ABSTRACT Soil pH and base saturation are important parameters for evaluating the effects of acidity on plant growth. However, the relationship between soil pH and base saturation (V%) has been poorly studied in Brazil. The aim of this study was to evaluate the influence on the cation exchange capacity (CEC) on this relationship for soils of Mato Grosso do Sul, Brazil, using a database consisting of 30,244 analyses of soils collected from 0–0.2- and 0.2–0.4-m depths. The CEC at pH 7 of analyzed soils varied from 3.4 to 40.5 cmolc·dm-3 and were grouped into four classes (< 6, 6–10, 10–20, and > 20 cmolc·dm-3) for each soil layer. The V% versus pH relationship was CEC dependent for both soil layers. For 0–0.2 m, V% was directly related to CEC (< 6, 6–10, 10–20, and > 20 cmolc·dm-3) and reached 50, 55, 60, and 74% at pH 5 and 66, 73, 81, and 88% at pH 6, respectively. Predicting V% in relation to pH was also affected by CEC, with a decrease in the correlation of p = 0.82 for CEC < 6 cmolc·dm-3 to p = 0.64 for CEC > 20 cmolc·dm-3. The decrease in this correlation was related to low clay content (17%) of soils with CEC < 6 cmolc·dm-3, which provided low buffering capacity. For soils with CEC > 20 cmolc·dm-3 and higher clay content (45%), greater buffering capacity was observed associated with kaolinite. Findings suggest that CEC should be considered to understand the relationship between V% and pH CaCl2. growth However V (V% Brazil (CEC Sul 30244 30 244 30,24 002 0 2 0.2–0.4m 0204m m 0.2–0.4 4 depths 34 3 3. 405 40 40. cmolc·dm3 cmolcdm3 cmolcdm cmolc·dm cmolc dm cmolc·dm- ( 610 10 6–10 1020 10–20 layer layers 0–0. 50 55 60 74 66 73 81 88 respectively 082 82 0.8 064 64 0.6 cmolc·dm3. 17% 17 (17% cmolc·dm3, 3, 45%, 45 45% , (45%) kaolinite CaCl2 CaCl (V 3024 24 30,2 00 4m 0204 0.2–0. dm3 61 1 6–1 102 10–2 0–0 8 08 0. 06 (17 (45% 302 30, 020 0.2–0 6– 10– 0– (1 (45 02 0.2– (4 0.2

ABSTRACT Phosphorus availability in soils is low due to its strong retention by inner-sphere complexation on minerals in the clay fraction with pH-dependent charges, such as goethite. On the other hand, sulfur has greater availability because it is retained mainly by electrostatic attraction. We evaluated the intensities of the inner-sphere complexation of orthophosphate and sulfate (H2PO4-/HPO42- and SO42- - generically treated as PO4 and SO4) under different experimental conditions (pH, goethite purity, and contact times) on synthetic goethite samples to establish the mechanisms and models involved in those bonds. Inner-sphere PO4 and SO4 were extracted using both HNO3 1 mol L-1 and USEPA 3051A methods. Inner-sphere complexation of PO4 and SO4 was highest at pH 5 in relation to pH 9. Attenuated total reflectance/Fourier transform infrared spectroscopy (ATR-FTIR) spectra showed inner-sphere complexation bands of PO4 on goethite in the protonated binuclear bidentate (pH 5) and deprotonated binuclear bidentate (pH 9) forms. Inner-sphere complexation of PO4 was much more expressive than that of SO4. Phosphorus and sulfur oxyanions displace the diprotonated ferrol ligand (-OH2+0.5 in -FeOH2+0.5), while the -OH-0.5 in the -Fe-OH-0.5 group are only displaced by PO4. The -O-1.5 ligand in Fe-O-1.5 group is not displaced by PO4 or SO4. The high surface negative charge density of PO4 defined its higher activation energy for exchanging -OH2+0.5 and -OH-0.5 on the goethite surface in relation to SO4. The proposed model can be used to reduce inner sphere phosphate adsorption in soils and improve P fertilization efficiency for farming.