A cry for kelp: a view on seaweed solutions

In my experience, the majority of enterprising farmers will take time to consider anything that promises to boost yield. It’s no surprise, then, that there’s been a lot of interest in crop biostimulants recently.

The term ‘biostimulant’ covers anything added to the plant or soil that stimulates natural processes and benefits crops, excluding any fertilisation and pesticidal action.

A common question I’m asked is: ‘which biostimulant is the best?’ I wish there was a straightforward answer to this, but there isn’t. There is limited unbiased and empirical evidence for biostimulant benefits in the UK.

At AHDB, we recognised the biostimulants trend early on and funded a research review (led by ADAS) to help sift the ones that work from the ones that do not.

It is estimated that more than 200 biostimulant products are available in the UK, whereas in France this number exceeds a whopping 300. To make the review manageable, we decided to categorise products by the way they work in a plant or in the soil.

To keep this blog short and sweet, I’m going to focus on seaweed extracts. These are, generally, marketed as foliar sprays to boost crop nutrition.

If you think biostimulants are a recent discovery, then I suggest you think again. Seaweed has been in agricultural use for centuries. Ancient Romans knew how to keep their seedlings green for longer; they used to wrap them up in the seaweed!

Another point – when you read a ‘seaweed extract’ label have you ever thought about which seaweed is used? Perhaps not. There are more than 10,000 seaweed species described in the literature but brown seaweed (Aschophylum nodosum) is the one used most often for its sap.

How the sap is extracted is vital, as it can affect the function and efficacy of the biostimulant. The sap-extraction process can be complex and expensive but better product quantity and quality is often associated with the more costly extraction routes. Cheaper extraction routes can include the use of hot or cold water, acids or alkalis to breakdown the tissues.

Let’s look at plant growth and development hormones as an example of how the extraction method can influence quality. Auxins are usually extracted with alkali, cytokinins with chilled ethanol and gibberellins with methanol. To avoid the complexities arising from the use of these chemicals to extract different products, hot or cold water is generally used. In a nutshell, methods of extraction with milder temperature and lower pH can retain the effectiveness of the key ingredients in the extract.

So how could the presence of hormones affect plant growth? Well, it’s known, for example, that seaweed can have large quantities of auxin and this hormone is known to promote rooting and shoot elongation. If crop establishment is poor, then an application of such hormone-containing extracts could be expected to boost growth and development.

Seaweed extracts can also be a good source of abscisic acid, a plant hormone that is associated with a reduction in the size of the aperture in stomata (tiny ‘breathing’ pores mostly found on the lower side of the leaf). By reducing the size of these pores, plants transpire less and this could help them survive better in drought conditions. Once again, clear effects on yield are hard to prove. One thing to bear in mind, however, is that excessive quantities of abscisic acid can cause slow growth under optimum soil moisture conditions.

Extracts can also boost nutrient (N, P, K and Mg) uptake, leaf chloroplast content and reduce chlorophyll (green colour) degradation – now we can see why the ancient Romans used to use seaweed to keep their seedlings green! Several other minerals and/or compounds, associated with changes to crop growth and development, such as trace elements and amino acids (e.g. proline), are also found in extracts.

Now back to the killer question: do they work? Our review looked for evidence of yield impacts associated with 11 broad biostimulant product categories. As limited data was available for UK field conditions, evidence from controlled experiments and non-UK field conditions was also used. This found that nine of the 11 product categories were associated with a statistically significant increase in yield in at least one experiment. For seaweed extracts, 3/7 experiments showed a statistically significant yield response. Data for oilseed rape was found to be lacking for all products and no firm conclusions could be made.

The evidence does suggest that, in some situations, biostimulant products could help boost yield. The lack of results from UK replicated and randomised field trials is a concern, as it means we have to make assumptions based on results from controlled environmental conditions (e.g. in glasshouses or pots) and field experiments conducted in other climates.

So, before you rush to apply any of these products, you may wish to test how they perform on your farm through a tramline trial or two (see the ADAS protocol). In short, this means selecting your most uniform field (e.g. crop, soil and topography) and applying product to part of it (following the manufacturers’ advice) and measuring the difference (compared with untreated crop) in performance. Carefully collected yield data from the combine is just a small part of what you can measure, however. Take a look at our wheat growth guide for inspiration on what you could measure. However, a word of warning: results from a single season are unlikely to be representative. The best on-farm tests are conducted over several years to help smooth out seasonal affects.

Access the review

Read the information sheet (summarises the key points from the review)

Image credit: Andrew Buchanan on Unsplash

Sajjad Awan

Sajjad Awan

Sajjad is a Resource Management Scientist AHDB. He specialises in crop nutrition, as well as precision technology.