Understanding fertilizers – How and when to feed your garden

fertilisant naturel bio permaculture afrique

The plants in your garden need food like you, it may seem obvious, but you will be surprised at how many people don’t bother to feed their plants and wonder why they are not beautiful or don’t grow well.
Even if you occasionally feed your garden, knowing what to give your plants and when to feed your plants can make the difference between a successful garden and a disappointing and discouraging result, so if we have to feed our plants, what do we give them to eat?

Understanding plant nutrition NPK

The majority of plant biomass does not come from the soil, but from air and water! Photosynthesizing plants use carbon dioxide from the air, in combination with water and sunlight, to produce sugars and carbohydrates.
Plants need various nutrients in different amounts, and can derive them from the air, water or soil.
The nutrients a plant needs in greater quantities are called macronutrients.
The macronutrients that plants obtain from air and water are carbon (C), hydrogen (H) and oxygen (O). (The capital letter in brackets is the scientific chemical symbol of the element)/
Although plants absorb only a very small amount of nutrients from the soil, these elements are very important for the growth and health of the plant.


The three main macronutrients that plants extract from the soil are nitrogen (N), phosphorus (P) and potassium (K).
If you look at the label of a fertilizer, 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 needed in smaller quantities, but which remain very important, are magnesium (Mg), calcium (Ca) and sulphur (S).
What do these macronutrients do? Rather than entering into the complex chemistry of plants that only concerns plant specialists and means almost nothing to the majority of gardeners, it is easier to explain the functions in general terms that are relevant to practical gardening.
Plants use:

  • Nitrogen for vegetative growth of green deciduous trees
  • Phosphorus for root formation, stem growth and fruitingPotassium for flowering and ripening of fruits, plant immunity/disease resistance
  • Magnesium for photosynthesis is the key element of chlorophyll, a pigment that makes plants green and allows them to absorb light energy.
  • Calcium for structural purposes in cell walls and membranes, mainly to maintain cell walls together, and also others for metabolic functions.
  • sulphur (sulphur) for the formation of amino acids, proteins, oils and chlorophyll


Micronutrients are nutrients that the plant needs in trace amounts, such as iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni) and chlorine (Cl), which are called micronutrients in fertilizer.
At least some of the most common trace elements are included in most complete fertilizers (they are listed on the ingredient label), but you can also buy the trace elements as a separate product, more often as a mixture of trace elements combined together, but in some cases, alone, as iron in the form of iron chelate.

The importance of soil

This complex ecosystem is made up of a multitude of organisms that transform materials to break them down and release nutrients in a form usable by plants.
In permaculture, there is a saying that says « feed the soil, not the plants », and the rationale for this healthy ecological perspective is very clear when we know that soil is not only an environment to anchor plants and retain water, but a complex living ecosystem that is much more complex than any other ecosystem above the ground !
When we add synthetic chemical fertilizers to the soil, water-soluble nutrients are transported with water, so that plants are abnormally « forcibly fed » when they absorb water. 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 that supports plants.
It kills them mainly by osmotic effect, just as salt kills bacteria when you use salt to preserve meat or other foods.
Soil is not a passive environment as some might think, it is an active ecosystem in which beneficial soil organisms play an active role in the suppression of plant diseases and parasites. We are all familiar with this concept, because it works in us!
Our stomachs are like the soil-feeding network, they are also living ecosystems, which have a direct influence on our health when they are balanced or unbalanced. Our stomach contains beneficial organisms that outnumber and keep harmful organisms under control. When we take antibiotics, they destroy the intestinal flora (the right organisms) and we are advised to take fermented probiotic products containing particular strains of Lactobacillus casei to restore balance. When the ecosystem is unbalanced, harmful organisms (pathogens) can multiply and have a negative effect on our health.
If we keep the living ecosystem healthy and balanced, our soil will manage some pests and diseases as it has done for hundreds of millions of years. When we destroy the ecology of the soil by digging, compressing the soil, using synthetic fertilizers and other toxic synthetic products such as herbicides, pesticides and fungicides, we lose this first line of defence and plants suffer.

Knowing plant foods

What is fertilizer and what is not?
One of the most common mistakes is to confuse soil amendment products with fertilizers.
The following products are all fertilizers, plant foods:
Blood and bone ManureManureManure Fish emulsionLombricompostBlood and bone are made entirely of nitrogen and phosphorus, just like poultry manure. animal manure is mainly made of nitrogen and a little phosphorus.
None of them contain potassium (potash), so it should be added as potassium sulphate (which is organically approved) or as a more natural seaweed extract.
Fish emulsions are essentially a source of nitrogen and phosphorus, and do not contain much potassium, which is usually added. Quality products will be improved with seaweed extract or potassium sulphate, but lower quality products will use potassium chloride, which is cheaper for manufacturers, but toxic to your plants and the soil!
Wood ash is also rich in potassium, it contains potassium carbonate, which is very strongly alkaline, so use it sparingly, a light watering is enough, the soil ecosystem will balance the pH change and restore the acid-alkaline balance alone, no need to add anything else to compensate for the alkalinity.
To summarize the guidelines for the use of potassium (potash) in the garden: –

  • Good potassium (potash): marine algae extract, potassium sulphate, wood ash (in small quantities) –
  • Bad potassium (potash): potassium chloride (to avoid!)

The following products are NOT fertilizers – they are not plant food :

  • Seaweed extract
  • Compost
  • Jus from vermicompost

Seaweed extract is a plant supplement. It contains large amounts of a macronutrient, potassium (potash). Coming from the ocean, it contains almost all the minerals, which helps to improve plant health very quickly. It also contains various hormones that act as growth promoters for plants, the main hormones in seaweed are auxins, gibbelerins, cytokinins and betaines. Seaweed extracts are also soil amendments, they contain alginates that create long chain molecules that improve soil structure and swell when wet to increase water retention capacity in soils that do not hold water well.
Compost is a universal soil amendment, it is what nature uses to build soil, in fact it is the ONLY thing it uses to build soil! Compost adds organic matter to the soil, both living and non-living. This organic matter is an essential part of healthy soil, without it, your soil is nothing but crushed minerals, sand and clay in various mixtures.
Tthe rules for using compost are very simple.
if your soil is too sandy, add compost, organic matter that improves soil structure and promotes the retention of moisture and nutrients.
If your soil is too clayey – add compost, it adds organic matter that improves soil structure and helps break up clay and improve drainage.
Gypsum will break up the clay, but if you don’t add organic matter between the broken clay particles, you will only transform the solid clay into sticky clay!
Vermicompost juice is actually a solution of beneficial microbes that will bring the soil back to life, restoring the soil-feeding system in the same way that we take probiotic supplements to inoculate good microbes into our stomachs. It contains some minerals but is certainly not food for plants.


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