Soil Acidification from Long‐Term Use of Anhydrous Ammonia and Urea

Abstract

Acidity generated by N fertilizers depends on factors such as the composition of the fertilizer, climatic and soil conditions, and the crops grown. Our objective was to quantify the acidifying effects of urea and anhydrous NH3 when used as fertilizers for cereal production in Saskatchewan, Canada. The fertilizers were injected annually (at 10‐cm depth) into a medium‐textured, moderately acid (pH ≈5.5) Typic Haploboroll, at rates of 0, 45, 90, and 180 kg N ha−1 for 9 yr. Soil acidity increased as N application rate increased, with anhydrous NH3 causing greater acidification than urea. Although pH values as low as 4.3 were recorded in soil treated with anhydrous NH3, KCl‐exchangeable acidity remained low. The major effect of acidification was a depletion of exchangeable Ca and Mg. The solubility of Mn (but not Al) increased substantially as pH decreased, with solution concentrations of almost 30 mg Mn L−1 being recorded 6 d after injection of NH3. Acidity generated by anhydrous NH3 compared well with values predicted assuming that all of the applied NH3 was oxidized to NO3− (with the production of 1 mol H+ mol−1 of N) and that these protons were partly neutralized by OH− released when NO3− was taken up and assimilated by plants. Acidification due to export of bases in grain was insignificant because wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) remove only a slight excess of cations over anions. Urea failed to realize its full acidification potential because of an apparent loss of urea‐N from the soil by NH3 volatilization.

Cation exchange and buffer potential of Saskatchewan soils estimated from texture, organic matter and pH

“…These estimates involve the assumption that exchangeable acidity (i.e., acidity extractable using a neutral salt solution) was negligible. This assumption is reasonable in light of previous work showing that exchangeable acidity is essentially absent from prairie soils even when pH is as low as 4 (Curtin et al 1984;Bouman et al 1995). Only seven of our samples had pH less than 4.…”

“…Cation exchange reactions are vital to such key processes as nutrient leaching, fertilizer-soil interactions, nutrient supply to plants, and pH-buffering (Binkley and Richter 1987;Bouman et al 1995). Soil CEC is a complex characteristic because the measured value depends on several factors, the most important of which are texture, clay mineralogy, organic matter content, pH, and electrolyte concentration of the soil solution (Helling et al 1964;St.…”

“…Acid soils occur in the Peace River region of Alberta and, to a limited extent, in west-central Saskatchewan (Rostad et al 1983). However, increased use of N fertilizers will likely accelerate the rate of soil acidification (Campbell and Zentner 1984;Bouman et al 1995). …”

Nitric and nitrous oxide emissions following fertilizer application to agricultural soil: Biotic and abiotic mechanisms and kinetics

“…Subsequent lowering of soil pH by as much as four units can occur due to nitrification of NH4 +, which can reach concentrations > 1000 ttg N g soil 4 near points of fertilizer application [Frederick and Broadbent, 1965]. Although pH values may return to nearinitial levels in the short term, more persistent acidification can be promoted by continued fertilizer use over several years [Bouman et al, 1995]. In acid-to-neutral soils which continue to receive intensive N inputs, the accumulation of NO 2-following fertilizer application may create optimum conditions for localized HNO 2 formation and NO production, as proposed by Nelson [1982].…”

Absorção de cátions e ânions pelo capim-coastcross adubado com uréia e nitrato de amônio

“…A acidificação do solo também está associada com a depleção de cátions básicos por causa da lixiviação de NO 3 - (Bouman et al, 1995). Grandes perdas de N pela volatilização de NH 3 da uréia podem explicar a diferença entre fontes na acidificação do solo, porque essas perdas reduzem o acúmulo de N inorgânico no solo, e menos nitrato é lixiviado com uréia (Cantarella et al, 2003).…”