Comparing the effects of prolonged fertilizer applications on the composition and structure of soils
The introduction of chemical fertilizers was promoted in the first years after World War I to give new impulses in agriculture, together with intensive cultivation and mechanisation. Under intensive land use, many soils are fragile, such as silty soils developed on aeolian loess that represent one third of the soil cover of France. Silt-textured soils are widely used for cereal production, due to their great chemical and physical fertility. Yet, in case of over-exploitation, such soils are subject to acidification and the formation of surface crusts enhancing surface erosion.
Based on that experience, Albert Demolon, Stéphane Hénin and Henri Burgevin started an innovative long-term research programme about the impacts of fertilizers on the structure and composition of loamy bare soils, instead of studies of their impacts on crop production being widely conducted elsewhere.
In 1928, they initiated at Versailles the long-term bare fallow experiment, also designated as the 42-plot trial, including 16 duplicated fertilization treatments and 10 unamended reference plots. Soil samples have been collected annually from the surface layer since March 1929, forming today a huge historical soil archive. We can compare, for equal fertilizer inputs, their effects on soil properties:
- nitrogen, phosphate and potassium fertilizers, used to supply nutrients to cultivated crops
- basic and organic amendments, favourable to soil structure stability
Soils may strongly acidify within one decade
Under application of ammonia-based fertilizers (nitrate, phosphate, sulphate, or chloride), the pH of the soil surface layer dropped in a short time span of 10 years by almost two pH-units, from about 6.5 to 4.3, a value more frequently observed for forest soils.
By contrast, the ancestral “liming” practice by application of quick-lime or calcium carbonate produced an opposite effect: today the soil pH reaches values comparable to those observed for soils developed on carbonate rocks. Such practices lead to increased soil aggregate stability with respect to heavy rain events and favour soil aeration and water infiltration.
Effects of road traffic, rain and bare fallow
The experiment and soil archives are now used for purposes that are quite different from the initial objectives:
- accumulation of metal pollutants in soils from atmospheric deposition, and in some cases, their amounts and sources may be identified (lead derived from petrol combustion)
- soil acidification induced by acid rain (drop of 1.5 – 2 pH units in 85 years)
- a declining organic matter content under bare soil management (no crops): mineralization of up to 75% from the initial organic carbon stock in 80 years.