The Organic Gardener’s Guide to Fertilisers and How to Use Them

feeding garden with fertiliser pellets

The plants in your garden need food just like you do! That might seem obvious but you’ll be surprised how many people never bother to feed their plants and then wonder why they aren’t flowering, fruiting, or growing all that well.

Even if you do give your garden the occasional feed, knowing what to feed your plants and when to feed them can make the difference between a successful flourishing garden and a disappointing, discouraging result.

If we must feed our plants, what do we feed them?

Understanding Plant Nutrition

The majority of a plant’s biomass does not come from the soil, it actually comes from air and water!. Plants photosynthesize, they use carbon dioxide (CO2) from the air, in combination with water and sunlight to manufacture sugars and carbohydrates, and produce oxygen.

Plants use the energy of sunlight to convert water and carbon dioxide (CO2) into sugars and carbohydrates

Plants require various nutrients in different quantities, and they may derive them from the air, water and soil. The nutrients which plants requires in larger quantities are known as macronutrients.

The macronutrients that plants get from the air and water are Carbon (C), Hydrogen (H) and Oxygen (O).

(The capital letters after the element names are their scientific chemical symbols)

Even though plants only take up a very small amount of nutrients from the soil, these soil nutrients are very important for the growth and health of plants.

Plant Macronutrients

The three main macronutrients that plants obtain from the soil are Nitrogen (N), Phosphorus (P) and Potassium (K).

If you look at the label any fertiliser you will see an NPK ratio listed such as ‘NPK analysis: 3.7 – 2 – 1.8’ which indicates the proportions of Nitrogen (N), Phosphorus (P) and Potassium (K) in the fertilizer.

The secondary macronutrients, which are required in lower quantities, but are still very important, are Magnesium (Mg), Calcium (Ca) and Sulphur (S)

What do these macronutrients do?

Rather than go into complex plant chemistry which is only of concern to plant scientists and means almost nothing to the majority of gardeners, it is simpler to explain the functions in general terms that are relevant to practical gardening.

Plants use:

  • Nitrogen for leafy green vegetative growth
  • Phosphorus for root formation, stem growth, and fruiting
  • Potassium for flowering and fruit ripening, plant immunity/disease resistance
  • Magnesium for photosynthesis, it’s the key element in chlorophyll, a pigment which makes plants green and allows plants to absorb energy from light
  • Calcium for structural purposes in the cell walls and membranes, basically to keep cell walls together, and also for other metabolic functions
  • Sulphur (Sulfur) for the formation of amino acids, proteins, oils and chlorophyll

Plant Micronutrients

Micronutrients are nutrients that the plant requires in trace amounts, such as Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Nickel (Ni) and Chlorine (Cl).

In fertilizers, these micronutrients are referred to as trace elements. At least some of the most common trace elements are included in most complete fertilizers (they’re listed on the label of ingredients), but you can also purchase trace elements as a separate product, more often as a mixture of trace elements combined together, but in some cases, as individual elements, such as iron in the form of iron chelate.

The Soil-Food Web and How Plants Feed

It is not actually possible to feed plants directly in any real sense, when you add natural fertilizers (as opposed to synthetic chemical fertilizers) to the soil, you add raw materials to the soil-food web, which is the complex ecosystem below the ground which serves the function of returning everything once living back into the soil. This complex ecosystem is comprised of endless multitudes or soil organisms which process materials to break them down, and release nutrients in a form that plants can use.

In Permaculture, there is a saying “feed the soil, not the plants”, and the rationale for this sound ecological perspective is quite clear when we consider that the soil is not just a medium to anchor plants in and hold water, but one of the most complex living ecosystem on the planet!

When we add synthetic chemical fertilizers to soil, the water soluble nutrients are carried with water, so plants are unnaturally force fed when they take up water, and forced to grow very quickly. This results in soft, sappy elongated growth which aphids and other sap-sucking insects absolutely love.

To make matters worse, synthetic chemical fertilizers are essentially simple mineral salts, much like table salt is, and their addition to the soil kills the soil organisms that make the soil a living ecosystem which supports plants. It basically kills them through an osmotic effect, much how salt kills bacteria when we use salt to preserve meat or other foods.

The soil is not a passive growing medium as some might think, it is an active ecosystem in which the beneficial soil organism play an active part in suppressing plant diseases and pests. We are all familiar with this concept as it operates inside of us in terms of gut health!

Our digestive tract is just like the soil-food web, it’s a living ecosystem which has a direct influence on our health when balanced or unbalanced. Our gut contains beneficial organisms which outnumber the bad ones and keep them under control. When we take antibiotics they destroy out gut flora (the good organisms), we’re advised to take probiotic fermented products containing particular strains of Lactobacillus casei to restore the balance. When our gut ecosystem is out of balance, harmful organisms (pathogens) can grow in numbers and have a detrimental effect on our health.

Soil works in exactly the same way. If we keep the living ecosystem healthy and balanced, our soil will manage certain pests and diseases the way it has done for hundreds of millions of years. When we destroy the soil ecology through digging, soil compaction, use of synthetic fertilizer and toxic synthetic agents such as herbicides, pesticides and fungicides, te soil loses that first line of defence and the plants suffer for it.

Know Your Garden Products, Which Ones Are Fertilizers?

One common mistake gardeners make is confusing soil amendment products with fertilizers. They are not the same thing.

Soil amendments improve the soil structure, soil chemistry or water holding ability in various ways.

Fertilisers are a food for plants, a source of plant nutrition, providing them with the nutrients they need.

The following garden products are all fertilizers:

  • Blood & Bone
  • Animal and Poultry Manures
  • Fish Emulsion
  • Worm Castings

Blood & Bone contains only nitrogen and phosphorus, and needs to be supplemented with a source of potassium for it to be used as a balanced fertiliser.

Animal and poultry manures are mainly nitrogen and phosphorus with very little potassium. If these are used as a fertiliser, it’s a good idea to also use seaweed extract, or potassium sulphate, also known as sulphate of potash (which is organic approved) to add extra potassium which is required for flowering and fruiting.

Wood ash is also high in potassium, it contains potassium carbonate, which is very highly alkaline (and contains lime, or calcium carbonate, which is also alkaline), so use this sparingly, a light sprinkle is all that is required. The soil ecosystem will balance the pH change and return the acid-alkaline balance on its own, so there’s no need to add anything else to counteract the alkalinity.

Fish emulsions are essentially a source of nitrogen and phosphorus, and they don’t normally contain much potassium, so it’s usually added to the product during processing to make it a more balanced fertiliser.

The quality fish emulsion products will be either enhanced with seaweed extract or potassium sulphate to add the extra potassium. The lower quality products will employ a cost cutting measure, and use potassium chloride insted, which is cheaper for manufacturers, but the drawback is that it’s toxic to plants and soil!

To summarise the guidelines for potassium (potash) usage in the garden:

  • Good potassium (potash) – seaweed extract, potassium sulphate, wood ash (in small quantities)
  • Bad potassium (potash) – potassium chloride (cheap and nasty, avoid!) 

How Different Manures Compare as Fertilisers

Manures vary in the amount of nutrients they contain. The table below lists most common manures and their respective nitrogen, phosphorus and potassium levels.

Comparison of Manure Nutrient Levels

Listed below is a comparison of the percentage of Nitrogen (N), Phosphorus (P) and Potassium (K) in various animal manures and worm castings.

  • Rabbit: 3-4.8 %N, 1.5-2.8 %P, 1-1.3 %K, medium release speed
  • Cattle: 0.5-1.5 %N, 0.2-0.7 %P, 0.5-2 %K, medium release speed
  • Cattle (dairy): 0.5–2 %N, 0.3-0.5 %P, 0.4-1.5 %K, medium release speed
  • Horse: 0.7-1.5 %N, 0.2-0.7 %P, 0.6-0.8 %K, medium release speed
  • Sheep: 2.2-3.6 %N, 0.3-0.6 %P, 0.7-1.7 %K, medium release speed
  • Poultry (75% water): 1.5 %N, 1 %P, 0.5 %K,
  • Poultry (50% water): 1.5-2 %N, 1.8 – 2 %P, 1 %K, medium to fast release speed
  • Poultry (30% water): 3–4 %N, 2.5 %P, 1.5 %K,medium to fast release speed.
  • Poultry (15% water): 6 %N, 4 %P, 3 %K, medium to fast release speed
  • Worm Castings: 1.5 %N, 2.5 %P, 1 %K

Poultry manure is listed in the above table with various percentages of water because fresh poultry manure is wet, but when dried it reduces in volume and increases in nutrient concentration.

Liquid Fertilizers vs Solid Fertilizers

Are liquid fertilisers a good choice for feeding the garden?

Solid fertilizers such as manures, blood & bone, blended fertiliser pellets are slow releasse fertilisers which release nutrients into the soil over a period of 6-12 month, and are the only fertilisers which should be used to give a garden its main feed for the season.

Liquid fertilisers are only meant to used to provide a quick extra supplementary feed if needed in the weeks or months after a proper feed with a solid fertiliser. All the liquid fertilisers are only suitable as a top-up feed, and not the main feed for the season. Being liquids, they do wash out after watering, and they wash out of pots and containers much faster than they do out of soil.

Garden Products Which Are Mistakenly Thought to Be Fertilisers, But Aren’t!

The following garden products are NOT fertilizers, they have other uses, and cannot be used to feed plants:

  • Seaweed Extract
  • Compost
  • Worm Casting Leachate (‘worm wee’)

Seaweed extract is a plant supplement. It is an excellent root growth stimulant to use after transplanting new plants into the ground or repotting them. It does contain high amounts of one macronutrient, potassium (potash). Coming from the ocean, seaweed extract contains almost every mineral, which helps boost plant health very quickly. It also contains various hormones which act as growth stimulants for plants, the main hormones in seaweed are auxins, gibbelerins, cytokinins and betaines. Seaweed extracts are also soil conditioners, they contain alginates which create long chains molecules which improve soil structure and which swell when wet to increase the water holding ability in soils that don’t hold water well.

Compost is a universal soil amendment, it’s what Nature uses to build soil, in facts its the ONLY THING Nature uses to build soil! The main purpose of using compost in the garden is to add organic material to soil, both living and non-living. It does contain a small amount of nutrients, but not enough to be used as the only source of nutrients to feed a garden. This organic matter is a critical part of healthy soil, without it the soil is just a mixture of sand, silt and clay in various proportions.

The rules for compost use are quite simple.

  • If your soil is too sandy – add compost, it adds organic matter which improves soil structure and helps with moisture and nutrient retention.
  • If your soil is too clay –add compost, it adds organic matter which improves soil structure and helps break up the clay and improve drainage.

Many people will try to amend clay soil with gypsum alone. Gypsum will break up sodic clay soils, but if you don’t add organic matter between the particles of broken up clay, all you will manage to do is turn solid clay into gluggy clay!

Worm Casting Leachate, also known as ‘worm wee’ is really a solution of beneficial microbes which will bring life back to the soil, restoring the soil-food web much the same way that we take probiotic supplements to inoculate our stomach with good microbes. It does contain some minerals but is definitely not plants food, in contrast to worm castings – the rich, dark material in your worm farm, which is one of the best fertilizers around – don’t confuse the two!

When Should You Feed Your Garden?

At the very least, feed your garden at the beginning of spring and the beginning of autumn, that’s twice a year.

Which months? Depends in which half of the world you’re in, the top or bottom half!

If you’re in the Southern hemisphere, feed your garden:

  • at the beginning of spring (September)
  • at the beginning of autumn (March)

If you’re in the Northern hemisphere, feed your garden:

  • at the beginning of spring (March)
  • at the beginning of autumn (September)

You can feed more often, make sure you follow the directions on the fertiliser product you’re using. Over-fertilizing is as bad as under-fertilizing, so don’t overdo it. Too much of a very strong manure such as chicken or horse manure can burn the roots of plants and trees, so go easy with these.

Generally, you can feed your garden with natural fertilizer every 6-8 weeks during the growing season.

For additional information, see article – Why You Shouldn’t Fertilise Gardens in Winter in Cool and Temperate Climates

Obviously, certain plants have specific feeding requirements, and you may have certain climatic considerations for your location to take into account, so please don’t try to fertilize your rare orchids the same way as your lemon tree!

Happy growing!

9 thoughts on “The Organic Gardener’s Guide to Fertilisers and How to Use Them

  1. I am confused as I’ve been told all manure (chicken, roadside farm manure etc), should be put in the compost to be broken down, prior to putting it on the garden.
    But here you suggest manure is nutrition for the plants, and compost (the product of the manure breaking down), would be the soil structure (if I understand correctly). Can you put chicken manure straight on your plants, in your pots and under your trees?

    1. All manure needs to first be composted (when sourced as bulk manure from a farm) and only then put in the garden, but ‘composting’ doesn’t mean it’s thrown into your regular compost bin with all the other bits and pieces, a more correct term is ‘aged’ – it is allowed to sit in a pile for some time, it is rested or aged, where it will dry out a bit or a lot, change consistency or release excess ammonia which can burn plant roots. Fresh cow pats are a sloppy mess, partly dried cow pats look like large disks of dried poo, and aged cow manure looks much like compost.

      If you have a chicken coop, you can put small amounts of chicken manure straight into your garden after you clean out your chicken coop, the manure will be mixed with straw, sawdust or other bedding material and you won’t have massive quantities of it.

      All manures that you buy from commercial suppliers and garden centres are aged already and can be used immediately, when you buy manure from roadside farm outlets, it needs to be aged first.

      1. Thanks, I had completely misunderstood, and the cow pat explanation is perfect. Cheers

  2. Excellent presentation! I am saving this as a reference for clients! We have something similar but not as detailed about fertilizer on our website. Thank you for sharing!

  3. Dont forget other vegetation that add nutrients to the compost pile such as yarrow, valerrian, dandylion, nettle and bits of oak bark all documented in Maye Bruces system the QR compost solution. I tried it and it works. so when ever I am composting I pay particular attention to those plants.

  4. Living near the coast I have been taking advantage of seaweed. I make teas, use it as foliar, use it in the compost pile, as a mulch, and in the holes I plant. Amazing stuff. It breaks down quick into black gold, I mix in what I use as top dressing which helps repel slugs and when I dig it up next year it is just so rich. Roots love growing around it, and soil structure is great. It definitely speeds up compost decomposition too, our pile seems to break down way faster now. I mix in a little in the spring and dump a gallon or so of tea on the pile and off it goes! I plan to make a pile that’s mostly seaweed and leaves this year and dry more to shred to make an actual kelp meal. A local bait shop sells 5gallons of bait fish cheap so I might try a fish emulsion but I know it’s going to stank lol. Check around or ask local farms for manure and they might already have aged stuff cheap or for free. I scored a heaping truck load of 5year aged cow manure that they already sifted and fluffed $40. Can’t beat that!

  5. Dear Angelo thank you so much for your well researched and clearly presented articles. I have learnt so much from you and Deep Green Permaculture is always one of the first places I look if I am stuck as to how to proceed. Fabulous work greatly appreciated thanks. Katy (Urban Permaculture Cape Town)

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