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How to make the most of your substrate

We are often confronted with growers’ biased and contradicting opinions on which substrate is ultimately the best. Whereas some swear on seeding substrate based on the finest white peats, other growers say that is the worst substrate ever, and praise black peats mixed with various materials (perlite, sand, etc.). These varying opinions stem from a lack of knowledge (or understanding) of the properties of individual raw materials, their application or shortcomings, and under-, or over-estimation.

Substrate production is still predominantly based on peat. It either forms a 100 % of the base – in pure peat substrates – or is accompanied by some other components enhancing or modifying its qualities. Propagation stations, sowing and cultivation of pot plants in greenhouses need substrates with high content of quality white peat. On the other hand, substrates with black peats, composts and other mineral components are better-suited for outdoor use.


White peat – this peat is only partially decomposed. There are many guidelines for choosing the best one. By extraction method, for example (block peat is considered of better quality than milled peat), by geographic position of the peat bog (the more north, i.e. the cooler climate the better), etc. Despite all these general principles we can get top quality peat that does not come from such sources. The quality of the peat must be always judged by the actual material.

As mentioned earlier, the highest quality of the used peat will be appreciated mainly by modern greenhouse growers, for propagation, sowing or potting plants. The difference from standard quality of the white peat may demonstrate in controlled cultures with 100 % optimum of all factors. If any of the factors is not in its optimum during cultivation, this difference may not demonstrate fully (e.g. temperature reduction by 2°C than optimum, or pattern fertilization without continuous monitoring of the used-up nutrient contents – i.e. not just conductivity = salt content, followed by correction in fertilization – has a significantly greater impact on the plants).


White peats are characterised by their great capacity to absorb water. These substrates require completely different irrigation. The more water they get, the longer the interval between watering. This is an advantage with top irrigation and reduces the risk of leaf fungal diseases. The substrate that does not soak up too much water needs to be watered more frequently. Leaves receive a lot of water and in humid air they may quickly develop fungal diseases. Growing plants in pure peat may be considered almost hydroponics. This means that all nutrients for the growth and development must be added via continuous fertilization.


However, the properties we value may be also risky. Many growers save electricity and therefore don’t maintain the optimum temperatures. It is necessary to watch out for weather and if a spell of cold and cloudy weather is expected, irrigation must be reduced in advance. A substrate fully soaked with water, in cool weather and without intensive sunlight, almost never dries up. In those days you have to heat up the greenhouse to a higher temperature and ensure ventilation, otherwise the plants stop growing and may die. Hand potting uses up more of such substrate as it gets compacted vey easily. On the other hand, machine potting followed by irrigation may result in the fluffy substrate settling too much, leaving an empty space in the pot.


Substrates have very low specific gravity, which is an advantage when importing substrates and shipping plants. But for nursery professionals and outdoor production this is not a welcome trait. Light pots are easily tipped over by the slightest breeze. A pot that lies on its side does not get any water and causes production losses. This problem can be sorted out by wind-breaker nets stretched across the direction of the wind and not far apart, by choosing special pots with broader bases, or by tying up the tall plants onto the top frame and introducing drip irrigation.


The structure is a key advantage of the white peat. Quality peat retains its structure throughout the entire crop cycle. This becomes useful with the ever more common tidal irrigation. The substrate does not get muddy and keeps its air content – pushing air out leads to root rot. Another option for increasing the lift of the substrate is to use peat in different size fractions. Substrates for tidal irrigation tend to have a greater content of coarser particles, which enhances the volume of non-capillary pores and faster drainage of surplus water.


The 8 – 20 mm fraction is used for most cultures. 20 – 40 mm and coarser fractions suit larger pots and plants that need a substrate with more air. With the increasing production of larger plants in nursery produce (2 litre and larger containers) there is greater demand for substrates with a coarser fraction (usually 20 – 40 mm or coarser). Of course these substrates do not tend to get muddy, and in rainy years the plants’s roots don’t rot. These substrates need to be watered more frequently. Higher air humidity combined with quick drainage of surplus water makes these substrates prone to leaching nutrients. Nursery professionals prevent this by using higher doses of cote-type controlled release fertilizers or by frequent feeding with low concentrations ofliquid fertilizers.


Black peat – in terms of structure it is the opposite of white peat. Black peat is highly decomposed peat with fine structure. Fine structure and ability of the particles to stick to each other comes handy when you need to fill trays with very small cells, and also for bag-grown seedlings. Gardeners, especially of the older generation, like the fact that they can quickly see whether the substrate is moist or dry. Due to lower water-holding capacity this type of peat tends to dry faster.

However, substrates based on black peat tend to get muddy if watered too much and cause root rot due to lack of air inside the substrate.


Clay materials are important components of peat and other types of substrates. They significantly increase the buffer capacity of the substrate (i.e. ability of the substrate to dampen nutrient or pH fluctuations caused by fertilizers or water), which is practically absent or minimal in pure peats. Long-lived crops benefit most from the use of clay minerals that stabilise the chemical properties of the substrate. Clay minerals also facilitate the fixation of the nutrients within the substrate. If irrigation exceeds evaporation, a buffer eliminates the loss of nutrients by leaching. Buffers also play a key role in modifying the moisture regime of the substrate and facilitate absorption of water into dried peat substrate. Peat substrates with clay are “more dense” – they have a higher volume gravity and therefore are not easily squished during potting.



Horticultural substrates are made mainly with two types of compost. Bark compost and green compost. The great advantage of the bark compost is that it is a standard material with stabilised properties. Green compost tends to be more varied, but also richer in nutrients.

Substrates enhanced with compost have better buffer abilities and they release the nutrients slowly. Quality composts pose no risk of biological nitrogen fixation. Compost is a source of readily available nutrients and trace elements. A high amount of nutrients is released during cultivation. Many experiments proved that e.g. in ornamental nurseries with lower fertilization intensity you can achieve larger and better-quality plants with this compost.


Other inert components may modify some physical properties. Sand, for instance, increases volume gravity and facilitates absorption of water by dry peat. Perlite increases aeration of materials without defined structure. Wood fibres are still considered an alternative to peat although they may significantly increase aeration and control substrate’s water capacity. Substrates with these fibres are usually not prone to water-logging. They drain quickly even after heavy rains or watering and ensure quick access of air to roots. Relatively quick leaching of nutrients from such substrate is one of the drawbacks.

As you can see on these several examples, every raw material has a number of advantages, which is why it is a good idea to use substrates containing a combination of different materials in order for the substrate to meet the needs of the plant as well as of the applied technology. It is the grower’s choice whether he will utilize these advantages or whether they will become risk factors influencing production.


Ing. Josef Vydlák
Technologist, AGRO CS a.s.

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