Synthetic or organic fertilizers?

Extract from the film: Les Gardiennes

The aim of agriculture is to produce crops that will be consumed by humans or livestock. These crops contain mineral salts taken from the soil, whose reserves are not infinite. For example, in the Mediterranean region, potatoes export an average of 3.2 kg of nitrogen, 1.6 kg of phosphorus, 6 kg of potassium, 0.4 kg of magnesium and 0.3 kg of sulphur per tonne of tubers harvested (1).

Mineral salts come from the breakdown of parent rock, the decomposition of organic matter, or the activity of specialised bacteria such as nitrogen-fixing azotobacter. The natural breakdown of parent rock is not sufficient to compensate for the loss of mineral salts caused by harvesting. This is particularly the case for certain major substances and trace elements such as potassium, magnesium, boron, zinc, molybdenum and others. If farmers do not take action to compensate for these losses, deficiencies quickly set in, resulting in lost production and susceptibility to disease.

Plants find it difficult to defend themselves against infections when they are poorly nourished. The healing of wounds and the regeneration of organs destroyed by pests are slowed down or even interrupted. Pathogenic bacteria and fungi establish themselves more easily. For example, in potatoes, a potassium deficiency not only reduces yield, but also weakens the potato’s defences, making it more susceptible to late blight. It is also known that plants with adequate potassium nutrition have thicker cell walls, which increases their resistance to lodging and to attacks by fungi and insects.

The use of chemical fertilisers in the hope of increasing soil fertility is said to have negative consequences on microflora, leading to an imbalance in biodiversity and soil depletion, ultimately reducing it to a skeletal state. How is this possible? A website provides an answer often cited in environmental circles: “It is one of the principles of natural gardening to nourish the soil, as opposed to chemical fertilisers, which nourish the plant but leave the soil poor.” (emphasis added by the author) (2).

Clearly, the author of this website is unaware that conventional agriculture mainly uses complete fertilisers containing urea or ammonium nitrate, which are gradually converted into nitrate by microflora organisms (bacteria, archaea, etc.).

Urea is a natural substance found in animal urine, which is present in manure. When manure is spread on fields (or added to compost), the urea is broken down into ammonium nitrate and then into nitrate, which can be absorbed by plants. Part of the urea and ammonium nitrate in compound fertilisers is mobilised by soil microflora for its own needs, as is the case with organic manure-based fertilisers.

Thus, complete fertilisers, by nourishing the microflora, contribute to the development of microbial biomass.

However, synthetic fertilisers do not directly add carbon to the soil. But cultivated plants add this carbon to the microflora through rhizodeposition (see the article on the rhizosphere), which can be supplemented by organic amendments (compost, green manure, etc.) and straw incorporation. Thus, mineral fertilisers increase plant production and produce more organic waste (straw, plant debris, root mass, etc.). Mineral fertilisers therefore indirectly generate humus.

Recycling organic matter; inevitable losses

Ideal recycling would consist of returning all the nutrients taken from crops back into the soil. This objective could be achieved through sustainable agriculture based on the model of crop and livestock farming. For amateur gardeners, and in line with this approach, the model of vegetable gardens and small animal husbandry (rabbits, chickens, etc.) is recommended. However, this is more difficult to implement in large-scale farming, especially for cereals, because it requires large areas of grassland and because losses during the various stages of organic matter recycling cannot be avoided for the following reasons:

• The fermentation of composted manure is accompanied by a loss of ammonia into the atmosphere.
• The mineral elements that remain in the soil are not fully absorbed by plants; some are lost through leaching or carried deep underground (lixiviation).
• The evolution of compost incorporated into the soil is also accompanied by gas losses during biological nitrification processes.
• Food consumption is always accompanied by losses of recoverable elements. Some of these are retained by animals and the bacteria they harbour, while others are lost in the form of gas and non-recycled excrement.
• With the widespread use of mains drainage in both urban and rural areas, human excrement is no longer recovered for agricultural use. After treatment in sewage treatment plants, it ends up in the form of nitrates, phosphates and other mineral salts that are released into the environment and end up in watercourses and groundwater. A gold mine lost forever. Barely 50 years ago, the contents of septic tanks were still a valuable source of fertiliser for allotments and farmers.

Note on the origin of organic fertilisers

It is not certain that organic fertilisers purchased from a specialist shop meet the criteria for organic farming in terms of their origin. Organic fertilisers purchased outside the “organic” farm, such as dried blood, ground horn and poultry manure and droppings, often come from conventional farms and are therefore ultimately the result of the conversion of synthetic fertilisers (used to produce animal feed) into organic fertilisers. Some inputs even contain imported GMO residues, which have been “cleaned” by passing through conventional livestock buildings