Lars Lund

08 Sep 2020 14:33

You can make your own fertilizer

Use all plants

Unfortunately, there are no books that clearly describes what a homemade fertilizer contains. This is obviously because it is homemade. Many uses for example stinging nettle fertilizer not knowing what it exactly contains. The books online are typically based on knowledge from another source, which again might be copied from another source and it is therefore without any documentation of the information being accurate. Typically, you can read that stinging nettles contain lots of iron, but there is no documentation confirming this. Contrary, using stinging nettle fertilizer shows that it works in accordance.


A real test

Luckily, there is a bright side in the fertilizer jungle. The Swedish University of Agriculture has in cooperation with the largest horticultural organization in Sweden, Riksförbundet Svensk Trädgård, analysed the homemade fertilizer, to investigate if the fertilizer does contain what plants need. The conclusion of the analysis confirms, what organic horticulturists also have learnt, that stinging nettle fertilizer works very well. Very well means that it contains the nitrogen that plants need. Though it varies depending on when you harvest the stinging nettles. Early summer stinging nettles contain the most amount of nitrogen. The stinging nettle fertilizer, which was used for the analysis, was made from 2.2 pounds of stinging nettles and 2.2 gallons of water. The analysis showed that stinging nettle fertilizer contained 92 micrograms of nitrogen per 0.22 gallons when harvested on the 11th of July. When harvested on the 31st of July the value dropped to 8.4 micrograms per 0.22 gallons.

The nitrogen drops

Nitrate concentration in fertilizer made before July is larger than the plants generally need, so you should mix half water and half fertilizer. When harvesting later, in late July, the nitrogen content is low but suitable, because the plants at this time need less nitrogen. The content of other substances, like potassium and calcium, is high, but not that high that it upsets the balance of substances the plants need. The amount of manganese, iron and boron is low, but it has no effect, as these substances are already stored in the soil. Besides stinging nettle fertilizer, fertilized water from comfrey, horsetail and other weeds were also analysed. All mixtures had a fine balance of the substances that plants need.


The right dosage

The dosage depends on which fertilizer you use, stinging nettle fertilizer or fertilizer from other plants. If you use stinging nettle fertilizer from the early summer harvest, it needs to be diluted and you need to irrigate with 0.4 gallons of water per 10 square feet per week. If you use other plant extracts, you need 2 gallons of water. A good combination is to start irrigating with 0.4 gallons of stinging nettle fertilizer early summer and then irrigate with 2 gallons of horsetail late summer. Thereby the risk of eutrophication is little. The principle is: little but often. In addition, you can see if the plants need nitrogen. If the leaves do not have the colour they usually do (often they turn bright), something is wrong with the amount of nitrogen and you need to fertilize more often. Moreover, it is recommended to use fertilizer as fresh as possible, so the fertilizer won’t get polluted. This is most important if you use it for consumable crops.

This is how you do:

The plants you are going to use should be soaked in some time, depending on which plant it is. You definitely need experience, but here are some rules of thumb.

If you want to make stinging nettle fertilizer, then mix 2.2 pounds of stinging nettles with 2.6 gallons of water. Cut the stinging nettles in small pieces and throw the ones with seeds away. Put them in a tub or barrel, but not of metal. If the barrel is made from metal, an unfortunate chemical process will happen. Plastic or ceramic is fine. Stinging nettles ferment, so leave room for foam in the surface. Place your barrel in full sun and stir it once a day. It doesn’t smell good, but after three weeks the mixture is dark and ready to use. Sieve the plant parts so you have a strong solution. It should be diluted before use. If you have a jute bag, you can put the stinging nettles in there, so you don’t have to sieve the mixture later.  


Try the method with other plants

You can also make a good fertilizer out of chicken manure, rabbit manure, sheep manure or cow manure. Put it in a sack which can absorb water, then put the sack in a barrel with water. Let it soak for three weeks and dilute 2.2 pounds with 5.2 gallons of water. If there is any manure left in the sack you can throw it in the compost.


Coffee grinds

Coffee tastes good but the grinds are thrown out. It is a shame because coffee grinds contain nitrogen, phosphor, potassium and different minerals. An old remedy says that it keeps vermin away from roses. It is true that the healthier your roses are, the better they stand against lice. Use coffee grinds in your greenhouse or garden. Scatter it with a light hand. Earthworms love it and your soil will get fertilized. Use it sparingly, which means four times a year. 

Om Lars Lund

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.  

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