Lime Effects on Soil Acidity, Crop Yield, and Aluminum Chemistry in Direct‐Seeded Cropping Systems

Soil acidification threatens dryland crop production in the inland Pacific Northwest (IPNW). Our objective was to assess the efficacy of lime to lower soil acidity, alter Al chemistry, and increase crop yield in a direct‐seeded system. Treatments of subsurface banded fertilizer (120–168 kg N ha −1 yr −1 ) alone or with subsurface banded lime (224 kg ha −1 yr −1 ), or a one‐time broadcast application of lime (7000 kg ha −1 ) or elemental S (1000 kg ha −1 ) were initiated in spring 2002. Grain yield was measured annually from 2002 to 2005 in a spring barley ( Hordeum vulgare L.)–spring wheat ( Triticum aestivum L.)–winter wheat rotation. Soil was sampled in spring 2004 to assess pH and model Al speciation. Low pH was evident at the depth of fertilizer placement (5–10 cm). Broadcast lime increased pH in the surface 15 cm, although reductions in Al 3+ activity [(Al 3+ )] occurred only in the 0‐ to 5‐cm layer. Relative to banded N, there was no increase in pH at the 5‐ to 10‐cm depth where banded lime was placed. At the 0‐ to 5‐cm depth, lower pHs were observed with broadcast S (4.6) compared with the banded N control (5.1) or banded lime (5.0). There was no effect of treatment on yield. Modeling suggests that soluble Al is dominated by organic matter–Al complexes (fulvic acid, FA; FA 2 Al + and FA 2 AlOH 0 ). Solid‐ and solution‐phase organic complexes may control (Al 3+ ) at pH <5.5. Although acidification is a concern in the IPNW, the high organic matter content in direct‐seeded soils may buffer against Al phytotoxicity.