Scientific registration : 1495 Symposium : 38 presentation : poster. LUZ MORA G.María de la, DEMANET F. Rolando

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1 Scientific registration : 1495 Symposium : 38 presentation : poster Gypsum And Liming Effect On Surface Reactivity In Chilean Acid Soils Effet de l'apport de gypse et du chaulage sur la réactivité de surface de sols acides du Chili LUZ MORA G.María de la, DEMANET F. Rolando Universidad de La Frontera, Temuco, Chile, Casilla 54-D, mariluz@werken.ufro.cl Abstract Changes in the chemistry of the acidic Chilean Andisols in response to various ameliorant treatments (gypsum, dolomitic and calcitic lime) and their effects on Lolium perenne ryegrass production were studied in laboratory incubation and greenhouse experiments. Dolomitic and calcitic liming alone and in combination with gypsum increased ph significantly and, at the same time, decreased exchangeable Al concentration to low concentrations. Gypsum alone increased ph slightly and reduced Al concentration by 50%, but its effect on ryegrass yield was similar to the calcitic and dolomitic lime. The Al/S ratio in the soil was a good predictor of Al toxicity for plant growth. The Ca + Mg + K content in shoots was highly correlated with dry matter yield. Introduction In the intensive farming systems used nowadays in Chile for dairy production, ryegrasses (Lolium perenne and Lolium multifloum) are the dominant pasture species grown. The major factors limiting plant growth on acid Chilean Andisols are aluminium (Al) and/or hydrogen ion toxicities. Especially in young plants, Al uptake can result in plant concentrations above 2000 ppm (Mora, 1993; Demanet and Mora, 1995). Lime and gypsum application used for rising soil ph and to reduce exchangeable Al to relatively low levels are used today has been a common practice producing changes in solid/solution interface characteristics. Consequently, acid-base behavior of these colloids would also be expected to changes. Gypsum applied to soil surface has been observed to be more effective than surface applied lime for improving crop yields Thus, gypsum is rapidly leached to the depth of soil where acidity is limiting root growth whereas lime movement into the subsoil is generally very slow. A number of mechanism have been proposed by which gypsum may ameliorate Al toxicity in acidic soils, although the actual mechanism which operates in soils remains poorly understood. 1

2 In these soils the high soil organic matter content may present opportunities for complex formation with its acid groups. For example, when Al is complexed with organic ligands its activity is decreased and it does not appear to be phytotoxic for plant growth The aim of this work was to determine the physicochemical changes produced by gypsum and liming and their effect on dry matter yield of ryegrass. Materials and Methods Incubation experiment Acidic Andisol belonging to the Barros Aranas Series from the South of Chile with 60% of aluminum saturation and 4.14 ph (CaCl2) was used in this study. Soil subsamples were incubated with 1000, 2000, 3000 and 4000 ppm of calcite, dolomite and gypsum. The soils were moistured approximately to field capacity and incubated for 48 hours in polyethylene bags with aeration provision, at a temperature of 60 C (Barrow and Cox, 1990), adjusted to Chilean soil condition (Mora and Barrow). Surface charge behavior was evaluated by means of potentiometric titrations, zero point of Charge (ZPC) and point zero salt effect (PZSE) were determined from potentiometric data and acidity constant were calculated at three ionic strength using the Capacitance Constant Model (Summ et al., 1980) Greenhouse experiment A greenhouse experiment was carried out with ryegrass Lolium perenne in pots. The treatments were calcitic (CL) and dolomitic liming (DL) and gypsum (G) applied at rates equivalent to 2 t ha -1 (subsequently referred to as CL2, DL2 and G2, respectively) and 4 t ha -1 (CL4, DL4 and G4). Other treatments were combinations of the amendments applied at 1 or 2 t ha -1 (CL1 + DL1, CL2 + DL2, CL1 + G1 etc.). The assay was performed in pots (containing 1 kg soil) using a randomized block design with three replications. Every pot received basal fertilization of N, P, K, B, Zn, Cu The herbage was cut five times during the experiment when ryegrass was approximately 250 mm in height. Each time, the herbage was cut to 25 mm above the soil surface, dried at 70º C for 48 h, and weighed to measure dry matter (DM) yield. The herbage was ashed at 500º C for 8 h and digested with hydrochloric-nitric acid. Solutions were analyzed for Ca, Mg, K and Al using an atomic absorption spectrophotometer and for P using a spectrophotometer by the colorimetric method of Murphy and Riley (1962). For sulfur, the ashed herbage was digested with nitric-perchloric acid and measured turbidimetrically.soil nutrients were determined before and after the greenhouse experiment. Results and Discussion Soil Chemistry In both incubation and greenhouse studies, application of calcitic and dolomitic lime significantly increased soil ph (by up to 1.5 units) and the concentration of Ca and Mg (Table 1). At the same time, the exchangeable Al concentration and % Al saturation decreased. The effects were similar regardless whether the calcitic and dolomitic limes were applied alone or in combination with other treatments. 2

3 Application of gypsum increased the concentration of soil Ca and S which led to a reduction in the Al saturation % in both soils. The effect of gypsum on soil ph was much smaller (only units) than that of the calcitic and dolomitic limes. Undoubtedly increases in ph when gypsum is applied are due to ligand exchange, whereby the sulphate ion replaces hydroxyl ions (Bolan et al., 1993), followed by hydrolysis and precipitation of exchangeable Al. However, it can be inferred that this mechanism only partially controls the process because the ph did not continue to rise as the rate of gypsum increased. It is observed in potentiometric titration that negative protonic charge was greater than calcitic or dolomitic lime when gypsum was used. (Figure 1), indicating inner sphere complex formation is not very important mechanism in this process.. This is probably attributable to the formation of AlSO 4 +, an insoluble complex of Al that is not available to plants (van Raij, 1988; Furlani and Berton, 1992). Other studies have shown that soil ph increases after gypsum application due to the displacement of Al 3+ and H + from the cation exchange sites into solution by Ca 2+ ions (Kamprath and Foy, 1985; Alva et al., 1988). Table 1 : Effect of applying lime and gypsum to acid soils on the soil chemical properties in a laboratory incubation experiment. Treatments ph Ca Mg K Al Al Sat. (%) cmol(+)kg -1 Control CL CL CL CL DL DL DL DL G G G G The ph changes produced by ligand exchange are regulated by zero point charge (ZPC) on the colloidal surface. Thus, sulphate adsorption on soil surfaces neutralizes the positive charge present in acidic Andisols, and generates negative charge until the surface reaches a new ZPC, where no further adsorption of this anion takes place. For this reason, the effect of gypsum may be to increase, decrease or not change the soil ph according to how close the soil ph is to its ZPC when gypsum is applied. In this study, the ph of the ZPC was near 4.4, measured by potentiometric titration. This indicates there was initially a low positive charge present in the soil, and so when gypsum was applied, there was only a limited exchange between the hydroxyl and sulfate ions. 3

4 Surface Charge cmol(+/-) kg -1 2,0 1,5 1,0 0,5 0,0-0,5-1,0-1,5-2, ph control yeso dolomita Cal Figure 1 : Potentiometric tritation in soil amendment. Temperature 25 C and ionic strength 0.1 N KCl The ZPC increased from 4.4 to 5.2 and 5.6 with calcite and dolomite respectively, due to negative charge increased but when gypsum was applied there were not changes at all, confirming that outer sphere complex formation wish does not alter protonic negative charge (Figure 1) suggesting ligand mechanism is poor in this process according to what is reported by Agbenin et al (1997) ; He et al. (1997). Application of calcitic amendments had little effect on the soil exchangeable K content in the incubation study. However, in the greenhouse experiment soil K was reduced in all of the treatments (including the control) compared to the start of the experiment due to uptake of the K by the plants ( Table 2). This reduction occurred despite the application of basal K fertilizer to the soil in the greenhouse experiment and illustrates the poor buffering capacity of these soils to removal of exchangeable K by plants. Effect on Ryegrass Since the acidity of these soils was very high, all the calcitic ammendments produced a significant increase in DM production. The increase was apparent at the first sampling date and continued throughout the experiment. Overall, increases in DM production relative to the control ranged from 50% to over 145% (Figure 2). Calcitic ammendment G DL CL DL+G CL+DL CL+G 4 t / ha 2 t / ha Relative dry matter yield Figure 2 : Relative dry matter yield influenced by application of lime and gypsum to acidic soils. 4

5 Table 2: Effect of applying lime and gypsum to acid soils on the soil chemical properties in a greenhouse experiment. Treatments S ph Ca Mg K Al Sat. Al (ppm) cmol(+)kg -1 (%) Control CL CL DL DL G G CL 1+DL CL 2+DL CL 1+G CL 2+G DL 1+G DL 2+G The DM responses to gypsum were similar to the other ammendments despite the smaller effect of gypsum on soil ph and exchangeable Al. These results confirm the theory that complexes of aluminium sulfate form species which are not toxic to the plant, but when extracting the soil with KCl it extracts these Al species. In addition, there was a high correlation (r 2 = 0.70 ) between the Al/S ratio in the soil and DM production (Figure 3); suggesting that there was a response to the applied S due to the initial low S level in these soils. Further evidence comes from the increase in S concentration in shoots from 2 g S kg -1 to 9.7 g S kg -1 with those treatments including gypsum. This was accompanied by a DM increase of 95 to 125 % relative to the control. There was also a close correlation (r 2 = 0.82) between herbage S and soil S. In general, a better balanced mineral absorption in the forage was produced with dolomite and gypsum treatments 140 % response over control y = -8,93x + 118,88 R2 = 0, Al/S ratio Figure 3 : Relationship between dry matter yield on perennial ryegrass and Al/S ratio in acidified soil. 5

6 Table 3 : Effect of appplying gypsum and lime to acid soils on the nutrient concentration of perennial ryegrass in a greenhouse experiment. Treatments Ca Mg K S (%) Control CL CL DL DL G G CL 1+DL CL 2+DL CL 1+G CL 2+G DL 1+G DL 2+G Calcium concentration in the shoots increased as soil Ca content increased following application of the calcitic amendments (Table 3). At shoot Ca concentrations >8 g Ca kg -1 the DM production decreased, indicating that luxury consumption of Ca by the plants produced an imbalance in plant nutrients. The relationship between the Ca + Mg concentration in the shoots and DM production was not better than Ca alone. But when K was added the relationship between Ca+Mg+K content in shoots with DM yield shows a high correlation in both soils ( r 2 = 0.91). Conclusions The use of gypsum as an ameliorate for acid Andisols has resulted in a substantial ryegrass yield increase although the effect on soil ph was small. Nevertheless, when it was used in a mixture with dolomitic lime, the yield increase was greater, as a result of the higher increase in ph and a greater decrease in exchangeable Al. Consequently the forage quality was improved due to a better nutrient balance in the plants. The high organic matter content in acidic soil impedes the soil Ca/Al ratio as a good predictor for Al toxicity. But Al/S ratio appears to be a good predictor of Al toxicity, especially in soils which are S deficient. Acknowledgments The research was supported by FONDECYT and DIDUFRO Grants and 9450, respectively. References Agbenin J Sulfate Retention by Kaolinitic Alfisols from Nigerian Savanna. Soil Sci. Soc. Am. J

7 He L., Zelazni L., Baligar K., Ritchey K., and Martens D Ionic Strength Effect on Sulfate and Phosphate Adsorption on γ-alumina and Kaolinite : Triple-Layer Model. Soil Sci. Soc. Am J Barrow N.J. And Cox, V.C A quick and simple methods for determining the titration curve and estimating the lime requeriment of soil. Australian Journal of Soil Research Bolan, N. ; Syers J.K.and Sumner, M.E Calcium-induced sulfate adsorption by soils. Soil Sci Soc. Am. J Demanet, R. and M.L. Mora Efecto de las fuentes de fósforo sobre la producción. II Praderas. Frontera Agrícola Falkengren-Gerup, Brunet J., Quist, M. E. and Tyler, G Is Ca:Al ratio superior to ph, Ca or Al concentrations of soils in accounting for the distribution of plants in deciduous forest. Plant and Soil 177: 21, 31. Furlani, P. R., and Berton, R. S. (1992). Atividade de cálcio e alumínio e desenvolvimento radicular. II Seminário sobre o Uso do Gesso na Agricultura, pp IBRAFOS, SP, Brazil. Manoharan, V., Loganathan, P., Tillman, R.W. and Parfitt, R.L Assessing aluminum Phytotoxicity in long-term phosphate fertilized pasture soils. Comun. Soil Sci. Plant Anal. 27: Mora, M. L Nivel de fertilidad de los suelos de la IX Región y su relación con la acidificación. Frontera Agrícola, 1: 5-12 Mora, M.L. and Barrow, J The effects of time of incubation on the relation between charge and ph of soil. European Journal of soil Science, Raij B. van Gypsum in the amelioration of the rooting environment of subsoils. Associação Nacional para Difusão de Adubos e Corretivos Agrícolas, São Paulo. 88 p. Stumm, W., Kummert R. and Sigg (1980). A ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide interface. Croat. Chem. Acta 53 : Keyword : Ryegrass, aluminium, sulfur, gypsum, calcitic lime, dolomite Mots clés : raygras, aluminium, sulfure, gypse, chaux calcitique, dolomie 7