Substrate composition and quality

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
How to identify good substrate composition, and how you can make this yourself.

Author: Ben Geijtenbeek – Senior Crop Technical Specialist – Syngenta Flowers

Ben Geijtenbeek has over 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”.

Source and physical composition

The growing medium in which we grow plants can be made from several ingredients, like peat, coco fiber, coco peat, composted bark or other organic materials.

Growers base their choice on what’s available on the market in a consistent, reliable and affordable quality. These criteria exclude topsoil and clay, as they do not provide a consistent and reliable growing medium. The most important sources are peat and coco coir. Both have been proven suitable for decades.

A third option is composted bark, sometimes with additives like vermiculite. You can also mix these sources. But keep in mind that plants do not ‘eat substrate’. It is important that the quality is high enough to give the plant what it needs. 

The substrate is a main quality driver and can be made in several ways and mixtures, as long it is controlled roughly according the following standards:

  • Moisture  60 – 75 %
  • Organic matter 80 – 90 %
  • Bulk density 100 – 125 kg / m3
  • Shrinkage (volume change) < 30 %
  • Pores > 70 %
  • Air porosity 16 – 25 %
  • Easily available water 25 – 35 %

Besides this, we never want to have weeds, nematodes, fungi, pests and bacteria in the soil. This is the main reason why local topsoil is out of scope.

Young plant production India pot flowers



RHP has been the European knowledge center for growing media since 1961 ( The RHP stamp, developed in The Netherlands, has been known since as a quality standard that gives plants the best possibility to grow quick and strong. RHP certified substrates are monitored on quality of growing media in the chain, from raw materials production until processing and delivery at the grower. Most of the substrates on the market are made according to these quality standards. Members of the RHP quality stamp can be recognized by this logo.
The closer you reach the above-defined standard values, the better your plants will grow. Notice that the amount of organic material is very high.
Poinsettia regular soil
Poinsettia regular proper composite
Both pictures show the same poinsettia variety, potted at the same date. The left picture shows a plant in regular soil, the right plant is in a proper composite. All other circumstances were kept similar. 

Do it yourself

If you have no option to buy substrates from a reliable supplier, you can make a substrate yourself. To do this, you need to follow this strict procedure.
The first step is to compost your organic material. If you have not fully composted your material or if the composing process was not properly executed, you can end up with all kinds of problems. Plants can be affected by lack of nutrients or can even suffer from poisonous nutrients or a wrong ph.
The composting process works as follows:
  • Buy or collect your organic material at least one year in advance.
  • Put the organic material, for instance leaves or bark on a heap, and make it wet with water.
  • The best is to add some Nitrogen (1-2 kg /m3) as Urea or Calcium nitrate to speed up the fermentation process, which is done by bacteria.
  • Cover the heap with plastic to prevent drying out.
  • After some time, the temperature inside the heap is going up.   
  • Temperature must reach around 60-70 degrees C. After a few weeks the temperature goes down.
  • At that moment the pile has to be rebuild again, and the outside changed with the inside.
  • By mixing it firmly, you give new oxygen and water to the bacteria. Add some more Nitrogen and let the process start again.
  • A good composting process takes around 9 to 12 months.
  • Due to the high temperature most diseases active in the organic material will be killed.
  • Also unwanted organic oils from the bark will be decomposed, as well as insect eggs.  
  • The last step is to mix lime and other fertilizers to balance the EC and pH before potting plants in it.
You need to build up experience to gain proper information on how to use these rules in your local circumstances. Generally speaking, we can say: “The basis of a healthy product is the substrate.”
Compare the quality of composition with your shoes.  You can’t walk a whole day on wrong shoes. This is the same for plants with their substrate. Spend enough time and money for a good base of your production. You will be paid back for this by having less problems later in the culture. 
Example of healthy root growth in good soil.

Water and air capacity

It is very importance that your pot soil contains enough oxygen. The water you give sinks down into the smaller pores of the soil. This means that the bigger pores can get access to oxygen again. But the roots can start to rot when there is insufficient oxygen at the bottom of the pot. To balance this, you need to make the soil lighter with cocopeat chips. Always remember that roots need air for a strong activity!
It makes sense to control the structure of the substrate after a longer period. Even after 5 or 6 months, the soil structure should be comparable with that of the start. Especially in warmer countries, there can be a risk that the decomposing process in the pot continues too much. This makes the substrate more compact and closed (so, less oxygen available for the roots). Even when the crop is ready to be sold before that time, it is important to maintain the structure. Because you want your plant to continue to grow and stay healthy throughout the sales period as well a long enjoyable period with the end-consumer. If the substrate does not meet this goal, you need to use different, less composting substitutes or mixed them in. An example is using bigger cocopeat chips. 

Chemical characteristics as EC and pH

As you might know EC is the number that we use to describe the salt level of the substrate. The EC shouldn’t be not too high at the start of potting, around 0,6 – 0,8 EC. It can go up to 1,0 – 1,5 EC in the substrate later, depending on the crop and growth of the plant. These values follow the 1:1,5 analyzing method.

The second question that need to be answered is the composition of the EC. Does the soil contain just sodium and chlorine, or does it also contain valuable elements as Calcium, Magnesium, Potassium, and Sulfate? You can see these valuable elements as food for plants, that they need to grow. Keep them in line with the needs of your plant, balanced to promote a strong and healthy plant. This topic is of as important to maintain plant quality as the EC and substrate quality. In my next article I will share you more details regarding that subject.

With pH we express the acidity level of the substrate. A neutral pH is pH 7.0. But most plants need a slightly acid substrate with a pH of around pH 5.0-6.5. What happens with a wrong pH?


When pH is too high 

When the pH in the substrate is too high, it’s mostly a result of development of the crop. When a crop is growing strong, especially during the start of the season, the substrates’ pH level can go up easily. This is because roots take up more nitrate-nitrogen, which increases the pH near the roots.

When pH is too low 

Many organic substrates have a low pH of itself, so they are limed during the preparation of the substrate-mix. In this way a substrate with the optimal characteristics is created for the crop. 

Other pH problem cause

The quality of the water plays an important role as well.  Most water used has a too high pH, which gradually increases the substrate pH as well. The problem only becomes visible at the end of the culture, when there is no time anymore to improve. Solution is knowing the water quality and acting accordingly.
The pH of a substrate is the typical parameter that is measured when checking the quality of a new growing medium at the RHP organisation. During the season pH can be low as a result of the start of flowering or when too much acidifying fertilizers like ammonium nitrate are used.
Below an example how the pH can change in the time by plant activity and (wrong) fertilisation regime.
pH Change
Below a short overview about the impact of pH on the availability of all different elements for plants. As you can see, the best strategy is to keep the substrate pH around pH 6.0 to make sure all necessary elements are available and absorbable for the plants. 
pH Range

Hygienic and disease free

The last important topic is the hygiene level of the substrate. Peat and coir are disease free, even as additives as Vermiculite. But bark, when not fully composted can contain insects or eggs from insects, or fungi spores. The biggest risks are with local topsoil. Unless the topsoil is steamed or disinfected before mixing, this can contain problems like weed seeds. But also eggs from snails or all kind of other insects such as black flies, or even nematodes can be found in topsoil. These weeds and insects can have a dramatic impact on root development and plant health.
So, despite the cost-efficient image, in all cases using topsoil turns out to be the more expensive route. Because you will need to be taking corrective actions afterwards to stop problems.
The picture below shows you the impact of a standard mixture, compared with cheap substrate. 
Standard mixture cheap substrate
Same crop, same treatment. The difference is obvious.
Average: 1.7 (32 votes)


Ben Geijtenbeek
Syngenta Flowers