How to Make DIY Plant Fertilizer for the Vegetable Garden

Make inexpensive environmentally-friendly homemade plant fertilizer for your vegetable garden from foraged and waste materials. Includes DIY plant fertilizers you can make from seaweed, comfrey, and nettles.

On its own, soil is a mixture of sand, crushed rock, and non-living substances. It takes decayed and broken down leaves, bark, and animal waste to transform dirt into rich and living garden soil. Without this organic fertilizer, your garden soil will quickly run out of nutrients, and your land and harvests will suffer. Though common, Factory-produced artificial fertilizers have a high environmental and financial cost. It makes no sense to waste your money on them when you can make your own DIY plant fertilizers from garden waste, cardboard, nettles, comfrey, and foraged seaweed.

There are various ways to create it and different uses for different plant fertilizers. What they have in common, is that they’re easy to make and useful for feeding your garden’s soil and plants for free. The benefit of using organic garden fertilizers goes beyond cost though. Each type helps the soil to retain its natural integrity, reduce pests and disease, and improve soil health. Healthy soils grow productive crops and your own homemade plant fertilizers are the best way you can keep your soil healthy.

Types of DIY Plant Fertilizer

Each type of fertilizer will have varying content of nitrogen, phosphorous, and potassium (NPK), the main three plant nutrients. They’ll also contain essential trace elements such as magnesium, calcium, iron, zinc, and sulfur. Shop-bought plant fertilizers will list the nutrient value as an NPK ratio. Nitrogen (N) is responsible for leaf growth, phosphorus (P) for roots, and potassium (K) is beneficial in the production of fruits and flowers. NPK = shoots, roots, fruits.

You don’t have to buy any of those packaged fertilizers though, because you can make your own. They include garden compost, comfrey fertilizer, nettle fertilizer, worm castings, and green manures. These and others can support different phases of a plant’s growth cycle by providing essential nutrients. Nitrogen-rich nettle feed will encourage early leaf growth whereas potassium-rich seaweed enhances fruit development. Fertilizer tea delivers nutrients straight to the plant in a liquid form which is easily absorbed by the roots.

Homemade Liquid Seaweed Fertilizer

If you live by the sea, foraged seaweed makes an excellent organic mulch, liquid fertilizer, and compostable plant. It’s high in around 60 trace elements and potassium (NPK: 1:0:4) and is great for feeding tomatoes as they’re forming fruit. To make concentrated liquid seaweed plant fertilizer add as much seaweed as you can squeeze into a bucket. Next, cover it with rainwater, and leave to soak for a month, stirring every few days. The fertilizer grows stronger with time, as does its smell, so store away from your house. After a month, strain and dilute five parts of rainwater to one of seaweed fertilizer. Water the base of plants or apply as a foliar feed spray which can even inhibit pests, viruses, and fungal problems.

There is much debate as to whether to rinse seaweed before using it to remove the salt. It is used unwashed though, and in coastal areas, farmers will even haul it up to their fields with tractors. If you’re worried about the salt, you can of course rinse it thoroughly before you use it as a mulch, or make a liquid seaweed plant fertilizer.

Another thing, avoid using tap water to make feeds or water plants since it may contain fluoride or chlorine. Chlorine is toxic to plants, and some sensitive plants can also develop leaf burn from the fluoride in tap water. 

Homemade Comfrey Fertilizer

Symphytum officinale, commonly called comfrey, is a tall perennial herb with bell-shaped purple flowers. These flowers are a magnet for bees and pollinating insects but can self-seed and the plant can quickly take over. To avoid this, grow ‘Bocking 14,’ a cultivar of Russian comfrey, S.x uplandicum, which does not produce viable seeds so won’t take over.

Comfrey makes great liquid plant fertilizer and is particularly high in potassium (NPK 1.8:0.5:5.3). It’s also very easy to make. Begin with a large container, squeeze in as many leaves as you can fit, and weigh down with a brick. Tearing or cutting the leaves is optional. Fill with rainwater, then leave sealed with a lid to avoid flies. It takes three weeks for the leaves to break down into a nutritious liquid. Be warned, it stinks! Comfrey tea can be sprayed on your plants as a foliar feed or watered into the soil. Diluted 1 part comfrey fertilizer to 10 parts water.

Another way to use comfrey leaves without the stench is to pull them and lay them on the ground as mulch. The hairy leaves can be laid directly at the base of plants, whole or chopped up. I recommend wearing gloves and long sleeves, these leaves can really irritate. Or line pots with a leaf before planting and let the nutrients slowly absorb.

Another use for comfrey is in healing salves. Use the leaves to make comfrey-infused oil, which is helpful in speeding up the healing of bruises, sprains, and pulled muscles.

DIY Nettle Tea Plant Fertilizer

Urtica dioica, a.k.a. stinging nettles often get a bad rep in the garden. They have plenty of things going for it including being an integral food source for butterfly caterpillars and a favorite place for ladybirds to lay their eggs. Nettles also make a nutritious tea for both us humans and our plants. In spring, they are particularly high in nitrogen, iron, magnesium, and calcium (NPK 5.6:0.7:3.7).

If you have space, you could leave an area of your garden for nettles to grow. Alternatively, look out for local nettles to forage. If it’s not on your property, always ensure you have the landowner’s permission before foraging. Pick young nettles before they flower or go to seed and pinch leaves hard to avoid being stung. Thick gloves and long sleeves are required when harvesting or you will be tingling for hours after.

It is easy to make a nutrient-rich nettle tea plant fertilizer. Crush the leaves into small pieces and fill a large bucket with as many leaves as you can cram in and weigh down with a brick. Fill your container with rainwater, cover, and leave for two weeks. It has a pungent and unpleasant smell so leave somewhere discretely out of the way. Dilute your concentrated nettle tea fertilizer one part tea to 10 parts water and apply to the base of your plants or spray as a foliar feed to the leaves. It is strong so don’t apply to young plants whose root systems have not fully developed. You can add undiluted nettle tea to your compost heap to speed up the decomposing process.

Animal Manure Fertilizer

Aged animal manure makes a great mulch applied neat in early spring and autumn and is such an easy DIY plant fertilizer. It improves soil texture and structure, conserves moisture, and suppresses weeds whilst slowly releasing nutrients. Use organic free-range chicken (NPK 4.2:2.8:1.9), cow (NPK 0.6:0.3:0.7), pigs (NPK 05:0.3:0.5), pigeon (NPK 4:2:1), sheep and goat (NPK 0.7:0.3:0.6), horse (NPK 0.7:0.3:0.6), llama and alpaca (NPK 1.7:0.7:1.2) or even your pet rabbit’s manure (NPK 2.4:1.4:0.6).

To age (compost) it, just leave the manure to rot down in a heap or compost bay made from recycled pallets for about four months. Nutrient levels vary but typical values are listed. You only apply aged manure to the soil as fresh animal manure is rich in types of salts that will burn plants. Fresh animal manure is usually filled with weed seeds that survive the digestive processes. By aging manure, the heat in the composting action kills seeds and breaks down the salts to safe levels.

Manure Tea as a DIY Plant Fertilizer

Animal manure tea is literally manure steeped in water to make a concentrated, nutrient-rich DIY plant fertilizer. To make manure tea, put a few spades of manure into a hessian sack, tie the top shut on a pole over a bucket of rainwater. The suspended sack allows the manure to steep like a teabag. When finished, you strain it into containers, then dilute it in rainwater and use it directly on the soil or as a foliar feed. Avoid using manure tea on root crops as the high nitrogen will encourage leafy greens rather than strong roots.

Organic animal poo is essential if you’re planning on using it in the garden. By only using organic, you are able to avoid aminopyralid herbicide, a hormone-type herbicide used in farming to kill persistent broadleaf weeds like thistles and docks. It’s often sprayed onto grass and grain crops, and then the resulting hay and straw are contaminated. The herbicide passes right through the animal and survives in the manure, and is not destroyed in the composting process. Aminopyralid herbicide will kill many of your vegetable garden crops, so be careful to only use organic animal manure.

Using Wood Ashes in the Garden

Wood ash contains variable quantities of potash and trace elements like calcium (NPK 0:1:4-10). This alkaline fertilizer can be used to reduce the acidity of soil and is a natural substitute for lime. Use sparingly though, best to apply a small amount of wood ash into your compost heap. Wood ash can be used as a mulch, just remember to fork it in well as nutrients leach away once wet.

When using wood ash, avoid applying it to acid-loving plants such as rhododendron, raspberries, and blueberries, or where potatoes are grown. That’s because alkaline soil can encourage potato scab. On a plus side, wood ash can help combat clubroot, the bane of brassicas, by raising soil alkalinity. When collecting wood ash, make sure to not use BBQ or coal ashes as these can contain harmful contaminants.

Vermiculture for making DIY Plant Fertlizer

Vermiculture is the rearing of worms to mix kitchen scraps and bedding materials into a water-soluble organic plant fertilizer and soil conditioner. All you need is a compost bin, special composting worms called brandling worms, bedding like newspaper or cardboard, and kitchen and garden vegetable waste. The DIY plant fertilizer you get out of it is ‘worm tea’ and worm castings. Both feed the soil, and the castings are marvelous at improving texture and water retention.

Making a wormery is a great way to produce homemade nitrogen and potassium-rich fertilizer. Use a recycled plastic bin or a wooden box with holes in the bottom for drainage. Stand it on a drip tray, to catch the worm tea, and make sure to start a second one before the first one is finished. Otherwise, the worms can all disappear, looking for organic material to break down. A good design is a purpose-built wormery like the one I have. It has several tiers, that contain various stages of composting kitchen waste. It also has a drain tap for collecting the ‘worm tea’.

Set up a Wormery

Whatever your set-up, position your wormery somewhere warm, dark, and moist — worms become inactive in low temperatures. Also, make sure to cover the wormery with a lid to stop fruit flies. There will usually be directions included in your purpose-made wormery, but homemade types are easy to use too. Line the container with layers of shredded newspaper, leaf mold, and a variety of food and garden waste. Avoid adding citrus fruits and onions as they increase acidity. The worm tea you get from your wormery is a fantastic DIY plant fertilizer.

Worm tea contains living microbes so it’s best to use fresh. Dilute it at the ratio of 1:10 to rainwater and apply it whenever you wish. It helps boost nutrients and prevent pests and diseases, and will not burn plants. You can use worm castings as a top dressing on the soil and they help regulate moisture to your plant roots.

Homemade Garden Compost

My absolute favorite DIY plant fertilizer is homemade compost. It’s relatively easy to make and can only benefit your garden. Soil is a complex mixture of organic matter (plant or animal origin, not necessarily certified organic), liquids, gases, minerals, and organisms. It’s a mixture of materials both alive and dead and is an ecosystem that supports life and is vital to our existence. Soil comprises three main layers, topsoil, subsoil, and parent material, the soil’s original mineral form. Depending on where you live our soil will have specific compositions such as sandy, clay, chalky, peat or silt, acid, neutral or alkaline, heavy or light, stony, free-draining, or prone to waterlogging.

In a garden environment, soil can over time become nutrient-depleted and compacted if organic matter is not introduced. Applied at least once a year, garden compost slowly releases organic nutrients which promotes balanced growth. Through the action of worms, soil organisms, and plant roots, it can be pulled down into the soil and improves aeration. This is essential for plant roots and soil life to thrive.

Use Garden and Food Waste to Make Compost

Homemade garden compost is the best DIY plant fertilizer and it’s free! The way you do it is by mixing green waste (nitrogen-rich), brown waste (carbon-rich), moisture, and air. NPKs of each ingredient will vary, but don’t worry about figures too much. A good mix of lots of different materials will make good well-balanced compost. You also need to pile the materials together in a space that keeps it all together. Though some create compost in heaps on the ground, most use a compost bin.

You can make a compost bin from recycled pallets, wood, wire, brick, or a barrel, alternatively buy manufactured wooden or plastic bins. So what can I put in my compost? Lay decaying fruit and vegetables, flower heads and leaves, coffee grounds, eggshells, tea leaves, wood chips, hay, straw and grass, soft prunings, fallen apples, seaweed, herbaceous plants, old potting compost, animal manure… the list could go on. You start to view all your organic waste as valuable when you make your own garden compost.

Different Ways to Make Compost

As the name suggests hot composting is a composting method that requires heat (around 54°C/130°F – 60°C /140°F). Soil temperature and moisture are essential for optimal microbial activity. It is hot enough to kill most weeds and harmful bacteria. As with all composting, it is important to have your material finely chopped to speed up the process. Select a compost bin around four feet by four feet, too small and the heap will not heat up enough.

Most gardeners will be familiar with cold composting. It’s the traditional method of throwing scraps in a bin in the garden, and either leaving it or turning it occasionally until broken down. For the majority of gardeners, cold composting is by far the easiest to achieve, it requires little effort or space, just the patience to wait for it to decompose.

When compost is finished, it smells sweet and earthy and has a fine, crumbly texture. When it gets to this point, apply compost in a 1-2″ layer of mulch on the soil. Used this way, it will improve soil structure and replenish nutrition. Consider homemade compost as soil food.

Homemade Plant Fertilizer for Vegetables

DIY plant fertilizer is easy to make. As you have read there are numerous to choose from and each boasts multiple benefits to your soil and plants, from improving leafy growth, roots, and soil texture to encouraging abundant crops of fruit and vegetables. You can even grow your own organic fertilizer as green manure on your veg plot to improve fertility and soil structure.  Some plants like red clover, legumes, and comfrey have nitrogen-fixing properties and once dug into the soil will slowly release. Plant fertilizers really are amazing!

Take care of your soil and the soil will take care of your plants. Using organic materials supports soil health and microorganisms in the long term and saves money; we just need to be patient for mother nature to slowly work her magic.

If you want to get creative with DIY plant fertilizers and concoctions I can highly recommend the book Garden Alchemy by Stephanie Rose. Another great book to draw frugal garden ideas from is Grow Food for Free by Huw Richards. You can also explore Lovely Greens for top tips for testing soil, controlling New Zealand flatworm, and growing seeds indoors:

• The Easiest Way to Test Soil pH and how to naturally amend it 
• New Zealand flatworm control in the garden 
• Tips for Starting Seeds Indoors: grow lights, propagators, and more 

Contributors for this piece