Soil Acidification From Ammonium‐Nitrogen Fertilization in Moldboard Plow and Stubble‐Mulch Wheat‐Fallow Tillage

Abstract

AbstractContinued use of ammonium‐based (NH4‐N) fertilizer can lower soil pH to levels deleterious to crop growth. The depth and intensity of acidification is affected by rate and type of N fertilizer, and differs with tillage systems. Change in soil pH in relation to applied N was determined for one conventional (moldboard plow) and two stubble‐mulch (offset disk, subsurface sweep) tillage treatments in a winter wheat (Triticum aestivum L.)‐fallow rotation receiving cumulative N application of 493, 728, 986, 1221, 1714, and 2207 kg N ha−1 over 44 yr. Nitrogen was applied as (NH4)2SO4 from 1940 to 1961 and as NH4NO3 from 1962 to 1983. The soil was a Walla Walla silt loam (Typic Haploxeroll) containing 200 g clay and 11 g organic C kg−1 in the upper 30 cm. Soil pH (1:2 soil/0.01 M CaCl2) decreased linearly with increasing N application in both conventional and stubble‐mulch tillage. Acidifying effects were concentrated in the top 7 cm of stubble‐mulched soil but distributed to 22 cm or below with moldboard plowing. The rate of pH decline in the top 22 cm of soil was greater for moldboard plow than for disk or sweep tillage, 0.38 vs. 0.21 and 0.28 units, respectively, per Mg of applied N ha−1. Linear regression analysis predicted that 2.7, 4.8, and 3.5 Mg N ha−1 would acidify the 0‐ to 22‐cm soil depth to a pH deleterious to wheat growth with moldboard plow, offset disk, and sweep tillage, respectively. However, the more rapid decrease in pH in the 0‐ to 7‐cm zone with disk or sweep tillage may sooner predispose stubble‐mulch systems to negative effects of acid conditions within the seed zone. Nitrogen applied as NH4NO3 produced more acidity per unit of N than did (NH4)2SO4, but NH4NO3 was applied at much higher rates at a later date so unbiased comparison is difficult.