Introduction to integrated methods in the vegetable garden
Chapter : Fertilization
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⇒ Synthetic or organic fertilizers?
Extract from the film: Les Gardiennes
About thirty simple elements are absorbed by plants, some of which are considered major elements. Among these, nitrogen, carbon, oxygen, phosphorus and potassium are consumed in fairly large quantities. Calcium, sulphur and magnesium, which are also considered major elements, are consumed in smaller quantities. Of all the nutrients, nitrogen is the only element absorbed by plants that does not pre-exist in the parent rock.
The major elements are responsible for more than 99% of plant weight. Other simple elements called trace elements are absorbed in very small quantities such as manganese, iron, boron, zinc, molybdenum....
Some trace elements: Cobalt, silicon, iodine and fluorine are only absorbed by certain plants. For this reason, they are not considered essential elements for all plants, whereas they are for humans. Constituent elements such as nitrogen, oxygen and carbon are used in the composition of plants to form proteins, carbohydrates and vitamins.
Non-constitutive elements such as potassium, chlorine, sodium, etc. are used during certain phases of the crop cycle to perform biological functions. All these elements are supplied in the form of mineral salts (nitrates, phosphates, sulphates...) or ionised atoms (K⁺ Ca₂⁺ Mg₂⁺ Na⁺) soluble in water.
All plants have different mineral requirements and these requirements change over time. It is therefore important that the farmer regulates the supply of these minerals throughout the crop cycle.
The introduction of mineral fertilisers in fertilisation more than 50 years ago has always been controversial. The lack of information and the fear of "chemicals" are at the origin of an aversion to anything synthetic, accentuated in recent years by the development of the internet with the proliferation of sites propagating inaccurate statements.
In the first half of the 20th century, the agronomist Sir Albert Howard was already questioning the artificial nature of synthetic fertilisers, which would disrupt the proper functioning of plants. For him, all inert elements such as minerals and chemical fertilisers interfere negatively with plant growth. Artificial fertility replacing natural fertility would be potentially harmful. Nowadays, this argument is still evoked in different forms in writings and websites.
For example, some naturalists believe that organic crops are in tune with nature because they are fed with manure and compost from the living world. But since such mystical claims cannot be made in every household, other proponents resort to more sophisticated claims that have the appearance of science to try to convince more educated sections of the population. Here is a significant example:
The use of chemical fertilisers in the hope of increasing soil fertility would have negative consequences on the microflora, producing in return an imbalance in biodiversity and a depletion of the soil, reducing it to a skeleton. How is this possible? One website gives an answer often mentioned in environmentalist circles: "It is one of the principles of natural vegetable gardening which consists of nourishing the soil, as opposed to chemical fertilizers which nourish the plant, but leave the soil poor." (bold underlining is by the author) (2).
Obviously, the author of this website is unaware that in conventional agriculture, complete fertilisers are used which contain urea or ammonitrate which are progressively transformed into nitrate by microflora organisms (bacteria, archaea...). Urea is a natural substance present in animal urine and is found in manure. When manure is spread on fields (or introduced into composts), urea is broken down into ammonium nitrate and then into plant-available nitrate. Some of the urea and ammonium nitrate in compound fertilisers is mobilised by the soil microflora for their own needs, as is the case with organic manure-based fertilisers. Thus, by feeding the microflora, complete fertilisers contribute to the development of the microbial biomass.
On the other hand, synthetic fertilisers do not bring carbon directly into the soil. However, cultivated plants provide this carbon to the microflora by rhizodeposition (see the article on the rhizosphere), which can be supplemented by organic amendments (compost, green manure, etc.) and the burial of straw. Thus, mineral fertilisers increase plant production and produce more organic waste (straw, plant debris, root mass, etc.). Mineral fertilisers therefore indirectly generate humus.
An ideal recycling would be to return all the nutrients removed by the crops to the soil. This could be achieved through sustainable agriculture based on the model of the crop and livestock farm. For the home gardener, the model of a vegetable garden and small animal husbandry (rabbits, chickens, etc.) is proposed. But this is more difficult to implement in arable farming, especially for cereals, because large areas of grassland are required, and because losses during the various stages of organic matter recycling cannot be avoided for the following reasons:
Fermentation of composted manures is accompanied by a loss of ammonia to the atmosphere.
The mineral elements that remain in the soil are not completely absorbed by the plants; some are lost through leaching or carried away to the depths (leaching).
The evolution of composts incorporated into the soil is also accompanied by gaseous losses during biological nitrification processes.
The consumption of food is always accompanied by losses in recoverable elements. Some of this is retained by animals and the bacteria they harbour, while some is lost in the form of unrecycled gases and excrement.
With the generalisation of sewage systems in cities and the countryside, human excrement is no longer recovered for agricultural use. After treatment in sewage treatment plants, it ends up as nitrates, phosphates and other mineral salts, which are released into the environment and end up in rivers and groundwater. A gold mine lost forever. Just 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 bought from a specialised shop meet the criteria for organic farming in terms of their origins. Organic fertilisers purchased outside the 'organic' farm, such as dried blood, ground horn and poultry manure, often come from conventional farms and are therefore ultimately the result of converting synthetic fertilisers (which were used to produce animal feed) into organic fertilisers. Some inputs even contain imported GMO residues, 'bleached' by the passage through conventional livestock buildings.
Synthetic fertilisers are refused in organic farming because the companies that produce them consume fossil fuels and emit greenhouse gases. Compost preparation produces just as much, if not more, notably through the volatilisation of ammonia. Industrial fertilisers contain less and less synthetic urea to avoid this ammonia volatilisation phenomenon. In France, ammonium nitrate has become the main form of nitrogen fertiliser used in conventional agriculture. Unfortunately, ammonium nitrate alone is now banned for home gardeners, forcing them to use urea when they need a nitrogen-only fertiliser.
For mineral fertilisation, compared to the other major elements contained in compound fertilisers, the industrial production of nitrogenous fertilisers consumes the most energy. This is supplied from natural gas (CH₄), two thirds of which is used as a raw material to manufacture ammonia with atmospheric nitrogen, and one third as an energy source. The energy expenditure for the production of nitrogen fertilizer from atmospheric nitrogen represents only 1% of the world energy expenditure (4). Abandoning this industrial production would therefore have no impact on the greenhouse effect.
On the other hand, if the organic farming model is imposed, it would be responsible for a significant increase in greenhouse gases, mainly in the form of methane and nitrous oxide from the composting of organic matter. To claim that organic farming is the inevitable solution to reduce greenhouse gas production in agriculture is therefore a sham. In turn, mineral fertilizers fix 4 to 6 times more CO₂ to form biomass, compared to the amount of CO₂ that is consumed in the production, transport and application stages of the same mineral fertilizers (5).
Mineral fertilizers thus allow crops to reach their maximum growth potential to fix more solar energy and CO₂ from the atmosphere. Note that there are alternatives when fossil fuels run out, such as hydrogen supplied by electrolysis of water.
Current land use in France
In 2015, cultivated agricultural land covers 36% of our territory, grassland 15%, woodland 31%, artificial land (buildings, roads, etc.) 13%, and the remaining 5% corresponds to moorland, scrubland, garrigue and other occupations (3). Taking into account that in France yield losses in organic farming are in the order of 40% to 50% compared to conventional farming, in order to produce the same amount of food, agricultural land would have to be increased to 50% or even 54%, the difference (from 14% to 18%) being taken from forest areas (which would increase to 22% or 16%). In order to make agriculture sustainable and no longer dependent on chemical fertilisers, this forest area would in turn have to be transformed into grassland to produce manure, which is largely insufficient. Ultimately, a return to the old model of crop and livestock farms will require a considerable expansion of cultivated areas at the expense of forests, which will inevitably lead to a loss of biodiversity; the very opposite of what the thinkers of the ecological movement want.
In France, because of the number of people that now need to be fed, it is likely that the transformation of all the remaining forests into grassland will not be enough to satisfy the organic input requirements of organic farming. Sustainable agriculture existed before the invention of fertilisers, but it was able to feed barely 20 million French people in the 18th century, to take one example, and when weather conditions were favourable with reduced pest pressure which was not always the case. Random and uncontrollable conditions that produced famines from time to time. Today, in France, we have to feed three times as many people with a global area that has not changed.
The consequences would be even worse if tomorrow countries like India, China and others with more than a hundred million inhabitants decided to convert their entire agricultural area to organic farming with the model of the crop and livestock farm. No one in these countries is seriously considering going down this road, except for a few ignorant and irresponsible extremists claiming to be part of the ecological movement.
The dream of self-sufficient sustainable agriculture remains a difficult goal to achieve unless major progress is made in new genomic editing technologies to increase production, reduce water consumption and create plants capable of surviving in poor soil. A technology that is also rejected by environmentalists.
Mineral fertilisers are said to be responsible for the degradation of soil biodiversity. But this is also the case for organic fertilisers. It is mainly the excessive use of chemical and/or organic fertilisers that has deleterious effects on soil microorganisms, which in turn degrades soil fertility and pollutes the environment. Careful use of fertilisers in accordance with the needs of the plants throughout their vegetative cycle does not have this disadvantage.
By increasing the fertility potential of the soil, all fertilisers have a positive effect on the growth and abundance of certain living organisms in the soil. But they also lead to a depletion of species adapted to nutrient-poor environments. Most crops need nutrient-rich soil, especially vegetable crops, which requires correction of soil fertility potential.
Regardless of how the soil is corrected with organic or mineral inputs, the result is always an increase in fertility potential, which in turn changes the environment. Irrigation also has an impact on biodiversity by creating negative competition with plants that survive in drylands. One would be entitled to be concerned about this evolution of biodiversity if it had the consequence of reducing the services provided to farmers by ecosystems, which has not been demonstrated. To sum up, should we abandon agriculture on the pretext of preserving natural biodiversity? Who is ready to go back to the prehistoric days of hunting and gathering?
It is said that mineral fertilisers acidify soils, which is not untrue, except that organic fertilisers produce the same effect. This acidification occurs when these fertilisers are transformed into nitrate (see the article: soil acidity and alkalinity).
1) Fatah AMEUR. Recherche de meilleures pratiques agricoles pour la culture de la pomme de terre - Ecole nationale supérieure agronomique El-Harrach Alger
3) Portail de l'artificialisation des sols ; panorama de base de données TERUTI LUCAS
4) Réponse à l’écologisme – comment la connaissance permet de réfuter les peurs entretenues