We’ll start of by looking at the different types of potting mix available for home growers and we’ve written a section about different types of peat. And we’ll tell you a little about the RHP, the industry’s certification organisation. But we have so much info for you that it won’t all fit in one article! In the next part we’ll look into the structure of potting mix and the different products obtained from the various methods of cutting peat. On top of all this useful information we’ll be giving you some valuable tips to reduce disease and increase yield and we’ll be drawing up a list of FAQs and growing tips.
Not All Potting Mix Is Suitable For Cultivating Short Cycle Crops!
Before we enter into a detailed explanation about potting mix, we should make one thing clear. Not all potting mix is suitable for cultivating short cycle crops. So you should select your growing medium according to what you want to grow and how long your plants will remain in the potting mix. The three most important factors you should take into consideration are structure, pre-fertilising and pH.
Good pre-fertilising and pH are of vital importance for a successful harvest. Potting mix, rock wool and COCO are the three most important growing media for fast growing plants. The big advantage of potting mix is that it is an easy medium to work with. Potting mix is principally biological and natural which accounts for its great popularity among organic growers. As well as this, when it is used in combination with CANNA nutrients there are almost no residues left in the mix which means that it can subsequently be used as a mix improver in an environmentally friendly way without any problems at all. Potting mix has the reputation for being ‘biological’ and ‘natural’ however this is not entirely justified since it often contains elements such as perlite and mineral nutrients that have been processed. Perlite is an inorganic material produced by an industrial process.
The history of peat
Approximately 12,000 years BC, the glacial tongues from the last ice age retreated northwards; they left a barren, sandy terrain behind. The first vegetation developed, died off and formed the first thin layer of peat, and this can still be found in what is known as the 'dark' layer. A long formative period interrupted by short ice ages around 6,000 and 3,000 BC followed, during which the black peat layer was formed. This marked the beginning of a new formative period during which the white peat layer was able to form. The intermediate layer between the white peat and the black peat is called ‘grey’ peat. The growth or ‘upper’ layer forms the profile’s top layer.
A lot of knowledge available
Peat is the basis for any good potting mix. Peat mainly comes from areas in which the rainwater has a low mineral content and therefore contains few nutrients. Because of this it is necessary to add all the nutrients that are needed for growth to the potting mix. This can be achieved by using specially prepared potting mix nutrients or biological ingredients such as algae granulate, blood meal, fish emulsion, dried cow manure, horn meal, bone meal, and worm casts. Because peat can also take up trace elements, in particular copper, it is important that these should also be added. The amount of trace elements that the peat absorbs depends on its type, sphagnum moss absorbs less than garden peat for example.
Most potting mixes that are available in the shops already contain nutrients and have a healthy level of acidity. This provides a stable root environment and reduces the chance of problems with nutrition. The big advantage that potting mix has is that it is an easy medium to work with. Another reason that makes many people choose potting mix is that, in comparison with rock wool and COCO, for example, there is a long tradition of using this medium and more knowledge and experience is available.
Different types of high peat
There are many different types and qualities of high peat available. The characteristics of peat depend among other things on the depth from which it is extracted, the method of extraction, and climatic circumstances in the peat region. Working through the peat profile from the top down, we will find the following types of peat:
‘Upper Layer’ Of Peat
The ‘upper layer' is the top few feet of the peat profile. According to German peat cutting regulations this layer should be laid on the sandy bed after the white and black peat have been dug out. When the peat cutting area became agricultural land this remaining peat was ploughed deeply into the sandy bed to mix it. At present, permission may well be given to use the 'upper layer' of peat in potting mix. A disadvantage of the 'upper layer' of peat is that it does not always have a homogeneous composition.
Sphagnum moss peat
Sphagnum moss peat is young, partially decomposed sphagnum moss that can retain 10 to 12 times its own weight in water. It has a light colour and is made up almost entirely of different types of sphagnum moss. Because sphagnum moss peat is a relatively young organic material, it breaks down more quickly than older types of peat. Originating in Northern Europe, currently, sphagnum moss peat is mainly used in the expensive sorts of potting mix.
Peat litter
Peat litter or peat dust is extracted from the top layer of the peat profile. The product is light brown and only slightly decomposed. Peat litter can retain at least 8 times its own weight in water. Water uptake and release are slower processes than in sphagnum moss peat. Peat litter is available as fine, normal and coarse the grade depending on the method used for extraction. The fine grade is extracted horizontally with the peat being cut out layer by layer and then dried and harvested. This is easily the cheapest method. In order to produce a larger coarse grade, the more expensive vertical pitch method of extraction has to be used.
‘Coloured’ peat
‘Coloured’ peat is extracted from the layer between the white and black peat layers. This layer has decomposed further than the white layer and its colour lies between the white and black peat. ‘Coloured’ peat can retain less water than peat litter and sphagnum moss peat.
Garden peat
Garden peat is an important resource for the potting mix industry. It is produced by allowing wet black peat to freeze. The quality of the garden peat depends on the extent to which it has been frozen. Freezing black peat improves its water retention qualities and reduces shrinkage. After drying, garden peat can take up at least 4 times its own weight in water. Garden peat is dark brown, which is a good indication that it has already reached an advanced stage of decomposition. It consists of very fine particles, which means it can hold relatively little air.
Black peat (non permafrost)
Non permafrost black peat, which is also known as old peat, champ peat or casing soil peat, is not suitable for potting mix because it shrinks a lot when it dries and subsequently has low water retention levels. If it is thoroughly dried it becomes very hard peat (pressed peat) that can be used as fuel.
Prevent plant damage
It's all in the mix
Peat is naturally quite acidic (pH 3.5-4.5) so lime must be added to raise the pH. Off the shelf potting mix always contains lime. The amount of lime that must be added depends on the composition of the peat. For example, garden peat always needs more lime than peat litter to raise the pH. On average, potting mix needs 5 – 6 kg of lime per m3 (1,000 litre) to increase the pH. After adding the lime it will take a few weeks for the pH to stabilise. If no lime is added, or not enough, high concentrations of elements such as manganese, iron and phosphate will be absorbed and this may lead to signs of over feeding being seen. The concentration of aluminium can rise to toxic levels for the plants, causing root thickening, which in turn will restrict food intake. If the pH of your potting mix is too low, adding 20 gram of lime (Dolocal) per 10 litres will raise the pH by one point. The correct degree of acidity for potting mix is between 5.3 and 6.2. Values higher than pH 6.2 can lead to phosphate being deposited in the form of calcium phosphate, which will make the phosphate less accessible to the plants.
What Is The RHP Quality Mark?
The potting mixes currently available on the market can vary greatly in quality. In the Netherlands the RHP foundation focuses on quality maintenance and control of peat products, soil components, potting mixes and substrates such as coco, perlite, pumice stone etc. Substrates and substrate components that have the RHP quality mark are safe mixes (few weeds and free of disease) but they do not offer any guarantee for a successful harvest. In fact, the RHP quality mark does not say anything about the precise structure and chemical make up of the potting mix. Potting mixes can vary considerably in price. Potting mixes based on garden peat are generally cheaper than mixes based on airy white peat, and it is also true to say that the coarser grades of white peat are many times more expensive than the fine grades.
Better results
As a result of previous research into the proportional balance of water and air in CANNA substrates, CANNA has set up practical tests using potting mixes based on high quality porous basic materials. These mixes were compared with the popular potting mixes containing perlite that are available on the market. In different growing rooms half of the plants were set in a potting mix containing perlite and the other half was set in the airier mix. Climate and feeding was the same for all the plants. After three weeks clear differences could be seen. The plants on the airy mix showed significantly better growth, on average 5 cm more development in the length and they had more robust stems. There were fewer limp hanging leaves during the night indicating that the plant was also receiving sufficient water in the dark, a prerequisite for optimal growth and flowering.
The results of these tests show the importance of making abundant air available to the root environment.
