syngenta does not accept any liability in any business damage resulting from the advice.

Author: Ben Geijtenbeek – Senior Crop Technical Specialist – Syngenta Flowers
Ben Geijtenbeek has forty years of experience as a technical expert in floriculture. He is specialized in educating growers in various cultures with new techniques and technologies. This way he revives the potential of Syngenta’s genetics, from the corporate philosophy “Bringing plant potential to life”.

Food for plants

The last time I wrote about different types of fertilizers, and the advantages and disadvantages of each type. This time I want to tell you something more about a quite difficult to understand subject: pH. I will focus on pot cultures with prepared or home made substrate. The following advices are general guidelines and plant behavior is always subject to local circumstances. Growers should always follow up on local legislation and consult a professional agronomist when in doubt. Syngenta does not accept any liability in any business damage resulting from the advice.

The influence of H+ ions in the sample

pH is an abbreviation of potency Hydrogen (H+). Good nutrition is essential for growing plants successfully. One of the first questions to consider when you want to improve your production, is: "Have you tested your substrate?" The pH is a measure of how acidic or alkaline (the opposite of acidic) your substrate is. This pH is important, since it affects the growth of plants and the severity of some diseases.

pH affects the ability of plant roots to absorb nutrients. Calcium, phosphorus, potassium and magnesium are likely to be unavailable to plants in too acidic substrates. Plants have difficulty absorbing copper, zinc, boron, manganese and iron in too alkaline substrates. By managing pH, you can create an ideal environment for plants and often discourage plant pests at the same time.

Accurate tests are an excellent management tool. They enable you to avoid increased production costs, yield losses, or both. Experience has shown that substrates differ greatly in their capacity to supply these elements. It depends on several factors. Two important ones are: 

  • The type of material from which the substrate was formed as % sand, % clay, peat for instance, or cocopeat or coco fiber 
  • Which treatment the substrate has received since being placed under cultivation. 
Not all particular elements in a substrate are available to a plant. Thus, the soil test must be able to predict whether it contains sufficient amounts of available nutrient elements for a specific crop. 
The pH level is important because also micro-organisms are responsive to their chemical environment.  Obviously, the pH is not a "cure-all" analysis, but may function as a possible problem indicator. These problems may then be further investigated with additional analyses. It is a general indicator of nutrient availability, presence of free lime (calcium carbonate), and excess hydrogen.

Factors influencing pH

Initially, factors such as previous crop, rainfall, and organic material in the substrate are dominant in determining the pH.  For instance, organic acids from plant roots, repeated use of acid-forming fertilizers, plant removal, and replacement of calcium and magnesium by fertilization eventually lowers the pH of substrates.


The pH is a result of many factors in the rooting area as substrate quality, plant activity, seasonal influents, fertilization ratio’s and irrigation water quality. Growers need to frequently check the pH to determine whether they are maintaining a proper acidity level or not.

Measuring pH

pH is measured on a scale of 0-14. A substrate or water pH reading below 7 is considered acidic, while a pH reading above 7 is basic (alkaline). A pH reading of 7 is neutral. The pH scale is logarithmic, which means that a pH reading of 6 is 10 times more acidic than a reading of 7.
You can measure the pH of your substrate, your spray tank water, or your irrigation/fertigation water.


Crops need careful pH management to maintain their best quality and appearance.  
A pH that's too high or too low can make disease, insect and weed problems worse.

Spray tank water

If your spray tank water is too acidic (low pH) or too basic (high pH), the pesticides you mix in can be deactivated and may even burn your plants.
Irrigation/fertigation water: The pH of water you apply to your plants should more or less match your desired substrate pH. Otherwise it will gradually change the pH, which becomes problematic just before sales. And because the problem goes up slowly, it can only be repaired slowly.

Desired pH levels

A normal pH varies by plant type. Generally, a pH between 5.0-6.5 is good. Some crops need something specific. Ericaceae needs for instance a pH lower than 4.0.
When you set a pH goal for your crop, you need to consider that acidic fertilizers can be used to lower pH whereas others work more alkaline. How quickly and how much the pH will change, is depending on the type of fertilizer applied, your substrate type and your water quality. For this reason, it is advisable to buy a good peat-based substrate from a reliable supplierf or your pot cultures.  And use a good quality handheld pH meter for the regular local testing. 

Testing your substrate (pot production) on pH yourself

I will now explain how you can test your substrate yourself. 

  1. Sample

When taking samples, use a probe or trowel, and a clean plastic bucket for collecting and mixing samples. Make sure that you use clean tools.
Take from 15 to 20 plants samples from random locations throughout the field and mix them together in the plastic bucket. Do not take samples from only one side of the field, only from the corners of the field, or from the same spot on each side of the field. Also, avoid taking substrate samples near lime or manure piles, fresh fertilized rows, low spots, fences and roads.
Take them from the total rooted area in the pot. 
  1. Mix

Mix the samples firmly to make one homogeneous lot.
  1. Measure

Take 1 part substrate from your mix and add 1,5 part demineralized or distillated water. For instance: take 100 ml substrate from your sample.  Press it lightly and add 150 ml water. 
Be sure you have both from substrate and for water proper proportions.
  1. Shake and wait 

Stir or shake the mixture vigorously. Then let sit for 1 to 2 minutes.
  1. Test

Turn on your pH meter, be sure you have calibrated your meter before running the test  (see owner’s manual). Remove the cap to expose the sensor and dip the sensor completely in the solution. Record the reading displayed on the meter.


This is a quick method you can do yourself, but be careful with the conclusion, the results can be varying quite a lot due to inaccuracy of the action. In case pH is an issue, I would advise you to use a good specific horticultural laboratory. 
If your pH is constantly out of balance, ask your substrate supplier or technician for a good advice how to solve this, and - even more important – how to avoid this in future.
Because only happy plants make a happy grower. 

Lack of iron
Lack of iron 2

Lack of Iron (Fe) due to too high pH in Petunia and Vinca.

MTS micronutrient toxicity syndrome due to too low pH.
Availability per element
Availability scale per element per pH level.
Overview pH
Overview of solubility of elements at different pH.


Ben Geijtenbeek
Syngenta Flowers