Fertiliser precipitate and biofilm in the water system can create a breeding ground for diseases and clog the system. Tomato nursery Varom is tackling these problems with a combination of a polyphosphate and biocide, focusing primarily on prevention.

The water system at brothers Guy and Dennis Van Rompaey’s nursery in Rijkevorsel in the Belgian horticultural area of Hoogstraten has grown as the business has expanded. “Originally we had a unit for 4 ha. When we added another 2.7 ha in 2008 we decided to use the same unit to service the whole area. For that we need a day’s supply, which we make up at night. It’s all a bit tight but we just about manage,” Guy Van Rompaey explains.
But this day’s supply has turned out to be a weak spot in the system. Bacteria can grow in the still water, giving rise to localised problems such as excessive root growth caused by Agrobacterium rhizogenes. “Two years ago we had such a severe problem that we suffered noticeable harvest losses,” he says. The brothers grow truss tomatoes (Merlice) and Coeur de Boeuf.

Two-pronged approach

Bacteria formation in the water system is a complicated matter. Fertiliser precipitate is a source of nutrients for biofilm, a slimy layer of microorganisms that are not hazardous in themselves. But the slimy layer forms a breeding ground and refuge for diseases. The problem can be tackled with a two-pronged approach: by preventing precipitation and by killing the biofilm-forming organisms and pathogens.
“At the end of the season we always used to clean the whole drip irrigation system through with nitric acid and bleach. We left each of them in turn in the system for 24 hours and then flushed it through. But nitric acid contains nitrogen which we aren’t allowed to discharge any more, so it’s better to avoid bleach if you can,” the tomato grower explains.
So he sat down with Guy Pluym, an advisor from suppliers Sanac, to take a closer look at the options. Besides the situation on site, there were also statutory measures and market developments to be taken into account.

Stricter laws

The EU Nitrates and Water Framework Directives are implemented in national law differently in the Netherlands and in Belgium. In Flanders, the Fertiliser Action Plan (Mest Actieplan, MAP) prohibits discharges of nutrient-rich water from greenhouses. There is a phasing-out plan in place for discharges in the Netherlands. So the law is currently stricter in Flanders than it is in the Netherlands, requiring Belgium growers to take action to be able to recirculate water problem-free.
“There is also pressure on the use of chlorine. The Knowledge Centre for Sustainable Horticulture in Belgium advises against the use of disinfectants containing chlorine unless the grower is certain that they won’t exceed residue limits if they do so. This advice was issued after the trade found chlorate in vegetables and fruit,” Pluym says.
Chlorate is a point for concern in Germany as well, where there has already been an outcry over this issue.


Having considered all the options, the grower and the adviser decided on a combination of the polyphosphate VitaPhos K and the biocide Huwa-San. The polyphosphate goes a long way towards preventing fertiliser precipitation and can carry any existing precipitate with it in solution down to the plant roots. The biocide kills the microorganisms.
Phosphate fertilisation can be done using two forms: orthophosphates and polyphosphates, Marco Molenaar of VitaPhos suppliers Haifa explains. The first form can precipitate with higher pH values and a high EC together with other elements such as magnesium, calcium and trace elements, making them less available to the plant. “You can counteract this by replacing some of these with polyphosphates. Not only do the elements remain in solution better, they are also available for uptake by the plant,” he says.
The ratio between ammonia and nitrate nitrogen also plays a role in the uptake of the various elements. The first has an acidifying effect while the second increases the pH around the root medium. Using more ammonia can encourage uptake of various elements. “We do that as well, but only to a limited extent,” the grower says. “We are very wary of blossom-end rot, you see. Coeur de Boeuf is extremely prone to that, and we are worried that a higher proportion of ammonia will cause problems.”

Blossom-end rot

This is a common concern among growers, Molenaar finds. “And yet we still advise people to use more ammonia, although you can of course stay on the safe side. But to avoid blossom-end rot it’s just as important to keep an eye on the potassium to calcium ratio. We recommend 4:7 or 5:8, in other words less potassium than calcium. Preventing precipitation while encouraging uptake at the same time therefore calls for an approach that not only involves phosphate but also nitrogen, potassium and calcium,” he says.

No chain reactions

The biocide they opted for is Huwa-San, a stabilised hydrogen peroxide. “Plain hydrogen peroxide also works as a disinfectant but it reacts away too quickly in the vicinity of organic contamination. So you will never get enough in the system to disinfect it right to the end,” Severina Windmolders of the product suppliers Roam Technology explains. “Our product is stabilised with a positively charged silver ion. The bacteria in a biofilm are negatively charged so they attract the positive charge. A reaction therefore only happens at that spot. You don’t end up with chain reactions as you would with ordinary hydrogen peroxide, and proportionally more active hydrogen peroxide passes through the system right to the end.”
Adviser Pluym confirms that there will still be active biocide at the end of the process. “If there is a big difference in levels between the end and the beginning of the system, that means your system is highly contaminated,” he says.

Starting early

The grower applies the mixture in three places: in the drain water, the mixing tank and the watering pipe. He does this to be on the safe side, as he is working with a day’s supply. He now has one-and-a-half seasons’ worth of experience to look back on. “We haven’t counted how many mats are affected by it, but I do think I had less damage from excessive root growth last season than two years ago,” he says. “I would have preferred to have been completely problem-free by now, but we are going in the right direction.”
Measurements show that Agrobacterium infections are significantly reduced, however. It may be some time before the old contamination has disappeared completely from the system. Windmolders emphasises the need to start early. “A few years ago, growers tended not to start disinfecting until around March or April,” she says. “We have taken measurements at nurseries and have seen quite a high degree of bacteria formation in still water as early as in January and February.” Pluym adds: “So it makes sense to start disinfecting when you fill the mats.”
The introduction of the system at the nursery was accompanied by regular measurements and advice. “This support is extremely useful,” Van Rompaey says. “The problem won’t be solved straight away, but we can see that it’s going in the right direction. I hope we’ve got a grip on it now.”


The Belgian tomato nursery Varom uses a combination of a polyphosphate and a biocide to control precipitation and biofilm formation in its water system. This method was chosen because of the risk of disease, the legal requirements and market demand. Disease problems have eased since they started using the new approach.

Text: Tijs Kierkels. Image: Wilma Slegers