Home made fertiliser and NPK levels

There are plenty of resources telling you how to make home made fertiliser and mostly they are right. It’s a complex topic and one that divides people due to the huge variety of ingredients that you can use to get a good NPK balance and a supply of the other trace minerals that plants need to grow healthily. I like to keep it simple as I’ve found that although there is a huge difference between using nothing at all and using something, the effective difference between the various concoctions is fairly small.


The essential bit is that you provide the plant with what it needs at the right time. NPK stands for Nitrogen, Phosphorus and Potassium and these are the three main elements that a plant needs to grow healthily. On most fertilisers you’ll find the levels of these three elements as percentages along with other trace elements that will be in them. I learned a little mnemonic when I was doing an RHS course that went ‘leaves, roots, flowers and fruits’, this referred to the benefits that each of the three elements provides. It’s very general however as there is a lot more to it than just making a plant grow or fruit at the right time but it’s a good rule of thumb.

Although plants need a good feed throughout their growth don’t be tempted to feed them too much as this will have an inverse detrimental effect. You may think that you’re being kind but it didn’t do Elvis any good.


Nitrogen is used by the plant to produce good strong healthy foliage and again a good rule of thumb is that the more leaf a plant has, the more Nitrogen it’s going to need. If you’re growing vegetables then things like cabbages are hungry for it. It does however leech from the ground easily and this is one of the problems with over fertilisation of land with chemical fertilisers in agriculture. There are a number of negative effects from this such as nitrogen hungry plants overwhelming native plants, excessive algal growth in water and poisoning of wild life from water contamination.

Symptoms of nitrogen deficiency

Usually a the symptoms of Nitrogen deficiency would be weak stunted growth and the yellowing of leaves. The latter can indicate a number of other issues though and shouldn’t be used alone as an indication. A good source of this is from organic matter such as well rotted manure or chicken manure pellets.


This is used by plants for good root and shoot development and for cell division. This is one that should be applied to young plants to give them good root establishment and to fruiting plants later on to aid in the production of the flowers. They also require some levels during normal growth however as without it they’ll be stunted and won’t grow as large as you’d hope.

Symptoms of phosphorus deficiency

Funnily enough you can see yellowing of the leaves if there isn’t enough Phosphorus in the soil but a deficiency in Phosphorus is quite rare. Also you’ll get stunted plants and without good cell division you’ll be getting no new growth. A good source of this is from bone meal or if you want vegetarian plants then use one of the organic fertilisers below.


Lastly is the K in the list, incidentally this come from kalium, the Medieval Latin for potash. This is the one that you’ll be putting on roses to make them produce the best blooms and fruiting plants like tomatoes to give you the healthiest crop. You’ll find a lot of rose and tomato fertilisers that high in potassium, they are pretty much the same stuff.

Symptoms of potassium deficiency

Again there is the yellowing of the leaves, this really is just a lack of chlorophyl so is indicative of a lack of many things and poor flowering and fruiting. Unfortunately the latter is only obvious once you’ve denied them the nutrients they need so it’s best to be on top of it earlier on.

Home made fertiliser

Now that we’ve got the scientific part out of the way here we are then onto the point of the post, how to make your own. Depending on how specific you want to get there are many different recipes that involve adding wood ash, egg shells and all sorts of other weird and wonderful ingredients to the pot. Mostly however people make do quite well with a mix of comfrey, nettles and dandelion.

I grow comfrey Bocking 14¬†as this is sterile so it won’t take over the area that it’s planted, and most likely all areas surrounding that and because it has a good NPK balance to do most of what I need. Nettles are higher in Nitrogen and so can be added to the mix early on to aid in good root establishment of young plants. In spring this stuff is really prolific do is easy to harvest although leave some for the butterflies. Dandelions are rich in Potassium and again are really getting going in the spring so are good to pick if just to stop the seed heads from spreading the plant too far and wide.

Comfrey Bocking 14

Comfrey Tea

Although this may sound like a delightful herbal infusion you’d be hard pressed to stomach a cup of this once it’s fermented for a few weeks, it smells awful but the plants love it. To make the fertiliser you just need to harvest as much comfrey as you can and stuff it into a watertight container, I just use a regular 3 gallon bucket but if you have more comfrey then go as big as you like. Then just top it up with water and put is somewhere for a few weeks, because of the smell though make sure it’s not near where you’ll be sitting for a gin and tonic in the evening. If you’re adding nettles or dandelion then just add them to the mx at this point. After a week or so you’ll see it start to look fizzy on top, this is good.

After 3-4 weeks you can strain the liquid into suitable containers and then use it in your normal watering routine, something like once a week during the growing season is fine. There are mixed views on the concentration levels of the tea in your watering but mostly I find that this stuff is fairly diluted already but if it’s really dark and concentrated then you can dilute it up to 10:1. What you’re putting on the plants should look no more concentrated than the picture below though. Comfrey will however make a more concentrated solution that nettles alone.

Comfrey tea

Another way that I’ve not tried is to make a concentrated mix by putting the comfrey in a vertical length of 4″ waste pipe sealed at the bottom with a smallish hole in it. Weight it down with a large fizzy drink bottle containing sand and wait until a thick black liquid oozes from the bottom. This stuff is really strong though and should be diluted in a 15-20:1 ration but apparently doesn’t have the same malodour as making it with water.

Other ingredients

  • Egg shells
    These contain a lot of calcium that is essential in cell functions. I can’t imagine that you’re going to get much out of them in 3 weeks so crush and spread around your plants, or simply compost them.
  • Molasses
    This contains carbon, iron, sulphur, potash, calcium, manganese, potassium, copper, and magnesium, a good allrounder really. If you buy some sea weed fertilisers they smell really delicious (they won’t be) because of the addition of this.
  • Wood ash
    Potassium simply. Take some from a wood burner or fire and add to the fertiliser mix or compost. Don’t take ash from anything else though, just clean burned untreated wood.
  • Milk
    Again a good source of calcium
  • Fish tank water
    If you have fish don’t chuck this stuff away, chuck it on the plants. Fish = poop = plant food.
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