Members' experiments
Does comfrey liquid feed live up to its claims?
Summary 🔗
Background
It is easy to make liquid plant feeds by soaking plants such as comfrey or nettles in water for a few weeks. There is lots of information in gardening literature and forums that states that comfrey liquid is high in potash so is good for feeding fruiting plants such as tomatoes, whilst nettle liquid is high in nitrogen so is good for feeding leafy crops. However, there is a lack of work that backs this up rigorously. There is very little work that makes a direct comparison between nettle and comfrey feeds produced under similar conditions.
Aim
The aim of this experiment was to test the differences in plant nutrient content between pairs of comfrey and nettle feeds grown at a range of locations around the UK to allow a fair comparison to be made.
Key findings
- Preparing a nettle or a comfrey feed by soaking the leaves in water for 1 month produced a feed that had a similar nutrient content to a commercial tomato feed. It should be used undiluted.
- Feeds made from nettles had more nitrogen than the comfrey, but the differences in potassium were only small and not statistically significant. Both nettle and comfrey produced a liquid that had a high potash content that would make it suitable for feeding tomatoes. If you only have a patch of nettles to hand, this would make a very good tomato feed.
- Brewing for longer than 30 days did not increase the nutrient content of the plant feed produced.
- There was considerable variability in the nutrient content of liquids obtained from different sites.
- Feeds made from compressing leaves were extremely variable and some had very high potash content relative to other nutrients. This makes it difficult to give advice on how to dilute them and how much to use.
Conclusion
- The concentration of nutrients in the home-made liquid feeds were higher than ready to use commercial feeds, so comfrey or nettle liquid prepared with water doesn’t need further dilution.
- The nettle feeds had a similar composition to the comfrey feeds, and were high in potash, so would also make a good tomato feed, contrary to some garden advice.
- Variability of the plant feeds can be caused by many factors especially soil conditions and age of plants leaves. To maximise the nutrient concentration of your feed, use younger leaves and remember that the soil will need feeding to replace the nutrients drawn out by the comfrey or nettle plants.
- The concentrated feeds made from compressing leaves were very variable and had a high concentration of potassium compared to other nutrients. This would make it difficult to make recommendations as to how much to dilute or how much to apply.