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Steam and fish fertilisers: A potential organic systemic herbicide?

A range of people, including growers, a steam weeder manufacture and some researchers believed that the use of a foliar fish fertiliser in conjunction with steam produces a systemic weed kill. If correct, this would be a truly enormous step forward for organics. It would allow the introduction of no-till/zero till and solve a considerable number of other weed management issues. That this could be done with relatively benign technologies such as steam weeding and a fish processing by-product would be even more amazing. However, despite considerable enthusiasm and anecdotal evidence no rigorous null control trials had been completed. Considering the enormous potential for this technique it was essential that these claims were properly tested. Therefore, the Biological Husbandry Unit (BHU) at Lincoln University successfully applied for a Sustainable Farming Fund grant from the Ministry of Agriculture to research this technology.

Advocates for the approach believed that the steam was opening up the plant tissues, thus allowing the fish fertiliser to get into the vascular system, translocating to the roots and thereby killing the whole plant. While this is not impossible, it seemed rather unlikely as the principle action of thermal weeders, both flame and steam, is to coagulate proteins and burst cell walls though water expansion. These processes would more likely inhibit material from entering the plant’s vascular system not enhance it. The mode of action was therefore unclear. However, the fish fertiliser contains around 15% fish oil which is very rich in the health giving omega-3 and omega-6 essential fatty acids. These compounds are known to be chemically very reactive, and are also found in vegetable oils such as flax. It may be possible that the highly reactive fish oil was responsible for the systemic effect. It was therefore decided to test two fish fertiliser products, ‘BioSea Omega +’ from Sealord Group and ‘Simply Organic’ from Fluid Fertilisers NZ Ltd. along with raw linseed oil from Functional Wholefoods which has high levels of essential fatty acids and a refined, bleached and deodorized (RBD) canola cooking oil which has very low chemical reactivity.

It was also unclear where the products should be applied to the vegetation in relation to the steam. Therefore, three approaches were tested, applying the products onto the vegetation immediately before the steamer, applied at the same time as the steam, or applied just after the steamer has passed. A randomized complete block (four blocks) split-plot design was used with product and application position as the two factors, application position as the sub-plot, with a steam only and null controls with analysis by ANOVA. Three trials were conducted; two on mixed pasture swards containing grasses, clover and chicory, the other was a predominantly red clover stand with some grass. For one of the mixed pasture and the red clover trails herbage samples were taken six weeks after treatment and dry weights recorded, unfortunately stock were mistakenly introduced to the other trial before samples were taken, but visual records were obtained. There was no statistical difference (p>0.1) between the dry herbage weights for all treatments and controls. Visual assessment on all three trials found no evidence at all of any systemic kill, with all steamed plots showing sustained plant regrowth. Considering the number of products trailed and different application positions it is clear that when used with steam none of the products had any systemic weed killing effect.

Advocates of the steam and fish fertiliser approach also believed that using steam with the organic certified contact herbicide ‘Interceptor’ from Certified Organics Ltd produced a systemic weed kill effect. With the failure of the fish fertiliser and oils to kill plants it was decided to test Interceptor with steam. A complete randomised block design was used, with the treatments being the three application positions, a steam only and null controls with analysis by ANOVA. The trial was on a mixed pasture, predominately grass and red clover with some chicory. Interceptor works by causing plants to desiccate by disrupting cell membranes, as such it is unusual, as most herbicides work by interfering with the plants’ biochemistry. Interceptors’ physical mode of action means that it works best in hot conditions. When used in conjunction with the steam it was very effective indeed, with the plant foliage dying off extremely quickly. However, it only took a few days for the plants to start regrowing with visual inspection revealing no difference between steam treated plots. Foliage samples taken five weeks after treatment found no difference (p>0.4) between any of the treatments. While the steam helped Interceptor kill the foliage it clearly did not result in a systemic killing of the pasture.

This lead to the conclusion that if steam was not making any of the products work as systemic herbicides then the nail in the coffin of this technique would be if steam stopped a known systemic herbicide from working. A randomised complete block trial (four blocks) was completed with four treatments of a null control, steam, glyphosate and steam + glyphosate, with the glyphosate being applied at the rear of the steam weeder. Three weeks after the trial visual inspection showed the steam treatment regrowing to match the control with the glyphosate and steam + glyphosate treatments both dead. Clearly the initial plant damaged caused by the steam was insufficient to stop the glyphosate working, contrary to expectations. That the plants are able to translocate material applied immediately after steaming is interesting and means that it is not possible to rule out using steam with other products, however it is considered very unlikely that steam would covert either a non-herbicidal material or a contact herbicide into a systemic herbicide. In conclusion, the use of steam in conjunction with foliar fish fertilisers, plant seed oils, and Interceptor clearly do not result in systemic plant death, contrary to previous claims by Welte (2005) and others.

There have also been claims by farmers that spraying neat foliar fish fertiliser onto weeds and pastures kill them. A trial on a well watered vigorous grass and red clover sward with undiluted FluidFert and BioSea and BioSea diluted 1:4 with water applied to run off showed that the products had the ability to desiccate and destroy foliage they touched but the pasture has continued to grow for two months post application so there is clearly no systemic kill.

There are occasional but regular anecdotal reports of salt, fertilisers, both organic and synthetic, killing plants when “applied” at high doses, i.e., typically when accidentally spilt on the ground. This is to be expected, fertilisers by their nature contain concentrated plant nutrients, which, if applied to the soil in sufficient quantities kills any plants growing there by causing severe osmotic and other soil imbalances. This does not however, make them suitable for use as herbicides. The rapid and severe imbalances to soil nutrient levels caused by applying concentrated fertilisers will have serious negative impacts on most soil properties such as microbial life, soil structure, pH etc. This is contrary to organic principals of good soil management and is restricted or prohibited under certification rules depending on the size of area being treated and alternative means of control.

These results are a good demonstration of why we need objectively measured scientific experiments. Anecdotal observations may be correct, but unless they are tested in a properly replicated and statistically analysed experiment there is no way to know. Without experiments, anecdotal observations that are wrong or even damaging can propagate far and wide potentially causing financial and environmental damage. Unless you are sufficiently qualified or experienced please get specialist advice before using products for anything apart from their designed use.


Welte, A. (2005). Organic weed control using Biosea product and Vaporjet steam system. Shaping Sustainable Systems: Proceedings of the 15th IFOAM Organic World Congress: , Adelaide, Australia, The International Federation of Organic Agricultural Movements.

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Copyright 2008 Charles N. Merfield.