Understanding Fertilisers – How and when to feed your garden



The plants in your garden need food just like you –  that might seem obvious but you’ll be surprised how many people never bother to feed their plants and wonder why they aren’t looking the best 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 your plants can make the difference between a successful flourishing garden and a disappointing, discouraging result.

So, if we must feed our plants, what do we feed them?


Understanding Plant Nutrition – NPK

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



Source image: Wikimedia Commons

Plants require various nutrients in different quantities, and they may derive them from the air, water or soil.

The nutrients that a plant requires in larger quantities are called macronutrients.

The macronutrients that plants get from the air and water are Carbon (C), Hydrogen (H) and Oxygen (O). (The capital letter in brackets is the scientific chemical symbol for the element)

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



The three main macronutrients that plants get 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 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 other for metabolic functions
  • Sulphur (Sulfur) for the formation of amino acids, proteins, oils and chlorophyll



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 fertilizer (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 Importance of the Soil – The Soil-Food Web

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 a complex living ecosystem which is way more complicated than any ecosystem above the ground!

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. The result is forced fast growth which is long, weak and sappy, which aphids and other sap-sucking insects absolutely love.

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

The soil is not a passive medium as some might think, it is an active ecosystem in which 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!

Our stomachs are just like the soil-food web, they are also living ecosystems, which have a direct influence on our health when they are balanced or unbalanced. Our stomach contains beneficial organisms which outnumber any bad ones and keep them under control. When we take antibiotics they destroy out gut flora (the good organisms) and we are advised to take probiotic fermented products containing particular strains of Lactobacillus casei to restore the balance. When the 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 with digging, soil compaction, synthetic fertilizers and other nasty toxic synthetics such as herbicides, pesticides and fungicides, you lose that first line of defence and the plants suffer for it.


Knowing You Plant Foods – What is Fertilizer and What Isn’t!

On e common mistake people make is that of confusing soil amendment products with fertilizers.

The following are all fertilizers – plants foods that provide nutrition:

  • Blood & Bone
  • Manures
  • Fish Emulsion
  • Worm Castings

Blood & Bone is all nitrogen and phosphorus, as are poultry manures. Animal manures are mainly nitrogen with some phosphorus. None of these contain potassium (potash) so this needs to be added as Potassium Sulphate (which is organic approved) or in the more natural form of seaweed extract.

Fish emulsions are essentially a source of nitrogen and phosphorus, and don’t contain much potassium, this is usually added. The quality products will be either enhanced with seaweed extract or potassium sulphate, but the lower quality products will use a cost cutting measure and use potassium chloride instead, which is cheaper for manufacturers, but is toxic to your plants and soil!

Wood ash is also high in potassium, it contains potassium carbonate, which is very highly 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, no need to add anything else to counteract the alkalinity.

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 (avoid!)


The following are NOT fertilizersthese are not plants foods:

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

Seaweed extract is a plant supplement. It does contain high amounts of one macronutrient, potassium (potash). Coming from the ocean, it 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! Compost adds organic material to soil, both living and non-living. This organic matter is a critical part of healthy soil, without it your soil is just crushed minerals, sand and clay in various blends.

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 clay, 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!


The Right Season for Feeding 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.

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! Smile



About Angelo (admin)

Angelo Eliades is a presenter, trainer, writer, permaculture consultant, urban permaculture pioneer and food forest specialist.
This entry was posted in Gardening Information and tagged , , , , , . Bookmark the permalink.

5 Responses to Understanding Fertilisers – How and when to feed your garden

  1. Imogen says:

    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?


    • Angelo (admin) says:

      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.


  2. Laurin Lindsey says:

    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. Florie says:

    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.


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