pH effect on soils properties: long time experiment of 42 plots in Versailles
In temperate regions, soil acidification is a real problem. The pH change causes transformations of both the chemical and physical properties of the soil. The 42 experimental plots at the Institut national de Recherche agronomique (INRA) in Versailles, France, were created in 1929. Since that time, each plot has received either fertiliser or amendments (Table 1). There are ten reference plots that have not been treated. Plots were dug up twice a year and left fallow. The results corresponding to 17 plots, 16 treated and one reference plot, are presented in this paper. The samples were collected in 1999 and compared to the 1929 soil reference. The pH was measured in water (AFNOR 1994), the cation exchange capacity (CEC) was determined at pH 7.0 (AFNOR 1994, CEC7) and at the soil pH by the cobaltihexamine method (Ciesielski and Steckermann 1997, CECsoil). Exchangeable cations (Ca2+, Mg2+, K+ and Na+) were measured after cobaltihexamine extraction and exchangeable aluminium by the KCl method (McLean, 1965). Water retention measurements were carried out with an apparatus developed by Tessier and Berrier (1979) and AFNOR (1996) at -10 kPa, with the Richards pressure plate cell at -1.6 kPa (Richards 1948) and with a controlled hygrometry dessicator at -107 MPa (Tessier 1984). The average pH of the reference plot collected in 1929 was close to 6.3. In 1999, pH values ranged between 3.5 and 8.2. For plots treated with ammonium fertilisers the pH ranged from 3.5 to 5.5. In contrast, plots with basic treatment or liming had a soil pH that was close to 8.2 and was equilibrated with carbonates. Intermediate pH values (from 5.5 to 7.5) were obtained for plots treated with neutral salts such as KCl or NaNO3, or with superphosphate. The cation exchange capacity in soils at the reference pH (7.0) demonstrated variations in soil composition. For example, the CEC7 varied with clay content as a function of depth or with organic matter loss or gain due to lying fallow or to manure application. When the CEC was measured at soil pH (CECsoil), strong differences were observed. Two groups of plots can be differentiated in 1999. One ranged between 11 and 16 cmol+/kg; this range corresponded to plots with amendments or with fertilisers containing bases. A lower range of CECsoil values (below 11 cmol+ /kg) was obtained with ammonium fertilisers. It is important to note that between pH=6.0 and pH=7.5 the CECsoil doubled. Six months after digging, the surface state of the soil was variable. Thick crusts dominated in acidic conditions, whereas the presence of dispersing cations (K+, Na+), and a smooth surface was present in soil treated with amendments. In the soil profile, bulk density increased with acidity and sodium or potassium cation concentrations. At -10 kPa, water retention measurements demonstrated that bulk density and sampling depth were linked with water retention: the higher the bulk density, the lower the water retention. At -1.6 MPa, water retention was correlated to the amount of clay and organic matter content. We also showed that the exchangeable calcium content and CECsoil influence water retention. At -107 MPa, water retention depends on pH as well as the CECsoil and exchangeable cation concentration. The main purpose of this study was to show that after 70 years, plots subjected to intensive fertilisation or amendments and lying fallow (i.e., without organic matter restitution), have dramatically different soil properties. The first indicator of this evolution was pH. Ammonium fertilisers produced very acid plots (pH 3.5 to 5.0). In the presence of neutral salts (e.g., KCl, NaNO3) and in the reference plot, the pH had decreased a little. In liming or basic treatment plots, the pH is controlled at 8.2 by carbonates. The second important factor to consider is the cation exchange capacity value. When measured at a reference pH (pH=7.0), the CEC7 reflects the natural components present in a soil, but doesn't consider the influence of physico-chemical factors that operate in the native soil. After 70 years without organic restitution, the soil has lost approximately half of its original organic matter content, and as a result the exchangeable sites were approximately 2 cmol+ /kg. The effective CEC at soil pH demonstrates the influence of pH on variable charges due to mineral constituents and organic matter. Plots with ammonium fertilisers have a low cation exchange capacity. Fertilisers with sodium and potassium cations have increased the exchangeable sodium (up to 11% of the CECsoil) or potassium (18%) respectively. The cation exchange capacity doubled in going from acidic plots to liming plots, and in the latter the CEC is mainly saturated with exchangeable calcium. In situ plot observations show that soil physico-chemical properties strongly influence not only soil surface state, but also soil profile porosity. Both acidification and dispersing cations are factors in soil degradation, whereas high pH values and calcium as the exchangeable cation produce strong structure stability and high porosity. It is also interesting to note that the CECsoil can double between pH=6.0 and pH=7.5, thus influencing the physical properties of the soil. Comparing CEC7 and CECsoil facilitates the prediction of the effects of fertilisation and amendments on soil properties. The cation exchange capacity at soil pH can be used as an excellent indicator of soil quality.