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Les pratiques de fertilisation et d'amendement peuvent contribuer à modifierprofondément les propriétés des sols. Ceci est mis en évidence par le dispositif des 42parcelles de l'INRA à Versailles qui reçoit chaque année les mêmes traitements depuis 1929.Il présente l'originalité d'être en jachère nue. Les résultats reportés ici reposent sur 16traitements que nous comparons à une parcelle témoin. Le sol est à caractère limoneux avec14 à 22 % d'argile. Le pH, la capacité d'échange au pH du sol (CECsol), etles cations échangeables ont été mesurés. Des mesures de porosité, de rétention de l'eau àdifférents potentiels de l'eau ont été effectuées.Des variations considérables de pH sont observées. Les engrais ammoniacaux conduisentà des parcelles très acides (pH 3,5 à 5,0), à faible CECsol,essentiellement saturée par des cations aluminium. Dans les parcelles chaulées ou recevantdes amendements basiques, le pH est tamponné à 8,2 par les carbonates. LaCECsol double du sol acide au sol chaulé où elle est majoritairementsaturée par du calcium échangeable. Avec les sels neutres (par exemple KCl,NaNO3) le pH n'a pas changé et pour le témoin il a légèrement baissé. Desobservations de terrain montrent que ces différents états physico-chimiques sont en relationavec les états de surface du sol et influencent la porosité du sol en profondeur.Ainsi dans les sols des régions tempérées la CECsol peut doubler dansun domaine très restreint de pH (6,0 à 7,5), orientant fortement les propriétés physiquesdes sols.In temperate regions, soil acidification is a real problem. The pH change causestransformations of both the chemical and physical properties of the soil. The 42experimental 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 oramendments (Table 1). There are ten reference plots that have not been treated. Plots weredug up twice a year and left fallow. The results corresponding to 17 plots, 16 treated andone reference plot, are presented in this paper. The samples were collected in 1999 andcompared to the 1929 soil reference. The pH was measured in water (AFNOR 1994), the cationexchange capacity (CEC) was determined at pH 7.0 (AFNOR 1994, CEC7) and atthe soil pH by the cobaltihexamine method (Ciesielski and Steckermann 1997,CECsoil). Exchangeable cations (Ca2+,Mg2+, K+ and Na+) weremeasured after cobaltihexamine extraction and exchangeable aluminium by the KCl method(McLean, 1965). Water retention measurements were carried out with an apparatus developed byTessier and Berrier (1979) and AFNOR (1996) at -10 kPa, with the Richards pressure platecell 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, pHvalues ranged between 3.5 and 8.2. For plots treated with ammonium fertilisers the pH rangedfrom 3.5 to 5.5. In contrast, plots with basic treatment or liming had a soil pH that wasclose 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, orwith superphosphate.The cation exchange capacity in soils at the reference pH (7.0) demonstratedvariations in soil composition. For example, the CEC7 varied with claycontent as a function of depth or with organic matter loss or gain due to lying fallow or tomanure application. When the CEC was measured at soil pH (CECsoil), strongdifferences were observed. Two groups of plots can be differentiated in 1999. One rangedbetween 11 and 16 cmol+/kg; this range corresponded to plots with amendments or withfertilisers containing bases. A lower range of CECsoil values (below 11cmol+ /kg) was obtained with ammonium fertilisers. It is important tonote 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 crustsdominated in acidic conditions, whereas the presence of dispersing cations(K+, Na+), and a smooth surface was present insoil treated with amendments. In the soil profile, bulk density increased with acidity andsodium or potassium cation concentrations. At -10 kPa, water retention measurementsdemonstrated that bulk density and sampling depth were linked with water retention: thehigher the bulk density, the lower the water retention. At -1.6 MPa, water retention wascorrelated to the amount of clay and organic matter content. We also showed that theexchangeable calcium content and CECsoil influence water retention. At -107MPa, water retention depends on pH as well as the CECsoil and exchangeablecation concentration.The main purpose of this study was to show that after 70 years, plots subjected tointensive fertilisation or amendments and lying fallow (i.e., without organic matterrestitution), have dramatically different soil properties. The first indicator of thisevolution was pH. Ammonium fertilisers produced very acid plots (pH 3.5 to 5.0). In thepresence of neutral salts (e.g., KCl, NaNO3) and in the reference plot, thepH had decreased a little. In liming or basic treatment plots, the pH is controlled at 8.2by carbonates. The second important factor to consider is the cation exchange capacityvalue. When measured at a reference pH (pH=7.0), the CEC7 reflects thenatural components present in a soil, but doesn't consider the influence of physico-chemicalfactors that operate in the native soil. After 70 years without organic restitution, thesoil has lost approximately half of its original organic matter content, and as a result theexchangeable sites were approximately 2 cmol+ /kg.The effective CEC at soil pH demonstrates the influence of pH on variable charges dueto mineral constituents and organic matter. Plots with ammonium fertilisers have a lowcation exchange capacity. Fertilisers with sodium and potassium cations have increased theexchangeable sodium (up to 11% of the CECsoil) or potassium (18%)respectively. The cation exchange capacity doubled in going from acidic plots to limingplots, and in the latter the CEC is mainly saturated with exchangeable calcium.In situ plot observations show that soil physico-chemical properties stronglyinfluence not only soil surface state, but also soil profile porosity. Both acidificationand dispersing cations are factors in soil degradation, whereas high pH values and calciumas the exchangeable cation produce strong structure stability and high porosity. It is alsointeresting 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 andCECsoil facilitates the prediction of the effects of fertilisation andamendments on soil properties. The cation exchange capacity at soil pH can be used as anexcellent indicator of soil quality

Influence du pH sur les propriétés des sols : l'essai de longue durée des 42 parcelles à Versailles