Water-dispersible clay in bare fallow soils after 80 years of continuous fertilizer addition

Abstract The 42 plots of the Versailles long-term bare fallow experiment offer exceptional opportunities to study the impacts of fertilizers and amendments on soil physical properties. In this experiment, continuous annual applications since 1928 of 16 different treatments – including nitrates, phosphates, manure, ammonium, calcium, and potassium salts – have led to strongly diverging physical and chemical properties in the soilʹs surface layer. In this work, the proportion of water dispersible clay (WDC), which is considered a good indicator of soil structural stability and sensitivity to crusting and erosion, was determined in all plots of the experiment. The results showed the influence of the valence of cations and pH on soil structure and clay dispersability. Application of sodium or potassium salts deteriorated the degree of soil aggregation, with increased bulk density and WDC values, whereas amendment with manure or calcium salts ameliorated soil aggregation, with decreasing bulk density and WDC. The strong acidification of the soils receiving ammonium-based fertilizers was another factor related to the increase of water dispersible clay, and therefore the deterioration of soil structure. This study emphasizes the role of potassium and ammonium, associated with fertilizer additions, on the deterioration of soil structure and on the increase of the risk of erosion.