Jan 10, 2017
Top principles for growing strawberries in substrate


The first modern production of strawberries in substrate started in Holland in the early 1980s. Early strawberry production in glasshouses suffered from soilborne diseases, mainly Phytophthora cactorum and verticilium. Methyl bromide was no longer allowed and other fumigants were expensive or not effective.

We started off with bucket (5–10 liters) cultivation with early varieties like Primella and Karola.

In this period, also the new variety Elsanta was introduced. It’s a fantastic variety that still is the most-grown variety in northern Europe. The big problem with Elsanta, however, was the susceptibility for a lot of diseases (P. fragariae, P. cactorum, etc..). Luckily, Elsanta performed very well in substrate cultivation.

BVB Substrates introduced the growbags for strawberries in 1987. This made the handling and growing process much easier than in buckets. In the early 1990s, soilless cultivation became the standard for glasshouse strawberries in Europe. All glasshouse strawberries are now grown out of the soil. In 1990 the growbag system was also introduced in Switzerland. In the mid ’90s, we saw a strong development in France and Italy, and a few years later in the UK.

In this period, the whole system was modified by growers and suppliers to a whole range of different systems, varying from growbags on raised beds up to all sorts of pots/containers on wire systems or steel gutters.

Why growing in substrate. While these systems were being developed, other advantages than only preventing soilborne diseases emerged. Nowadays, for many growers, soilborne diseases are no longer the main reason to start these production methods. Growing strawberries in substrate has many other advantages over soil plantings.

Labor. Substrate grown strawberries produce higher yields and better quality. With table top systems, the berries are ideal height for the pickers. Modern systems separate the fruits from the leaves. The fruits are very well exposed. Not only does this lead to increased picking speed, but it also leads to much better labor conditions.

Yield. In substrate growing systems, the plant density is mostly much higher than in soil plantings. This leads to a higher assimilation surface per hectare. Higher assimilation leads to higher yields, and better quality. In table top systems, all berries are hanging free in the air. There is less loss of berries due to botrytis or anthracnose but also from snails or birds.

Reduced use of pesticides. The use of fresh clean substrate and the protected environmental conditions of the crop can reduce the use of chemicals dramatically. Yields are typically higher in substrate systems. When spraying fixed rates per hectare, the usage per kilogram produced berries decreases.

Water and fertilizer. Most systems collect the runoff water. After disinfection, this water and fertilizers can be reused. This brings massive savings in water usage per kg of strawberries.

Infrastructure. By growing in substrate, the grower has the assurance that he can grow on the same spot every year again. This makes it worthwhile to invest in a good infrastructure with paved roads, high tech irrigation systems and stable, low labor tunnel structures.

Growth control. The limited root volume enables the grower to react very quickly to weather changes and it is also possible to steer the plant into vegetative or generative stages. Because the irrigation and fertigation have a very direct and controllable effect, it is possible to increase fruit size and to improve fruit quality.

Limitations. Of course, the cultivation of strawberries in substrate also comes with some limitations. The initial investments are higher than traditional soil planted crops. The grower will have to adapt new knowledge for growing in substrate and for steering the crop with the right fertilizer schedules – EC levels – pH – irrigation volumes and other new parameters. The water quality, that is already very important with soil plantings, is even more delicate in substrate cultivation.

Water quality. One of the most important conditions to grow in substrate is the availability of sufficient water of good quality. Before even thinking of starting to grow strawberries in substrate, please first have your fresh water analyzed. Important parameters for judging the water quality are EC, pH, bicarbonate, ballast salts and iron.

  1. EC. Conductivity (EC) is a useful parameter for the total dissolved salt content of water. These salts can either be useful nutrients like calcium, magnesium or sulphate but they can also be ballast like chloride or sodium. Sodium and chloride can become toxic to plants when present above certain levels. It is important that all these salts and the total EC level are taken in account when composing the fertilizer mixture.

pH. The right irrigation water pH is essential for good availability of the nutrients. Growers can be misled in thinking the pH of the fresh water is useful to calculate the amount of acid that is needed to get the right pH. However, the absolute pH tells not even half the story. Much more important than the pH is the concentration of bicarbonate in the water that needs to be neutralized by acid addition. As long as bicarbonate levels are present, the pH will not change significantly when dosing acid. High bicarbonate levels are commonly found in bore waters. Very high levels may make it necessary to pretreat the water before use.

Iron. Iron is common in bore or ground water. Water containing iron above 10 micromol/lt can, upon exposure to air, change to iron oxide which is a common cause of blocked drippers and brown staining on surfaces that it comes into contact with. Water with high iron levels needs to be de-ironed before use.

Substrate. The substrate that is used for the strawberries needs to meet several conditions. The substrate is the base to anchor the plants. It must take up and hold the water and nutrients and also set these available for the plants when it is needed. The tension of binding and releasing the water and nutrients is very important. To create a good and active root system, the roots must be provided with enough oxygen.

Therefore, it is essential that the substrate contains enough pores of the right size to allow a free air exchange throughout the whole pot volume. For long lasting optimum characteristics of the substrate, the durability or structure stability of the substrate is a key factor. We can use a wide range of raw materials to design this optimum conditions. Important parameters to design the optimum substrate are AFP (air filled porosity), WOK (water uptake characteristic), water release pattern, nutrient absorbrelease pattern, chemical stability, structure stability and more. The most important raw materials that are used in the soft fruit industry are peat, coir and perlite.

Peat. Peat is bog moss that has decomposed under low-oxygen conditions. Depending on climatic conditions and age, the structure ranges from more or less decomposed plant remains to a fine dark mass. Peat is produced primarily in the northern hemisphere, from Ireland to Russia and from Sweden to Canada. All these different origins have peat with their own specific characteristics. Also the harvesting method as well as the handling and screening have a big effect on the physical characteristics of the peat.

For instance, the vertically harvested sod peat gives a much better structure stability than the horizontal harvested milled peat and older, more composed black peat has a lower air content than younger white peat. The humic acids in the peat act as a pH buffer, meaning that it stabilizes the substrate against sudden change in pH. This function is important since substrate pH controls nutrient availability for plants. As their name suggests, they are weak acids that donate hydrogen ions to the solution if the pH rises too high and pick up protons if it falls, thereby buffering against pH change.

Coir. The interior of the outside husk of the coconut is filled with fiber and a “coconut pulp.” This coir pith, also known as coir dust, can be used as a growing media. Coir has a relatively good structure stability and is very easy to handle. Coir has a big CEC (cation exchange complex). This is a negatively charged complex, that is naturally saturated with sodium (and potassium). Calcium and magnesium however have a stronger attraction to the coir complex.

Fertilizing these elements can lead to lockup in the complex, while sodium and potassium will be displaced, come into solution and be taken up by the plant instead of calcium and magnesium. This leads to all sorts of crop problems – for example, excessive salt uptake to calcium deficiencies. Therefore, it is of great importance that the coir is always pre-buffered with calcium and flushed after that. Coir has a very low pH buffering capacity. This makes coir, used as a pure substrate, very susceptible for pH related nutrient deficiencies.

Perlite. Perlite is a volcanic glass (silicon dioxide) that is mined in Greece, Turkey and Italy. When heated the structure softens and the enclosed water vaporizes and causes the perlite to pop-up to 15 to 20 times its original volume. Perlite has a very high structure stability. It is used as a component in substrates where it provides aeration and increases the speed of moisture retention.

Substrate production. At BVB Substrates we produce about 2,5 million cubic meters of substrate every year. The production process is fully computerized. Every individual batch has its own unique recipe and is checked by our in-house state-of-the-art laboratories. Together with our R&D department, researchers, consultants and of course the growers, our advisory team designs substrate recipes for all crops and growing systems.

We have 30 years of experience with substrates for strawberry production. The cheapest substrate options are milled white peat mixtures or straight coir. Non-compromising high-quality growers use mostly sod peat mixtures (with or without perlite addition) or recipes containing coir, sod peat and perlite.

Structures. Over 95 percent of the substrate grown strawberries are protected by some kind of covering. This can vary from simple single row plastic covering up to high tech glasshouses. Generally, we can say that bigger volumes and higher top-ventilation capacities lead to the best crops.

Systems. We can distinguish three basic set-ups. The simplest systems are the soil-contact systems. These are bags or containers on the ground or on ridges or substrate-filled elevated-bed systems. These systems are widely used in Germany because they want to combine the advantages of production in substrate with the advantage of earliness thanks to the soil contact. As soon as the substrate is used without soil contact, some earliness is lost. The big disadvantage of these soil contact systems is of course that the strawberries are growing on ground level. Picking efficiency and labor conditions are the same as in soil plantings.

If earliness is not the major issue (i.e., everbearers) or if additional heating is available, it is always more economic to produce on a higher level. In basic polytunnels, we use mostly the table top or high bed system. Poles in the ground support the substrate carrier. The height of these systems vary from 80 cm for picking while seated up to about 1,4 meter for stand-up picking. If the construction is strong enough, the best way to support the substrate system is by using a hanging system. Hanging systems make it easier to move around in the production area. Transporting substrate, plants, fruits and spraying equipment is easier if there are no obstacles in the way. Also putting a ground cover or moving around with mowers is better without the poles in the way. Please bear in mind that a complete growing system gives a huge weight pressure on the construction.

Substrate carriers. There are different systems for holding the substrate itself. We see the growbags, pot/container systems and substrate-direct-systems. It is important to have a good free draining system. In some substrate-direct-systems, the drainage channels are very small and they tend to block very easily. The roots could come into direct contact with the run off water. Substrate direct systems need special machinery (or a lot of labor) to fill. They can not be used for crops that need to overwinter at minus temperatures and they offer no flexibility in production scheduling.

Pots and containers have an open surface. This is initially an advantage because of the good microclimate directly after planting. Later on this can turn into a disadvantage when humidity gets too high or fruits can lay on top of the substrate. Therefore, container systems with open tops should not be too wide (less than 18 centimeters). This makes it easier to get all the fruit away from the substrate surface. Filling the containers should be mechanized to ensure all containers get exactly the same volume.

Growbags do not have this problem. They are mostly a bit wider (20–26cms), giving the plants a bit more space. Growbags are ready filled, drainage- and planting holes are pre perforated on demand. Growbags are however more expensive, must be cut open after use and leave a lot of plastic waste.

We have a preference for systems with higher connected volumes. Ten plants in 20 liters will perform better than one plant in 2 liters. Moisture and nutrient availability are more constant and the plant roots have more space to grow. Unequal drippers and different plant sizes are compensated better in longer substrate units. Longer substrate units also allow more flexibility in steering the plant density.

The volume of the substrate may vary from 15 up to 30 liters per linear meter.

Irrigation. Based on the substrate quality and quantity, the optimum watering volume per irrigation loop can be calculated. The frequency of these irrigations depends on a complex of parameters like temperature, humidity, radiation, plant size, growing stage and many more.

Fertilizing. All water and nutrients need to be given by drip irrigation. The influence of the water quality on the final irrigation solution is very big. All elements that are present in the fresh water and also the EC of the water must be taken in account when composing the fertilizer recipe. The higher the EC level of the fresh water and the lower the EC level of the required irrigation, the bigger the influence of the fresh water composition is on the final irrigation water. Changing irrigation EC levels do not only change the total amount of required fertilizer but it also changes the required ratio between the nutrients in the fertilizer.

Technical installations. Water and fertilizer needs to be dosed very precisely. Depending on parameters like climate, variety and crop situation, the required EC level varies between 1,2 and 2,0 and the required pH is commonly between 5.5 and 6,0. To cope with these changing requirements and sometimes also varying water composition, the use of proportional dosing systems is not advised. The irrigation system should have an automatic EC/pH control with the so-called A and B tank system with a separate acid dosing system.

Plant material. When substrate systems are installed, this needs major investments. We need to avoid any risk of failure where possible. Good plant material is essential. We want to use strong, homogeneous, healthy plants with a high production potential. To guarantee plant health, we see an increase of mother plants grown in substrate and young plants tipped in all kind of trays or plugs. Especially with the use of short day high chill varieties, the production potential at the moment of planting is very important. Therefore, we use more and more tray plants (European tray plants, with designed flower mapping). In the glasshouses, this is a standard for many years already, but nowadays they get more popular in low cost structures as well.

Conclusions. The way forward for soft fruit production means, for many growers, growing in substrate. Mostly, the start of growing in substrate is initiated by the ban on chemicals for soil disinfection. Over the past decades, we have learned, however, that this is not the most important advantage of the substrate systems. The main reason to grow in substrate and also the choice of how to grow in substrate may vary per region, per market or per grower. The substrate production has outgrown its infancy — we left the trial and error period behind us. To all growers that want to start to grow in substrate, we are there to help.

Eric Boot, BVB Substrates, Swissgrow






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