COMPARATIVE STUDY OF THE FITNESS OF VARIOUS LIMESTONE AND GYPSUM BYPRODUCTS FOR AMENDING Al TOXICITY IN A PLINTHIC PALEXERULT R. Espejo Serrano1; F. Peregrina Alonso1; J. Santano Arias1; I. Mariscal Sancho1; P. González Fernández2 1
: Dpto. Edafología. ETSI Agrónomos. Ciudad Universitaria. 28040 Madrid; Spain, email:
[email protected] 2
: CIFA Alameda del Obispo. Apdo 3092, 14080, Córdoba
ABSTRACT The topmost part (Ap and AB horizons) of a Plinthic Palexerult from the Cañamero raña (western Spain) was reconstructed in percolation columns in order to compare the effects of the application of various byproducts including sugar foam waste (SF), phosphogypsum (PG) and red gypsum (RG) on the dynamics of Al3+ ion in the soil solution. Also, the action of these byproducts was compared with that of conventional amendments such as quarry limestone (QL), and gypsum (QG) Both QL, and SF were found to dramatically decrease the Al3+ content of the soil solution by raising the pH and precipitating the ion as insoluble hydroxides. Both (QG), and the gypsum-based byproducts increased the content in total Al upon application of the first few percolation rates; however, a sizeable fraction of Al was in the form of ion-pairs with sulphate and fluoride ions —the latter were displaced from the soil matrix by sulphate ions from the gypsum materials in the QG case—; this decreased the concentration and activity of Al3+ in the soil solution relative to the blank columns. With PG, F– displaced from the soil added to that contained in this byproduct, which increased the amount of Al–F ion-pairs formed relative to the other two gypsum-based byproducts. Unlike the calcareous amendments, the gypsum-based amendments also affected the AB horizon. After treatment with the amount of water equivalent to one year of precipitation in the field, soil from the Ap horizon in each column was subjected to a productivity test in pots that were sown with Triticum durum sp and received the same NPK rates used in the field tests but no external calcium or sulphate. The favourable effect of the calcareous and
gypsiferous amendments resulted from the increased Ca and decreased Al content of the exchange complex and a increased Ca/Al ratio in the soil solution. INTRODUCTION Xerults (Soil Survey Staff, 1999), are Mediterranean Ultisols developed under a xeric moisture regime One common feature of Ultisols is that their acidity, and hence the Al3+ saturation in the exchange complex, is higher in their subsurface horizons, where root development is minimal as a result. Water reserves in this type of soil concentrate in Bt subsurface horizons, where few roots are present. In drought periods, which are rather frequent in Mediterranean regions, the water stress usually experienced by crops is a serious problem adding to those derived from the unfavourable chemical properties of these soils. The problems arising from excessive acidity in their surface horizons have traditionally been alleviated by adding lime amendments; subsurface horizons, however, require gypsum amendments, which are more easily transferred to the soil surface on account of their increased solubility. Gypsum has been used to amend acid soils since the late 1970s; its beneficial effects on productivity-related characteristics are well documented. Both lime and gypsum influence soil pH, exchange base dynamics, and macro- and micronutrient bioavailability. Because of the balance between the ion composition of the exchange complex and the soil solution, the addition an amendment will inevitably alter the latter. In this work, we conducted laboratory tests involving soil columns that were leached over two cycles with a view to studying Al composition changes in the soil solution from the Ap and AB horizons of a Plinthic Palexerult, which is where most root development occurs, following application of lime or gypsum amendments to the Ap horizon. MATERIAL AND METHODS Soil Table 1 summarizes the properties of the soil profile studied (Peregrina et al., 2004). The values are normal for this type of soil. Worth special note is the low base content in the exchange complex and the fact that the pH decreased and Al saturation increased with increasing depth.
Table 1: Selected data of the soil profile Depth(cm) Ap (0-24) AB(24-43) Bt1(43-77) Bt21(77-132) Bt22(132190)
Sand 684 642 460 488 526
USDA loam silt g/kg 233 83 181 177 97 443 132 380 142
332
OM
pH(H2O) pH(ClK)
Ca
Mg
Na
0,18 0,12 0,16 0,21
cmol(+)/kg 0,08 0,09 0,06 0,07 0,04 0,04 0,03 0,04
1,02 1,37 3,29 3,67
2,4 1,8 4,7 5,2
43 60,35 70 70,85
0,95 0,48 0,29 0,28
0,05
5,33
6,6
80,63
0,18
41 7,4 nd nd
5,12 5,05 4,9 4,7
4,21 4,11 3,92 3,54
0,97 0,52 0,96 1,02
nd
4,5
3,11
0,97 0,18
K
Al(ClK) CICe
0,05
100Al/CICe Ca/Al
Amendments The industrial byproducts used as amendments included phosphogypsum (PG), red gypsum (RG) and sugar foam waste (SF). Their performance was compared with that of other, traditional amendments such as limestone (Gil lime) and high-purity building gypsum (G). The composition of PG and RG (Garrido et al., 2003), and that of SF, Gil lime and gypsum (Espejo et al., 2004), is shown in Table 2. The relative richness in CaSO4 of the gypsum amendments was estimated by dissolving an amount of 0.5 g of G, PG or RG in 1 l of distilled water and stirring at 25 ºC for 48 h, after which the supernatant was analysed for Ca and sulphate. Table 2: Chemical composition of amendments Component PG* RG* QG** g/kg CaO 361 334 388 SO42652 538 701 14,4 3,7 2,1 SiO2 F 11,6 nd nd 6,2
