Le phosphore dans les sols

Commissariat général au développement durableMay 7, 2019

Le phosphore caractérise, avec l’azote et le potassium, l’un des trois nutriments indispensables à la croissance des végétaux. Concentré dans la partie superficielle du sol, le phosphore provient de l’altération des roches (forme minérale) ou de la dégradation des végétaux par la faune et la flore du sol (forme organique). Les agriculteurs recourent également à des apports de fertilisants phosphatés minéraux ou organiques (fumiers, lisiers) dans les sols appauvris en phosphore. Ils utilisent cependant peu de phosphates naturels réservés aux sols acides. Associé aux nitrates en excès, le phosphore contribue à l’eutrophisation des eaux de surface. Par ailleurs, certains engrais minéraux phosphatés peuvent introduire du cadmium dans l’environnement. Enfin, les boues de traitement des eaux usées recyclées en agriculture pour leur qualité fertilisante, peuvent polluer les sols (micropolluants organiques, micro-organismes pathogènes, métaux).

Phosphorus, along with nitrogen and potassium, characterizes one of three nutrients essential for plant growth. Concentrated in the topsoil, phosphorus comes from rock weathering (mineral form) or plant degradation by soil fauna and flora (organic form). Farmers also apply mineral or organic phosphate fertilizers (manure, slurry) to phosphorus-depleted soils. However, they use little natural phosphates, which are reserved for acidic soils. Associated with excess nitrates, phosphorus contributes to surface water eutrophication. Furthermore, certain mineral phosphate fertilizers can introduce cadmium into the environment. Finally, wastewater treatment sludge recycled in agriculture for its fertilizing quality can pollute soils (organic micropollutants, pathogenic microorganisms, metals).

4 chapters

Phosphorus levels in soils
Median levels in France
• In mainland France during 2010-2014, median soil phosphorus levels range from 14 to 172 mg/kg for cantons with sufficient data, representing just under 80% of all cantons. • The highest levels (over 81 mg/kg) affect one-fifth of these cantons.
Regional disparities
• Locally, the proportion of cantons with the highest phosphorus levels is significantly above the French average in certain regions: Nord-Pas-de-Calais (77% of surveyed cantons), Brittany (69%), Alsace (31%). The causes of these high levels are varied: intensive livestock farming for forty years (Brittany), historical use of slag, a byproduct of metallurgical production (Alsace, Nord-Pas-de-Calais). Conversely, in many regions, the majority of cantons have low phosphorus levels to ensure good yields without fertilizer input in cultivated soils, regardless of crop type.
Data sources
• These levels represent the state of phosphorus reserves accessible to plants in agricultural soils. They come from agricultural soil analyses performed primarily at farmers' request to optimize fertilization of their crops. The GIS Sol land analysis database (BDAT) compiles results from analyses conducted between 1994 and 2014 on 2.7 million samples, yielding 30 million results, of which 2.2 million concern phosphorus. Aggregated by canton, these results indicate a trend but may mask significant intra-cantonal diversity.
Variation in phosphorus levels in soils
Downward trend
In terms of temporal variation, soil phosphorus levels have declined over the past two decades, raising questions about soil fertility in the medium term. Between the 1994-2004 and 2005-2014 periods, nearly half of all cantons experienced declining soil phosphorus levels.
Figures on change
• They decline for more than half of cantons with sufficient data. The decline exceeds 10% of the initial median level for 46% of cantons, corresponding to 46% of utilized agricultural area (UAA). Conversely, it increases for only 2.2% of cantons, or 2.5% of UAA. Finally, phosphorus levels remain stable for 51% of cantons, representing 46% of UAA.
Phosphorus availability in soils
Bioavailability and plant demand
• However, phosphorus bioavailability depends on soil characteristics and plant demand—that is, plants' ability to extract phosphorus more or less easily. While some plants are highly demanding (sugar beet, rapeseed, etc.) or moderately demanding (silage maize, peas, etc.), others are far less so (oats, grain maize, etc.). In cantons with low phosphorus levels, for example, phosphorus availability in many cultivated soils may be insufficient to ensure adequate yields regardless of crop type.
Availability trends
• Between the 1994-2004 and 2005-2014 periods, only 4.5% of cantons with sufficient data (4% of UAA) experienced increasing soil phosphorus availability, while it declined in one-third of cantons (35% of UAA). The decline affects 32.5% of cantons, corresponding to 35% of utilized agricultural area (UAA). Finally, it remains stable for 15% of cantons, representing 18% of UAA.
Phosphorus input to soils
Causes of decline
More judicious fertilization practices, sharp increases in the cost of mineral phosphate fertilizers, and diversification of inputs (increased use of wastewater treatment sludge, etc.) likely underlie this downward trend in soil phosphorus levels and availability.
Reasoned fertilization and fertilizer deliveries
• By using phosphorus fertilization omissions, farmers adopt reasoned fertilization. They better integrate the dynamics of phosphorus transfer from soil to plants based on their phosphate demand and application frequency. Phosphate fertilizer deliveries have in fact declined by 75% in forty years, progressively falling from 31 to 7 kg/ha arable between 1972 and 2016. These phosphate fertilizers come from industrial processes in the form of single fertilizers or combined with nitrogen alone or with nitrogen and potassium.
Global phosphate resources
• Unlike nitrogen fertilizers, which can be synthesized from atmospheric nitrogen, phosphate resources are limited. Global mineral resources are estimated at over 300 billion tonnes in 2018. The largest sedimentary deposits are found in North Africa (Morocco), China, the Middle East, and the United States. Exploited phosphorus deposits come primarily from sedimentary or marine deposits, as well as from magmatic deposits or guano from seabird droppings.

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