Attention to precision fertilization: leaf stem research in potatoes (part 2)

Last week we focused on mineralization under varying weather conditions and on the soil’s nutrient retention capacity and fluctuating nitrogen levels in the soil. This week we will discuss the possibilities of measurement methods.

In this graph, two systems of measurement are compared. The soil scan with the colored fields indicates differences. There was a dark green part and a yellow part to be recognized. Petioles were collected from the dark green part, with the most biomass, and also from the yellow part. The blue line of the petiole test is from the petioles that were picked in the green part and the red line from the yellow part.

The graph above compares two systems of measurement. A potato plot was monitored during the entire growing season with the leaf stem examination and a soil scan was made at the start of the season. The soil scan with the colored fields indicates differences in the biomass. In the petiole chart, nitrate levels are interpreted as high, too low, or good. Which is then converted into a fertilization advice.

At the arable farm VOF Ipenburg of the family de Geus in Mookhoek (Hoeksche Waard), the development in nitrogen availability will be monitored in the coming months with the leaf stem method. Joke is a soil and fertilization expert and works for the Groen Agro Control laboratory in Delfgauw. She follows and describes the developments on the Innovators plot for TopBodem.

Measuring is not always knowing

The beauty of this picture is that the scan makes differences in the plot visible. Leaf stems were sampled and analyzed on these different fields throughout the growing period. The biomass of the potatoes that you measured with the scan has been observed, but may be difficult to interpret. That would be like putting a sick person on the scale instead of measuring his fever. However, with the leaf stem examination we could see that both the green part and the light part were sufficiently fertilized. And both parts of the lot had no shortages.

Innovator, first measurement

Now to the plot that we are following this year: The first measurement is shown as a green triangle in the small graph. In March, 34 cubic meters of CZAV mixed fertilizer was applied to this plot. This took place in beautiful dry conditions. This resulted in a grant of 114 kg N/ha. Together with the basic rate of 63 kg N, this comes to 177 kg N/ha. A petiole sample was taken on Monday 13 June. That is 23 days after emergence. The advice is: additional fertilization is necessary, + 75 kg N/ha.

In the coming months, arable farmers Joke and Leen de Geus will monitor the development in nitrogen availability using the leaf stem method. Joke is a soil and fertilization expert and works for the Groen Agro Control laboratory in Delfgauw. She follows and describes the developments on the Innovators plot for TopBodem.

Publication date: Jun 18, 2022
Author: Joke de Geus
Image: Joke de Geus

Attention for precision fertilization: leaf stem research in potatoes

Every potato grower would wish for this: a beautiful crop and a good yield and the desired potato quality at harvest. He also wants to meet all environmental standards. At the start of the growing season, we should consider what will happen in the soil in the coming period.


Nitrogen availability fluctuates through the season.

Animal manure is an important supplier of main and trace elements. But it is not always immediately available. A large part of the nitrogen in the manure is not directly absorbable and only becomes available after mineralization. It is not easy to predict when that mineralization will occur and how much nitrogen will then be released. This requires sufficient moisture and a high temperature.
Soil processes

To learn more about this, we followed the course of the nitrogen contents on a number of plots for several years. We did this on several plots and at two depths, namely 0-30 and 30-60 cm deep. An important conclusion we drew was that a nitrogen peak was visible on 17 May on all plots. That peak was not caused by a fertilizer application, but by the weather conditions. It is temperature and moisture that stimulate mineralization.


Nitrogen levels in the soil fluctuate strongly. The potato grower wants the nitrogen to benefit the crop. And that the nitrogen is preferably absorbed as much as possible and converted into kilograms of potatoes. Nitrogen that denitrifies or leaches is lost.


The peak in nitrogen availability depends on temperature and moisture.


The CEC is the soil’s ability to hold these minerals. Those minerals are slightly bound to the negatively charged clay particles and organic matter particles. It is the positive ions such as potassium, magnesium, calcium and ammonium that remain available to the plants. Plants make use of this and absorb those nutrients. Unfortunately, this does not apply to nitrate, because it is negatively charged and will be able to wash out unhindered if it is not directly absorbed by the plants.


We will follow how plants absorb the nitrogen with the leaf stem study. Next week we will start in a lot of Innovators. It is then 28 days after the emergence date. A good time for the first measurements.

In the coming months, arable farmers Joke and Leen de Geus will monitor the development in nitrogen availability using the leaf stem method. Joke is a soil and fertilization expert and works for the Groen Agro Control laboratory in Delfgauw. She follows and describes the developments on the Innovators plot for TopBodem.

Publication: June 12th 2022
Author: Joke de Geus
Images: Joke de Geus


We welcome our new operational director

On 1 June Dirk-Jan Hemmelder will start as Operational Director at Groen Agro Control. In this role, he is responsible for operational development, among other things.

Dirk-Jan has more than 20 years of experience as director of a number of food producers, Eurofins Foodlabs and an international operator of bio-fermentation plants. “I am happy to contribute to a sustainable and safe food supply for now and in the future. Groen Agro Control is a great company with many opportunities. Giving substance to this together with the team so that we can serve our customers even better is a wonderful challenge,” says Dirk-Jan.

Groen Agro Control is a very innovative laboratory with customers who belong to the top of the international fruit and vegetable and horticultural world. Groen Agro Control has offices in the Netherlands and Peru and an extensive international network of agents. In addition, the company carries out chemical, microbiological and botanical research projects. With this, Groen Agro Control supports its customers so that they can further strengthen their worldwide leading position. Groen Agro Control is part of Normec Foodcare.

Testing Salmonella spp in chocolate

Salmonella spp in chocolate is uncommon, but when it is, people can become ill and experience abdominal pain, diarrhea or fever. The symptoms usually disappear after a few days. In high-risk groups, this can pose a problem, even leading to death.

During the preparation of chocolate, Salmonella spp can end up in the chocolate from the raw materials (e.g. eggs) or contamination via the production process. When ingesting this chocolate, the fat of the chocolate protects the Salmonella spp against stomach acid and then enters the intestines unaffected. There it will settle on the intestinal wall and cause the health problems mentioned above.

Within 2 days

If you, as a producer of chocolate, want to have your product checked for Salmonella spp or other micro-organisms such as E coli, then Groen Agro Control is the right place for you. Within 2 days of receipt we can indicate whether your product is suspicious for the presence of Salmonella spp.

If you have any questions or comments about this, please contact our Foods Product Manager: Michel Witmer. Or call +31(0)15 257 25 11.

Counts on useful fungi and bacteria show real values of resilience

Fusarium, rhizoctonia, pythium, these are all root diseases that are sometimes difficult to control. The preventive use of useful fungi and bacteria can improve plant resistance. Groen Agro Control provides analysis to quantify these organisms in the supplied material, or in the substrate or in water. This way you know what quantity is in the packaging and, for example, what is still alive in the substrate or in drain water. This way you can determine whether a new dose is already needed or whether there is still enough life.

Groen Agro Control mainly measures in bacterial counts, so that means the real living values in colony-forming units per ml or gram. The sample is placed on culture medium and after a few days it is possible to count how much Bacillus spp., Trichoderma spp., lactic acid bacteria and the like are present. “We also recommend having a count of general bacteria, fungi and yeasts done. And always the analysis for important root diseases. The use of useful organic products should of course be measurable in a lower infection pressure of harmful root diseases. sides in the root environment to see how the deployment of beneficial organisms for your crop works.”

For more information, please contact Ines van Marrewijk, productmanager at Groen Agro Control, Distributieweg 1 – 2645 EG Delfgauw, telf. +31 (0)15 2572511

Publication date: Wed 6 Apr 2022


Residue determination in potatoes and onions

You want best the quality of the product in your storage shed.
Be supported in this by the residue determinations of Groen Agro Control.

Measurements on Maleic Hydrazide and 1.4 Sight
The active ingredients maleic hydrazide and 1.4 Sight are used for the storage of potatoes.
The potato storage with 1.4 Sight (dimethylnaphthalene) is accompanied by measurements in the skin. The decrease of 1.4 Sight concentration in the peel is a measure of the effect of the sprout inhibitor. By measuring you know whether the storage is in order and whether a new application is necessary.
Maleic hydrazide forms the basis. MH is sprayed on potato plants before the potatoes are harvested. It is taken up by the leaves and translocated to the tubers. It does not affect the quality of the potatoes, but prevents the harvested potatoes from sprouting prematurely. We measure maleic hydrazide in the entire potato.
Measurements on chlorpropham
If you stopped using it years ago, there is a risk of contamination of potatoes and onions in storage sheds in which chlorpropham was previously used.
To ensure that you comply with the MRL, it is advisable to analyze your products accordingly. If your products do not contain chlorpropham, this could mean that adjustments in your storage shed are not necessary.
Groen Agro Control is an accredited laboratory (NEN-EN-ISO 17025). And can analyze your products on this substance for you.

Ask about the 1.4 Sight, maleic hydrazide and/or chlorpropham tests at Groen Agro Control.
Contact us and submit a sample, you will receive the results within two working days.

You can take a sample yourself and send it to our laboratory.

If you prefer to be sampled by our samplers within the Netherlands, please send an email to or call (015) 25 725 11.

Please note, you must always enclose an order form:

Health-promoting properties of fruits and vegetables

The demand for healthy food, and with it the analysis of healthy ingredients in food, is increasing. That is why our nutritional analyzes provide reliable information about the health of fruits and vegetables.

Variation in nutritional value

The nutritional value of fruit and vegetables varies and depends on the variety, cultivation method and season. Until now, there was insufficient control over this. We convert knowledge about nutritional value into reliability about the health or nutritional value of your fruit and vegetables.

The value(s) of fruit and vegetables

More and more people are confronted with lifestyle diseases such as type 2 diabetes or cardiovascular disease. Population research shows that eating fruit and vegetables contributes to health. That knowledge only increases through research.
Consumers are also becoming increasingly aware of the influence of food on their health. But because it is now difficult for companies to explain exactly what is in their product and why it is therefore healthy, the knowledge about health-promoting ingredients in fruit and vegetables is not yet used enough.

Innovative measurement methods

In order to appreciate the positive effect of fruit and vegetables, it is important to know more about these products. In our analyses, we use innovative measuring methods to measure ingredients in fruit and vegetables. We can also develop new analyzes for you on request.
Ultimately, only reliable information about important ingredients can contribute to strengthening consumer confidence.
With this in mind, we also support various initiatives, such as the research projects “Green Health Solutions” and “The Values ​​of Fruit and Vegetables”. In this, we work together on new crops or on research to enable faster claims on the health-promoting effect of products.

Our analyzes and investigations

Groen Agro Control has developed a number of analyzes that allow us to reliably measure what is in food. We also do this for research so that the health effect can be better mapped out. Below is an overview per product of the most common analyses.

Vitamins Additional Ingredients
Potato C
Strawberry B3, B6, B9 (folate) en C Anthocyanins*1
Avocado B11 (folic acid), E
Blueberry Pro-vitamin A (Beta carotene), B1, B2, B3, B5,

B6, B8, B9, B12, B11 (folic acid), C, E, K

Beans C
Broccoli C Glucosinolaten
Champignons D2 beta-Glucans
Cress A, B1, B2, B6, B11 (folic acid), C, E Glucosinolates
Dates A, B1, B2, B6, K
Grapes C Anthocyanins
Grapefruit C
Cucumber B, C
Mais B1, B2 and B6
Mango B6, C Carotenes, Glutamic Acid
Melon C Carotenes
Paprika C Carotenes
Pumpkin A Carotenes
Raisins B
Orange C
Lettuce B11 (folic acid), C, K Luteine
Tomato Pro-vitamine A (Beta carotene), B11, C Glutamic Acid, Lycopene
Carrot Pro-vitamine A (Beta carotenes) Carotenes
Sweet potato A, C Carotenes

Discover the value(s) of your product

Please contact us and consult with our colleague Michel Witmer. Or call to +31(0)15 257 25 11.

Extension of our BNN recognition for mycotoxins and heavy metals

Groen Agro Control has received extension of BNN recognition. In addition to pesticides (fruit and vegetables, grain, oilseeds and tea), also for mycotoxins and heavy metals. That is why you should contact Groen Agro Control for analyzes on organic products.

We recently received an extension of our BNN certificate. Our laboratory currently meets the requirements set by BNN for the analyzes of residues of pesticides, mycotoxins and heavy metals for biological products in the following groups:
  • fruit and vegetables
  • processed fruit and vegetable products,
  • cereals, grain products, rice, oilseeds and vegetable oils
  • Tea and spices
The Bundesverband Naturkost Naturwaren (BNN) is an organization that safeguards the interests of the organic sector. The quality of BNN-recognized laboratories is guaranteed by strict requirements that they must meet. With the renewed and expanded scope of our BNN recognition, we hope to be able to serve the organic sector even better.

Water-side system in horticulture companies reviewed

This spring, there was little rainwater, so reserves are already being drawn on. In a previous Horticultural Newsmail, Groen Agro Control already drew attention to rules on human pathogens in water according to GlobalGAP. That is about human health. But of course, your crop also requires ‘healthy’ water. “The whole water system is quite complex on many farms. In every pipeline, storage silo or in the unit, or with water treatment via technique or with agents, the water can change and with it the properties on the crop,” says Ines van Marrewijk, product manager at Groen Agro Control.

The focus in microbiology is on monitoring water at a number of places in the system. At the very least, one should regularly check the starting water before and after disinfection and certainly the drip water for germ counts. This can be a limited check on bacteria and fungi alone, but more and more companies are finding it important to have what is known as an ‘Extended Germ Count’ determined, which also shows the lowest possible contamination of Fusarium, Pythium and Phythophthora. And that can be critical for a good assessment of the system, because even though the grower checks and disinfects the water, microbiology can unfortunately multiply again in storage or in the system because a few spores have been left behind.

Sufficient oxygen in water and substrate will be a challenge in summer, with higher temperatures. Measuring is knowing when it comes to oxygen and that can be done very easily with sensors. It is good to realise that as a lab, we normally determine bacteria as aerobic germ count at 300 degrees Celsius. Under anaerobic, i.e. somewhat more oxygen-poor conditions, anaerobic bacteria develop more. If required, we can also count them via anaerobic bacterial count in water. Do this determination additionally and not as a replacement of the aerobic bacterial count. Biological and chemical oxygen consumption as an analysis can also give an impression of the activity in water.

It is often not immediately clear what kind of bacteria it is. “An example: For many years, Groen Agro Control named certain root growth ‘beaded roots’ and yet another image ‘thick roots’. Both appear to have a bacterial cause. It is not yet known that many bacteria can be harmful to plant and/or root growth. We now know more, but by no means everything. The bacterium Ralstonia was also such an ‘unknown factor’ when it suddenly appeared in roses and anthuriums; it had not previously been described as a disease in those crops worldwide.”

Don’t just check nutrients in drain water or output water, but also regularly take a sample of the toxic water, is the advice. That way, you can see that the settings and adjustments actually reach the plant. And take a sample of the poison as it reaches the plant or from a day’s supply. Preferably, do not take a sample from the mixing container, not while it is running and certainly not while it is standing still. The mixing of A and B tank with drain water and fresh water is done by pulses and is often not very homogeneous in the mixing tank itself.

Through drift, crop protection agents and even herbicides in the open rainwater basin occur more often than you might think. Therefore, ditch water is sometimes not a safe source. When in doubt, do not only have screening residue done but a more extensive analysis for inhibitors and herbicides.

“We have suitable packages for every type of water. So ask in advance what your goal is so that our lab can use the right analyses. Also take note of the information on sample materials and amount of sample required through our Materials for Sampling page.”

For more information, please contact Ines van Marrewijk, product manager at Groen Agro Control, Distributieweg 1 – 2645 EG Delfgauw, phone 015 2572511

Publication date: Tuesday

Uitbreiding capaciteit pathogeen analyses bij Groen Agro Control

Expansion of capacity for pathogen analyzes

Groen Agro Control is further expanding its testing capacity in the field of the pathogens Salmonella spp and Listeria monocytogenes.

This means that even more samples can be used daily and it can be communicated the next day whether the PCR method is positive or negative.

Positive PCR samples have a further confirmation step, but otherwise the customer may receive a report stating absent in 25 grams of product.

Companies within the fresh produce and food industry that have large numbers of samples for these pathogens, and also want a fast service, can contact our colleague Michel Witmer.