In Part 1 of this series, we explained how microbial life develops in stone wool, and shared the latest insights into the various functions of ‘good, bad and ugly’ microorganisms. Clearly, it is beneficial to actively manage the microbiome in a controlled way to contribute to a stronger, healthier, more resilient and more productive plant. But how can growers influence the microbial balance in the root zone? And what are the advantages of stone wool in this context? Read on to find out.
“A greater focus on the microbiome supports the shift to resilient growing, as registration for traditional plant protection products become scarcer for niche use cases such as in greenhouse horticulture and in the longer term could hold the key to steering a crop to even higher yields and faster cultivation cycles.”
Figure 1. Principal components analysis (PCA) of bacterial community structure in stone wool and coco coir with addition of biochar or spent mushroom substrate (SMS) during cucumber cultivation. Results were obtained within a PPS project Rootzone supported financially by Dutch Topsector Starting Materials & Horticulture , Club of 100 of Wageningen UR Greenhouse Horticulture & Flowers Bulbs and Foundation Knowledge in greenhouse (KIJK).
The root zone microbiome changes over time in both organic and non-organic growing media such as stone wool, according to Marta Streminska, a microbial ecologist based in the Wageningen University & Research , Business Unit Greenhouse Horticulture and Flower Bulbs (WUR) in Bleiswijk, the Netherlands: “The plant itself actively steers this to some extent by producing different root exudates to attract the most beneficial microbes at different times of the cultivation cycle (Part 1: An introduction to the root zone microbiome). Many people don’t realise that root exudates are not merely a waste product from photosynthesis. Instead, the plant intentionally sends 20-30% of the products of photosynthesis to its roots in order to feed the microbes.” So in effect, the plant plays an active role in shaping its own root zone microbiome.
Influencing the abiotic factors
Clearly, however, it would be beneficial if growers could give the plant a helping hand by being able to control the microbiome. One way they can do so is by influencing the various abiotic factors – such as temperature, pH, oxygen and water content levels that can directly affect the composition and functions of the root zone microbiome. “Different microbes prefer different temperatures, but happily many grow very quickly at 20 to 30 ⁰C, which are common temperatures in high-tech greenhouses. Fungi also influence the pH of the system by degrading complex organic matter. That is why in nature soil with high carbon content like forest soil very often have low pH” comments Marta.
“Meanwhile, bacteria can thrive in different ranges of pH, with some specific groups preferring pH levels of 7 or 8. But as a general rule of thumb, just as with plants, stability is key for a healthy root zone microbiome. Avoid big fluctuations in moisture or temperature that can cause stress,” she emphasizes.
Advantages of stone wool
Stone wool offers several key advantages in this context. Firstly, it is a highly steerable substrate, enabling growers to precisely manage and control the key abiotic factors affecting the microbiome in the root zone, such as WC, EC and pH.
“Additionally, unlike organic substrates such as soil or coco, stone wool does not naturally contain a lot of complex organic matter such as cellulose and lignin. As mentioned this is preferred by fungi, because they produce extracellular enzymes which break down this complex compounds. In fact in stone wool growing media fungal populations are often outcompeted by bacterial population as the later thrive on simpler less complex compounds such as glucose and organic acids which are excreted by the plant roots. Still some fungi and oomycetes survive also in stone wool”. Explains Marta.
“Moreover, in contrast to soil and coco peat, stone wool does not come with a ‘history’; it is clean after manufacturing and it doesn’t contain microbes left behind from the previous crop activity. Therefore, stone wool gives us the opportunity for a clean start. But of course, you have to take the right measures to ensure that the system remains free of bad microbes, by using healthy plant material and a pathogen-free nutrient solution. Remember if you want to actively manage the microbiome, you need to start as soon as you put seed in the stone wool propagation block; don’t wait until halfway through the cultivation cycle,” she advises.
The importance of oxygen
The most important factors affecting microbial growth are availability of organic carbon and oxygen, she says. “However it is important to understand that microbes do not always rely on oxygen for respiration they can also use nitrate or sulphate. So if there is too much water in the substrate they will use the remaining oxygen quickly and then revert to nitrates or sulphate. This will then reduce the availability of nutrients for the plant. That’s why it’s important for growers to maintain sufficient oxygen levels in the root zone by avoiding over watering not only for the plant’s roots, but for the microbes as well.” However, it is important to understand that anaerobic respiration doesn’t start until the oxygen levels in the root zone have dropped well below 1 mg O2/l. Microbes only switch to anaerobic respiration when they have no other choice. Respiration with oxygen still provides the most energy,” adds Marta.
This is another area where stone wool growing media can be advantageous, because it provides the opportunity for optimum control over the water content (WC%) in the root zone. Moreover, stone wool is designed to distribute the water and therefore the oxygen uniformly throughout the whole volume of the growing medium. “This is important, because research has shown that while the rhizosphere, . the area within approximately 2mm of the roots has the largest number of microbes, the microbial diversity can be actually higher beyond that area and the plants will select specialised microorganisms from the whole root zone to colonise their roots. For a healthy crop, you therefore need to keep all of the good microbes in the whole root zone active, not just the ones closest to the roots,” states Marta.
Can beneficial microbes be added?
Taking things a step further than abiotic steering, is it also possible to positively influence the microbiome by adding mixtures of beneficial microbes to the feed water? “Some growers are already doing that and there are various probiotic products on the market, but they have varying effectiveness,” replies Marta. “The products must be compatible with both your plant species and your system. In some cases, you may find that Bacillus or Trichoderma remain as resting spores rather than being active in the system. The added microbes have to establish first in the cultivation system. Sometimes they are unable to do so because there is not enough food source available to them. Also some microbes cannot survive in certain cultivation systems as they lose to competition from the microorganisms already present in the system,” she continues.
Probiotics for plants
“That’s why we’re now studying ways to positively influence the natural microbiome instead. Because if the microbes are already there, we at least know that they can survive in the system. So if we can give the good ones something extra to feed on, similar to how prebiotics work in humans, we can encourage the dominance of beneficial microbes in the root zone,” adds Marta. “Then by maintaining stable root zone conditions the root exudates can do their work.”
All about balance
Despite considerable fundamental research on soil microbiomes in recent decades and strong progress of microbiome research in soilless growing media in the past five or six years, there is still a lot to learn about the complex world of the root zone microbiome. “However, it’s now clear that a plant can’t function without microbes, and simply disinfecting everything isn’t the way to go because it kills off the good ones as well as the bad. Moreover, there is a lot of variation between the efficacy of different disinfection methods,” states the microbial ecologist. “Instead, the aim is to promote disease suppression in the rhizosphere and wider root zone. This is all about balance; ‘bad’ microbes in the root zone microbiome are not a problem, as long as the system is stable enough so that the ‘good’ microbes and the plant’s own responses, prevent disease from taking hold.”
Therefore, a greater focus on the microbiome supports the shift to resilient growing, especially in view of the ongoing phase-out of chemical-based plant protection products in Europe and elsewhere. This is a good fit with Controlled Environment Agriculture, particularly in combination with stone wool. As a pathogen-free growing media, Grodan’s products not only provide a clean and consistent start for the crop, but also support the grower in actively managing a healthy and balanced microbiome by steering the abiotic factors such as water content, pH and oxygen levels.
Potential for higher yields and faster cultivation
“In fundamental research, it has already been seen that some microbes can trigger the production of plant hormones leading to more generative plant growth. In other words, in the longer term, microbes could hold the key to steering a crop to even higher yields and faster cultivation cycles. Placing more emphasis on the root zone microbiome has the potential to improve the productivity of horticultural practices in the future,” concludes Marta.
