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 ²⁷Al 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 ²⁷Al-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(H₂O)_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(H₂O)₆³⁺, 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.

铝毒性是酸性土壤作物产量的关键决定因素。用于表征土壤可交换铝含量的技术通常会产生不一致的结果，从而阻碍纠正措施的实施。我们批判性地比较了三种这样的技术——反滴定、ICP-AES 和²⁷ Al NMR——用于长期（8 年）免耕栽培的热带 Oxisol，涉及四种不同的种植系统和三种 pH 控制方案。ICP-AES 在准确度、定量水平 (LOQ)、可行的 pH 范围、速度、成本和多功能性方面都非常出色。相比之下，反滴定始终高估了可交换的 Al，而²⁷由于 Al(OH) _n (H ₂ O) _{6− n} ^{(3̶− n )+} ( n  = 1–3) 信号。在任何土壤提取物中通过 NMR 检测到的唯一 Al 物质是有毒的 Al 六水合阳离子 Al(H ₂ O) ₆ ³⁺，尽管存在明显的溶解有机物。应用白云石石灰或硅酸钙镁 (CMS) 作为 pH 校正剂显着降低了上层土壤中的可交换铝。然而，没有任何减少是由于 CMS 溶解而形成的不溶性铝硅酸盐。小麦 ( T. aestivum ) 和艳丽响尾蛇 ( C. spectabilis ) 的轮作作物消耗了上层土壤中的可交换铝，而栅栏草 ( Brachiaria brizantha ) 对铝的可用性没有明显影响。当 pH (1 M KCl) 超过 4.9 ± 0.2 且铝饱和度低于 25 ± 5% (Al_ICP ≤ 22 ± 21 mg kg ^-1 )，平均而言，在 0-20 厘米的土壤层中。