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Soil amendments, including lime, biochar, industrial by-products, manure, and straw are used to alleviate soil acidification and improve crop productivity. Quantitative insight in the effect of these amendments on soil pH is limited, hampering their appropriate use. Until now, there is no comprehensive evaluation of the effects of soil amendments on soil acidity and yield, accounting for differences in soil properties. We synthesized 832 observations from 142 papers to explore the impact of these amendments on crop yield, soil pH and soil properties, focusing on acidic soils with a pH value below 6.5. Application of lime, biochar, by-products, manure, straw and combinations of them significantly increased soil pH by 15%, 12%, 15%, 13%, 5% and 17%, and increased crop yield by 29%, 57%, 50%, 55%, 9%, and 52%, respectively. The increase of soil pH was positively correlated with the increase in crop yield, but the relationship varied among crop types. The most substantial increases in soil pH and yield in response to soil amendments were found under long-term applications (>6 year) in strongly acidic (pH < 5.0) sandy soils with a low cation exchange capacity (CEC, <100 mmol c kg−1) and low soil organic matter content (SOM, <12 g kg−1). Most amendments increased soil CEC, SOM and base saturation (BS) and decreased soil bulk density (BD), but lime application increased soil BD (1%) induced by soil compaction. Soil pH and yield were positively correlated with CEC, SOM and BS, while yield declined when soils became compacted. Considering the impact of the amendments on soil pH, soil properties and crop yield as well as their costs, the addition of lime, manure and straw seem most appropriate in acidic soils with an initial pH range from <5.0, 5.0–6.0 and 6.0–6.5, respectively. [Display omitted] • Soil amendments increased soil pH and crop yield by ca. 13% and 36%, respectively. • Amendments increased soil CEC, OM and base saturation and decreased bulk density. • Lime is the most economical while biochar is the most expensive for raising pH. • Lime, manure and straw are recommended at pH < 5.0, 5.0–6.0 and 6.0–6.5, respectively. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilizer-induced soil acidification in Chinese croplands is well-known, but insight in the impacts of different N fertilizer management approaches (fertilizer type and rate) on soil acidification rates is very limited. Here, we conducted a field experiment on a moderate acid soil to quantify soil acidification rates in response to N fertilization by different fertilizer types and N rates through monitoring the fate of elements (mainly nutrients) related to H+ production and consumption. Two N fertilizer types (urea and NH 4 Cl) and three N rates (control, optimized and conventional, 0/120/240 kg N ha−1 for wheat, 0/160/320 kg N ha−1 for maize) were included. Nitrogen addition led to an average H+ production of 4.0, 8.7, 11.4, 29.7 and 52.6 keq ha−1 yr−1, respectively, for the control, optimized urea, conventional urea, optimized NH 4 Cl and conventional NH 4 Cl plots. This was accompanied with a decline in soil base saturation of 1–10% and in soil pH of 0.1–0.7 units in the topsoil (0–20 cm). Removal of base cations by crop harvesting and N transformations contributed ~70% and ~20% to the H+ production in the urea treated plots, being ~20% and ~75% in the NH 4 Cl treated plots, respectively. The large NH 4 + input via fertilization in the NH 4 Cl treated plots strongly enhanced the H+ production induced by N transformations. The low contribution of N transformations to the H+ production in the urea treated plots was due to the limited NO 3 − leaching, induced by the high N losses to air caused by denitrification. Increased N addition by urea, however, strongly increased H+ production by enhanced plant uptake of base cations, mainly due to a large potassium uptake in straw. Our results highlight the important role of optimizing fertilizer form and N rate as well as straw return to the field in alleviating soil acidification. • Soil acidification rates in response to N fertilizer management were quantified. • N transformation processes dominated soil acidification by NH 4 + based fertilizer. • Base cation uptake by crop was the main contributor to soil acidification by urea. • Straw return is an effective method to mitigate soil acidification in case of urea. [ABSTRACT FROM AUTHOR]