It was 75 years ago that Lawrence Hills first started working on comfrey. His work showed it was possible to produce a viable cost-effective liquid feed with low ecological impact using comfrey leaves.

Since then, there’s been an ongoing debate about which feeds are best for which purpose. Many experts have suggested comfrey liquid feeds are high in potash so are suitable for tomato crops, while nettle feeds are high in nitrogen so are suitable for rapid growing leafy crops.

However, there have been no studies where direct comparisons of comfrey and nettle feeds grown at the same site under similar conditions were made to back this up. So we set up our own experiment.

Our study makes a direct comparison between pairs of comfrey and nettle liquid samples taken from a range of sites. At each site, the comfrey and nettles were grown under similar conditions so we could make a direct comparison. We asked people to record how they prepared their samples, and we considered any differences in dilution.

The aims of this study were to:

Make a direct comparison between comfrey and nettle feeds grown under similar conditions.
Obtain a better idea of the variation that we might expect from comfrey and nettle feeds grown at different sites.

Liquid feed comparisons

Our study found differences between comfrey and nettle feed grown at the same site - but these differences were quite small. The important finding was that, on average, both feeds have a composition that makes a good plant feed, including for tomatoes.

All the feeds presented in table one were made by steeping the plants in water to make a feed that’s ready to use without further dilution. We also received a few samples made by compressing the leaves, but these were very variable, so we cannot draw conclusions from those samples.

Our results showed:

Nettles had significantly more nitrogen than comfrey. Nitrogen is important for leafy growth.
Comfrey had slightly more potash than nettles, but the differences were small and were not significant. Tomato feeds often contain high amounts of potash as it’s important for fruit development and flavour.
Nettles had significantly more phosphate than comfrey. Phosphate is important for root development.

Comparison with other feeds

Lawrence Hills undertook many experiments at Bocking, comparing the yields of crops treated with compost, comfrey leaves and manures. He also prepared comfrey liquid feeds, which he sent off to be analysed. This was expensive at the time, so the number of samples were quite limited. At the time there wasn’t the resources to compare comfrey liquid prepared at different sites or compare it with liquid feeds made from nettles.

Using feeds in practice

From a practical view, the study allows us to make recommendations for making plant feeds at home. The concentration of nutrients in the homemade liquid feeds were similar to commercial feeds when mixed for use, so comfrey or nettle liquid prepared with water doesn’t need further dilution.

The nettle feeds also had a similar composition to the comfrey feeds, and were high in potash, so would also make a good tomato feed. This is contrary to some garden advice.

Variability of the plant feeds can be caused by many factors, specifically soil conditions and age of plants leaves. To maximise the nutrient concentration of your feed, use younger leaves. It’s also recommended that you feed your comfrey plants if you regularly take harvests, otherwise you’re just continuously removing nutrients and not putting anything back.

Both comfrey and nettle feeds are high in potash so make a good feed for tomatoes if used in the ways directed. We think the suitability of nettles as a tomato feed has been underplayed. So while comfrey remains an excellent tomato feed, and an excellent source of pollen and nectar for bees, if you only have a patch of nettles on your site, we would not hesitate to use it as a liquid feed for tomatoes.

Thanks to the Centre for Agroecology, Water and Resilience for carrying out the analysis.