Organic fertilizer may kill plants
Tomatoes in the greenhouse have been hit particularly hard in many private greenhouses.
By Lars Lund
If your tomatoes did not succeed this year, the leaves curled strangely and the plants withered completely, then it might not be your fault nor your greenhouse’s. Over the last year or so, there have been more and more reports from garden owners about mysterious damage to their plants. The damage is consistent with similar symptoms that may occur with the use of the herbicides aminopyralid or clopyralid. These messages have reached the Ministry of the Environment. The plants become malformed with coarse, curly leaves and the growth stops. The damage has been seen both on tomatoes, which are particularly sensitive, and peppers, which have been grown in different types of commercial soil with added liquid fertilizer (which have even been approved for organic use).
Tomatoes and other plants can easily get curly leaves for other reasons, so in some cases it can be difficult to be sure of the cause, but for experienced horticulturists, one can see the difference in curly leaves and if the growth stops, and the plant has been cared for correctly, the alarm bells should ring.
The phenomenon was first observed in 2019 in Norway and Sweden, and not least in England. Morten Linnemann, editor-in-chief of the Danish magazine Haven, tells the Greenhouse Forum that in Norway they have investigated and found clopyralid in Norwegian organic fertilizer, admittedly in small quantities, but probably enough to damage, for example, tomato plants. At the same time, several Swedish retailers withdrew, in June 2020, organic fertilizers from the market, presumably in recognition that there may be herbicides in the fertilizer.
It has aroused a lot of wondering, that it is the organic fertilizers that seem to be the source of harmful substances. Morten Lindemann says that the explanation seems to be that it is actually legal to use a certain amount of non-organic plant material in products approved for organic farming.
The Danish plant expert Magnus Gammelgaard has taken a closer look at the problem. It seems, according to Swedish studies, that the problem is mainly products containing vinasse, which is a residual product from sugar beets and sugar canes. In some products with vinasse, however, no traces of toxins have been found.
“One of the problems in this case is that it today is allowed to sell liquid fertilizer under the name "approved for organic farming", even if the fertilizer is made from non-organically grown plants or plant material. It is important to know that there is a big difference between whether the fertilizer is organic with an ecolabel on the packaging, or whether the fertilizer is approved for organic production,” says Magnus Gammelgaard.
Om Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.