Temps de lecture :4 minutes
Global use of fertilizers has more than doubled in the last thirty years. More than 200kg per hectare is used in Europe, China, Japan, and South Korea; 50kg in Australia, 10-20kg in sub-Saharan Africa and central Asia, and an average of 100kg in other countries. This is partly due to the collapse of areas devoted to growing vegetables. During the 2006-2007 season, 164 million tonnes of fertilizer were used, of which 98 million tonnes were for nitrogen, 27 million tonnes for potassium and 39 million tonnes for phosphates. China is the world’s largest mineral fertilizer consumer using 49 million tonnes, followed by India (22 million tonnes), the United States (21 million tonnes), Brazil (9 million tonnes), Indonesia (3.5 million tonnes) and France (3.4 million tonnes). According to the United Nations (UN), since 1960 the number of dead zones in seas and oceans has doubled each decade, particularly because of nutrients such as nitrogen and phosphorus coming from rivers and streams full of agricultural fertilizer and manure. In 2005, 75% of Chinese rivers were contaminated by fertilizers.
Fertilizers are substances intended to give plans nutritional supplements in order to improve their growth and increase their yield.
In 2006, fertilizer production rose to 170.4 million tonnes, with 89 million tonnes of nitrogen fertilizer, 29.4 million tonnes of potassium fertilizer and 52 million tonnes of phosphate fertilizer. According to the FOA (Food and Agricultural Association of the UN), these levels will exceed 206.5 million tonnes in 2007, and 241 million tonnes for the 2011-2012 season. Estimated potassium stocks have climbed to 8 300 million tonnes, more than half of which is in Canada. China is the world’s largest producer of nitrogen fertilizer, Africa the world’s biggest exporter of phosphate, and Canada and Russia of potassium. These are products of the chemical industry, from the extraction of natural mineral deposits (potassium and phosphate mines). The increase in consumption is strong in developing countries, but is decreasing elsewhere. Thus in Europe, from 1996 to 2006, when the global increase in consumption was 2.5%, there has been a decrease of 3.5% for nitrogen, 12.1% for phosphate and 10.7% for potassium for 15 member states, and the opposite, an increase of 32% for nitrogen, 17% for phosphate and 18% for potassium for new member states.
Mineral fertilizers have significantly contributed to the growth in yield per hectare since 1950. Wheat production, for example, has quadrupled from a fivefold increase in fertilizer use. The highest production levels are only reached where there is sufficient water supply, which explains the growing need for crop irrigation.
Not all of the mineral fertilizer is absorbed by plants. Some of it finds its way into ditches and rivers, or seeps into the water table; some of it disperses in the air and ends up in urban areas, vouched for by the number of organisms following pollution. This atmospheric pollution, according to studies, relates to 30 to 60% of mineral nitrogen spread, and is linked to climate conditions and fertilisation techniques. A surplus of nitrogen combined with excess phosphorus (urban pollutants for washing and other domestic waste) encourages eutrophication, suffocating aquatic environments and producing toxins.
The agricultural sector, directly or indirectly, produces from 17 to 32% of the total global emissions of greenhouse gases caused by humans. To produce one kilogram of fertilizer, 1.5 litres of fuel must be burnt. More than one third of agricultural emissions come from over-fertilized soils. The processes of denitrification and evaporation of ammonia that are found in nitrogen fertilizers generate greenhouse gases 23 to 300 times more active than carbon dioxide.
The intensive use of mineral fertilizers tends to lessen the number of micro-organisms (bacteria, fungi etc.) and worms in the soil, essential for plant growth. The soil is lacking in organic matter, so more must be brought in from elsewhere.
Organic fertilizer, from plant or animal origin, comes from manure, guano, algae and various other organic waste, the most common being compost, as well as plants or plant waste straight from the earth, whether used spontaneously or cultivated (green fertilizer). One tonne of cow manure contains about 5kg of nitrogen, 2.5kg of phosphorus and 6kg of potassium. Organic fertilizing with manure has the advantage of reinforcing the life and structure of soil, and notably reduces the risk of erosion. The excess can also return to the soil and water. However, the global effects of organic fertilizers on the environment are much less than those of mineral fertilizers, in particular in terms of greenhouse gases. Organic fertilizers form a cornerstone of organic agriculture.
Using the Nitrogen in the Air
Certain plants, with the help of micro-organisms, can absorb, or fix, nitrogen from the air. Clover can fix 150kg per hectare per year, and alfalfa 180kg. Sesbiania rostrata, a leguminous plant that fixes 200 to 300kg of nitrogen per hectare in 50 days, can be used to increase the yield of rice cultivation from 2 to 4 tonnes per hectare, the equivalent of 60-80kg of chemical fertilizer. 75 million tonnes of nitrogen could be fixed by plants that would be the equivalent of 160 million tonnes of chemical fertilizer.
The Pacific island of Nauru, which became a State in 1968, has for a long time (since 1840) extracted the phosphate discovered there. From 1968 to 2002, it exported 43 million tonnes of phosphate, becoming one of the richest States per captia in the world. This resource has now been exhausted, and 80% of the island devastated. The country is bankrupt and accused of supplying equipment for money laundering operations on an international level, and financing terrorism.