Comparative analysis of exchangeable aluminum in a tropical soil under long-term no-till cultivation

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Aluminum toxicity is a key determinant of crop yield for acid soils. Techniques used to characterize a soil’s exchangeable Al content often yield discordant results, thus impeding the implementation of corrective measures. We critically compared three such techniques – back-titration, ICP-AES, and 27 Al NMR – for a tropical Oxisol under long-term (8-years) no-till cultivation involving four different cropping-systems and three pH control regimens. ICP-AES was superior in terms of accuracy, level of quantification (LOQ), practicable pH range, speed, cost, and versatility. By comparison, back-titration consistently overestimated exchangeable Al, whereas 27 Al-NMR, though accurate, had a higher LOQ and is inherently non-quantitative between pH 5.4 and 6.3 owing to extreme quadrupolar broadening of Al(OH) n (H 2 O) 6− n (3̶− n )+ ( n = 1–3) signals. The only Al species detected by NMR in any of the soil extracts was the toxic Al hexaaqua cation, Al(H 2 O) 6 3+ , despite an appreciable presence of dissolved organic matter. Application of either dolomitic lime or calcium-magnesium silicate (CMS) as a pH corrective significantly decreased exchangeable Al in the upper soil layers. None of the decrease, however, was attributable to insoluble aluminosilicates having formed as a consequence of CMS dissolution. Rotation crops of wheat ( T. aestivum ) and showy rattlebox ( C. spectabilis ) depleted exchangeable Al in the upper soil layers, whereas palisade grass ( Brachiaria brizantha ) had no discernable effect on Al availability. Plots which incorporated any one of the three rotation crops provided equally high yields of soybean in year-8 when pH (1 M KCl) exceeded 4.9 ± 0.2 and Al saturation was less than 25 ± 5% (Al ICP ≤ 22 ± 21 mg kg −1 ), on average, in the 0–20 cm soil horizon. • Three techniques for Exchangeable-Al determination in acid soils were compared. • ICP-AES was superior in terms of accuracy, LOQ, and pH range for Al determination. • The toxic Al hexaaqua cation, Al(H 2 O) 6 3+ was the only species detected by 27 Al NMR. • The 1 M KCl solution inhibits the prominence of DOC to bind Exchangeable-Al.