Organic substrates and natural fertilizers for garden plants

Last updated on October 23rd, 2023 at 08:19 pm

In organic farming it is essential to use appropriate substrates and fertilizers, organic substrates and natural fertilizers that do not contain chemicals and that allow plants to grow healthy and strong.

Characteristics of ecological substrates

To be able to say that we are using an “organic substrate”, first of all it must lack any type of synthetic chemical fertilizer (organic fertilizers or natural fertilizers must be used to fertilize it) and it must not have been subjected to any artificial disinfection (ionizing radiation to disinfect the soil, for example, is not allowed in Organic Agriculture).

Organic substrates and natural fertilizers for garden plants

In addition, the use of ecological substrates has positive effects or benefits for the conservation of the environment. For example, ecological substrates are those:

  • Recyclable : by-products of other activities or industries that are themselves biodegradable or recyclable.
  • That limit the development of pathogens, which often avoids their treatment with artificial chemical products: composted pine bark to prevent/eliminate some fungal diseases.
  • Avoid nutrient washing.
  • Optimize water consumption: substrates with high water retention and cation exchange capacity.

Most common substrate components


We will dedicate another article to explain how to choose and/or combine properly the different materials and the techniques of preparation and maintenance of the substrate.

You also have more information about the proper characteristics of the substrate in this post I wrote about the “ideal substrate”.
Natural fertilizers: organic fertilization

We have already seen that, in addition, if some time has passed since we established the garden or if the nutrient supply of the substrate is low, we must fertilize it. We are going to see the different fertilizers or fertilizers that we can use in our organic vegetable garden.

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Organic fertilization: compost, green manures, manure, liquid fertilizers…


It is the most used fertilizer in organic farming. It is very interesting because we can make homemade compost ourselves (so we save having to buy fertilizer) and also take advantage of domestic organic waste and those of the garden itself.

Organic substrates and natural fertilizers for garden plants
Compost to fertilize the organic garden. On the left: mature compost, on the right: compost in formation.

Composting is a technique of controlled decomposition of waste to obtain humus. This transformation is a biological decomposition, that is, it is carried out by microorganisms (fungi, bacteria and small insects or arthropods) so the conditions (humidity, temperature, air, size of the waste, time of the process, etc.) have to be within certain limits (we will dedicate an article to the technique of traditional composting).

We can also buy industrial compost or, as an alternative to traditional compost, use vermicompost or worm compost, obtained thanks to the action of the red California earthworm (Eisenia foetida).

Green manures

These are crops of different fast-growing plants (legumes, grasses, cruciferous plants…) that are sown, cut and buried to improve soil properties. Not only do they fertilize the soil with humus from the buried organic matter, they also improve its structure and favor the growth of beneficial microorganism populations.

Organic substrates and natural fertilizers for garden plants
Clovers for use as green manure

The crops most commonly used as green manure because of their rapid growth and the amount of nutrients they provide are: legumes such as clover, alfalfa, vetch, peas, beans, lupin, cereals or grasses (usually associated with legumes) such as rye, oats or barley, and some cruciferous crops such as forage rape, white mustard or fodder radish.

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For proper humus production, the crop should be cut between flowering and fruit formation, and buried, not immediately but after aeration for a few days.

Instead of burying the residues, we can also use them to create a mulch, which will protect the soil.


Waste from extensive or extensive ecological livestock farms is used. The most commonly used are manure from cattle, sheep, goats, horses, pig slurry, poultry manure, guano…

It is very important the proper handling of manure before its application on the soil to favor the formation of a good humus, the absence of weeds, seeds or pathogens and to avoid the loss of nutrients. Before adding it to the soil it should “mature” in a well compacted heap without oxygen (anaerobiosis) or in a composter under aerobic conditions.


There are many types of peat coming from peat bogs, where sphagnum moss accumulates. This organic fertilizer is composed of organic matter, water and minerals. In addition to providing organic matter, it improves soil structure and promotes the slow release of nutrients. It is a non-renewable natural resource, as the rate of extraction of peat reserves is exceeding the rate of natural generation.

Prepared organic fertilizers and humic amendments 

There are many types of natural organic fertilizers already prepared from algae, plant remains… They can be liquid or solid (crushed).

We must make sure that the product is suitable for use in organic agriculture, since not all natural products are. It is necessary to follow the manufacturer’s recommendations and pay attention to whether it is a fertilizer to be applied on the substrate or on the leaves of the plant.

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Mineral fertilization

Since organic fertilization is essential for the supply of nitrogen, microorganisms and because it favors soil structure, it is also possible to use mineral fertilizers to complement it and make up for any deficiencies, directly providing useful nutrients for the plant.

In organic farming, mineral fertilizers must be natural: synthetic products cannot be used, nor with the addition of chemical products.

Below are the types of natural materials for natural fertilization allowed in organic farming according to the most important nutrient they provide:

  • Potassium: wood ash, potassium salts such as sulfates or potassium chloride (sylvinite, carnallite, kainite…).
  • Phosphorus: calcined natural phosphates (such as aluminium-calcium phosphate, which also provides aluminum and calcium), Thomas slag…
  • Magnesium and Sulfur: it is usually enough with the amount provided to the soil with organic fertilization, but if it is not enough, natural mineral amendments from dolomite, magnesite, epsonite (with magnesium) or sulfur can be used.
  • Silicon: crushed siliceous rocks.


  • James Jones

    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.