It’s fine for a tomato plant to have a few spots on its leaves. But it’s a different matter when it comes to flowering pot plants. Visible defects detract from the ornamental value and therefore the price of the product. Aspects such as feeding and climate therefore need to be carefully controlled. By taking leaf samples, growers can find out precisely how much of the nutrients they supply actually gets taken up by the plants. They can then optimise their fertilisation based on the data the samples provide.

Yellowing leaf margins, discolouration, speckling or dwarfism – these are just some of the symptoms that indicate an excess or deficiency in the nutrients a Spathiphyllum crop needs. They are all easy to avoid by providing the plants with a constant supply of the right nutrients. But that’s precisely where the trouble lies, because plants’ needs differ depending on the cultivar, the stage of growth, between crops and even between nurseries.

Growers can base their fertilisation strategies on the theory from past research and on their own experience, of course. But it’s only leaf samples that will give them a clear answer to the question “What is actually happening in the plant?”.

Collecting data

And that’s exactly the reason why Olaf van der Voort, a Spathiphyllum grower and breeder based in Honselersdijk, the Netherlands, has been using leaf samples for as long as he can remember. He has collected a huge amount of data over the years which he uses as a reference framework for his current production and any problems that arise. “Because let’s be honest, every pot plant nursery gets the occasional speckling on a plant. The important thing is how you deal with it. Look, around nine years ago, when we joined the Decorum sales organisation, we decided to only sell plants of the very best quality. So I wanted to know what was causing that speckling and what I could do about it.”

To get to the bottom of this, the nursery takes soil, water and leaf samples at least once a month and compares all the results. “The soil and water samples show the current availability of nutrients and the leaf samples indicate what the plant has done with them over the past few weeks. I want to see numbers and collect as much data as possible. Log, log and log again. That gives you a much better understanding of the plant’s processes and enables you to respond faster to any problems.”

Quality inside and out

Delphy House Plant Consultant Aad van Holsteijn understands this only too well. “If you want good quality foliage and flowers, you must make sure you’ve got your nutrients absolutely right. The nutrient supply – fertilisation – plays an important part in the internal quality of the plant’s cells and therefore of its leaves or flowers. Good quality on the inside is important for growing fast. And growing fast is, in turn, an integral part of good production. In other words, if the quality of every leaf is good, you can grow more quickly and therefore increase your yields. You see big differences between nurseries in this area, particularly in crops grown in shade, such as Phalaenopsis, Spathiphyllum, Bromelia and Zamioculas.”

In a fast-growing shade crop such as Spathiphyllum, an imbalance in fertilisation impacts directly on the purity of the leaves. The main causes of visible defects are boron or manganese toxicity, manganese or magnesium deficiency and an incorrect K:Ca ratio, but also potassium or nitrogen deficiency. Constant monitoring is therefore called for, not only to enable you to grow as quickly as possible but in particular to control the crop better, van der Voort says. “You want to be able to deliver a beautiful product all year round.”

Zinc deficiency

In Van der Voort’s case, that’s a lot of different products. The company supplies pot sizes ranging from 6.5 to 17 cm and bowls from 17 to 23 cm, plants in various heights, as well as sleeves, labels and trays. Van der Voort works with his advisors to constantly push the boundaries – always bearing the controllability of the crop in mind, of course. “Capitalising on these small points for improvement can really make you stand out from the crowd,” the grower says. “And the same applies to fertilisation. By keeping a close eye on the take-up figures in the plant, I can quickly tell whether levels of certain nutrients are too low or too high. When that happens, we can adjust our fertilisation and improve the quality of later leaves.”

During Olaf’s father’s time at the helm of the company, the crop suffered from zinc deficiency. That was what prompted van der Voort to work with van Holsteijn. The advisor: “They had random plants lagging a long way behind in growth, particularly in the winter. In addition, the tips of the leaves had distorted colours instead of being uniformly green. We took leaf samples and discovered that zinc levels were very low. They made adjustments to the nutrient supply and the problem disappeared.” Since then, they have been taking leaf samples regularly at Van der Voort. For example, they take a flower or leaf sample of every growth defect as a basis for possible adjustments to the nutrient supply. “But remember, this grower has specifically opted to have soil, water and leaf samples analysed,” van Holsteijn says. “The combination of all three gives the best picture.”

Set protocol

With so many test results, the grower now has a relatively complete picture of what the nutrient levels should be. But as mentioned earlier, the target values depend on a range of parameters. “And there are more and more to take on board. For example, I have plants under 600 and 1,000 watt lighting and yet I still find that nutritional needs vary between different sections of the greenhouse. I learn something new with every new development. I want to understand it all. There is no constant, so we will keep on taking leaf samples,” the grower says.

This not only helps them avoid defects preventively but also has a curative function, highlighting problems that need dealing with. “It’s important but it doesn’t come cheap. It really is a conscious decision and you do have to take the outcomes seriously. And it has to be implemented properly too, of course. So we have drawn up a protocol for sampling. It’s always the same person who takes the leaf samples, and the leaves mustn’t be too young or too old. I get the results in an Excel spreadsheet. This isn’t ideal, though: at the moment I have to enter all the figures manually. Please could someone write a program that automatically feeds the data into my management software? Data is becoming increasingly important, but you can’t do very much with it without the right ICT tools.”

Fertiliser balance

The leaf samples give growers hard data that they can use to optimise their fertilisation. These figures and insights are becoming increasingly important in pot plant cultivation, not least because of the rapid developments going on in the sector.

Van Holsteijn: “Take recirculation of water flows in house plant crops like Phalaenopsis, Spathiphyllum, Bromelia and Zamioculas. If you want to keep the nutrients in balance, you constantly have to adjust the supply of fresh nutrients. A grower can only monitor the consequences at plant level and avoid a potential imbalance in the nutrient supply by analysing.”


Growers of pot plants steer for ornamental value and growth rate. An important aspect of this is fertilisation. Leaf samples provide a better picture of the actual uptake of nutrients in the plant itself. By comparing the results against the target values, growers can adapt their fertiliser supply and optimise leaf quality. Target values vary depending on the crop, the species and the growth stage.

Text: Jojanneke Rodenburg.
Images: Studio G.J. Vlekke.