Amelioration of Subsoil Acidity through Surface ...

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2: catechol + A1CI3 + CaCI. 2. ~ in either order. 1. 3. 3:catech01. + AIC13. 250. 360. Wavelength, nm d. 1: salicylic + H:O, KCI, or CaCl. 2. 2: salicylic + AICI a.
Amelioration of Subsoil Acidity through Surface Application of Organic Manures N. V. Hue* and D. L. Licudine ABSTRACT

ciably, pH-dependentcharge on the soil surface remains unchanged, enabling Ca supplied by gypsum to move through the soil in larger quantity and at greater speed than that supplied by lime (McCrayand Sumner, 1990). Furthermore, SOl- ions can reduce A1 toxicity by forming either aluminum-hydroxy-sulfate minerals (Hue et + ions (Kinraide and al., 1985), or the less-toxic ALSO4 Parker, 1987; Tanaka et al., 1987). This explains the improved performance of crops grown on acid soils amended with surface-applied gypsum (Sumner et al., 1986). Surface application of biowastes (e.g., animal manure or sewage sludge), which have relatively high Ca contents and can release organic molecules/anions with strong affinity for Al, should be effective in correcting subsoil acidity. This is due to the high mobility of most complexed Ca species and the nonphytotoxic nature of organically chelated AI species (Hue et al., 1986; Hue and Amien, 1989; Hue, 1992; Liu and Hue, 1996). In fact, Tan et al. (1985) showed that a combination lime and sewage sludge applied to the surface of an Ultisol mobilized significant amounts of Ca to below a 45 cm depth. Recently, Liu and Hue (1996) used fulvate solutions to effectively increase exchangeable Ca and decrease A1 saturation percentage in the subsoil of an acid Ultisol of Hawaii. These effects may explain the results of Wright et al. (1985) whoreported maximumroot growth and rooting depth in an acid soil amended with animal manure. The objectives of this study were to: (i) comparethe effects of chicken manure and sewage sludge with those of lime and gypsum, all surface-applied, on chemical properties, especially Ca and A1, of a simulated acid soil profile, and (2) demonstrate the interactions among A1, Ca, and selected organic ligands that maybe derived from biowaste decomposition.

Subsoil acidity is a serious constraint to crop production, and is difficult to correct by conventional liming practices. Thus, different approachesto ameliorating acid subsoils are needed. A columnleaching study was conducted to comparethe effects of chicken manure and sewage sludge amendments,each at 20 g kg-1, with CaSO4and CaCO3,each at 5.56 cmol (~Caz+) kg-1, on AI and Ca concentrations in the subsoil of an Ultisol having pH 4.4, 80%AI saturation and 2.78 cmolc kg-~ exchangeable AI. Treatments were incubated moist for 14 d with the top 5 cmsoil of 50 cmlong soil columns,then leached with 37.8 cm of water in 5 d. The soil columns were then cut into four equal segments for chemical analysis. Results showed that the manureswere effective in increasing soil-solution pH and Ca, and decreasing soil-solution Al and percentage of AI saturation in the subsoil, particularly below the 15 cm depth. Soil-solution C, nearly constant through the soil profile, averaged 2.8 mMin the CaSO 4 treatment and 7.6 mM in the chicken manuretreatment. A subsequent solution experiment on the competitive complexation of AI and Ca with modelorganic ligands supported the contention that (i) manurederived organic molecules facilitated the downwardmovementof Ca as Ca complexes, and (ii) along their downwardpath, these complexes reacted with AI, releasing Ca that would becomeplantavailable, and forming complexed AI that may be nonphytotoxic.

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UBSOIL ACIDITY is a serious constraint to crop production in manyparts of the world, especially in the humid tropics (Cahn et al., 1993; Sanchez and Logan, 1992). Toxic levels of soluble and/or exchangeable A1 and low levels of Ca impair plant root development, making water and nutrients, that may be abundant in the subsoil, inaccessible to plants (Adams,1984; Farina and Channon, 1988). Downward movement of surface-applied CaCO3 (lime) is very slow (Pavan et al., 1982; Ritchey et al., 1980). This is probably because lime-released OH-ions 2+ are quickly neutralized by the soil acidity, leaving Ca unaccompanied. These Ca2+ ions are then adsorbed by exchangesites on the surface soil. Thus, surface incorporation of CaCO3has little effect on subsoil A1 and/or Ca (Liu and Hue, 1996; Mathewsand Joost, 1990). Deep incorporation of lime by physical meansis effective (Farina and Channon,1988), but is difficult and costly (Cassel, 1980). Furthermore, in established orchards, deep liming is not possible because the tree roots wouldeither be destroyed by or present an impenetrable barrier to the lime-incorporatingaction. Similarly, no-till fields are not amenable to deep liming. Gypsum and phosphogypsum have been proposed as effective amendmentsto subsoil acidity (Alva et al., 1990; Shamshuddinet al., 1991). Since gypsumis much more soluble than lime and does not alter soil pH appre-

MATERIALS AND METHODS Column Leaching Experiment The soil used was a subsoil (40-60 cm) of a highly weathered, strongly acid Ultisol (RhodicKandiudult,Paaloa series), whichhad an original pHof 4.45 (1:1 in water), and contained (in g kg-1) 11.5 total organic C, 650 clay, 280silt, and 70 sand. The soil’s field water holding capacity was 260 g kg-L Kaolinite, Fe oxides, and gibbsite were the dominantclaysized minerals, and the zero point of net charge was pH3.85. Leaching columns were made of rigid polyvinyl chloride (PVC)50 cm long and 5.8 cm inside diameter. The column interior was lined with hard plastic sheeting to allow easy removalof the entire soil columnfor segmentationand analysis. The columnbottom-endwas filled with a 2 mmlayer of acid-washedsilica sand, underlainby a thin layer of glass wool to facilitate drainage.Finally, a fine-sized nylonnet wastaped over the bottom-endto hold the soil in place and allow drain-

Dep. of Agronomyand Soil Science, College of Tropical Agriculture and HumanResources, Univ. of Hawaii, Honolulu, HI 96822. Received 24 Feb. 1998. *Corresponding author ([email protected]). Published in J. Environ. Qual. 28:623-632 (1999).

Abbreviations: UV, ultraviolet.

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age. Thelower 45 cmof all columnswas filled with 1350g of moist, unamendedsoil screened to pass a 2 mmsieve. To avoid segregation of aggregatesand fines, small quantity of the moistsoil wasaddedat a time, followedby gentle tappings. The top 5 cmsection of each columnwas filled with 150 g soil, whichhad been incubated moist for 14 d at 24 _+ 2°C with one of the following treatments: (i) unamended control, (ii) CaCO3at 2.78 g kg-1, providing1.11 g kg-1 Ca2+ and 5.56 cmol (OH-) -1 ortwotimes exchangeable AI (a p ractical lime rate), (iii) CaSO4.2H20at 4.80 g kg-1, providingthe same amountof Ca2+ as treatment no. 2, (iv) 20 g -1 chicken manure(broiler type, ground to pass a 1 mmsieve), which contained(in g kg-1) 31.2 N, 18.9 P, 20.2 K, 16.3 Ca, 6.5 Mg, and (v) 20 g -~ anaerobically digested sewage sludge, which contained (in g kg-1) 43.6 N, 6.5 P, 0.8 K, 20.6 Ca, and 5.4 Mg. (The organic manureswere selected for their potential uses in agriculture; and their application rates, thoughrather high, werewithin the practical range.) Finally a thin layer of glass woolwasplaced on the columntop to help facilitate the distribution of the leachingwater. The experimentwas set up in the greenhousewhereaverage day temperature was 27°C(range 20-35°C)and the night temperature was 20°C (17-24°C). There were three replications per treatment, whichwere arranged in a completely randomized design. Leachingwas done daily by slowly pouring 200 mL(50 mL in 30 min) of deionized water onto the surface of each soil column. After the water was added, the columnwas covered loosely with parafilmand allowedto drain overnight. Aplastic bottle wasplaced under each columnto collect the leachate. The leaching process was performed for 5 consecutive days until a total of 1000mL(equivalent"rainfall" of 37.8 cm) had been added. The amountof leaching water was deliberately kept slightly