Exchange Properties of an Argiustoll: Effects of Long-Term Ammonium Nitrate Fertilization
- A. P. Schwab ,
- M. D. Ransom and
- C. E. Owensby
Soil acidification can result from the application of high rates of ammonia-based fertilizers, but little information is available concerning the effects of long-term, annual applications at moderate rates. In this study, plots that were mapped as Smolan silt loam (fine, montmorillonitic, mesic Pachic Argiustolls), 1 to 4% slopes, containing an established stand of smooth bromegrass (Bromus inermis Leyss.) were fertilized continuously with rates of 0, 22, 67, 112, 157, and 224 kg N/ha for 40 yr, or for 20 yr followed by 20 yr without N fertilizer. The objective of this research was to determine the effects of such long-term applications of NH4NO3 on exchange properties. In the continuously fertilized plots, pH and exchangeable Ca2+, Mg2+, and K+ decreased in the upper 20 cm of the soil, and neutral-salt cation exchange capacity decreased in the upper 10 cm, whereas exchangeable Al (1 M KCl) and BaCl2-triethanolamine (BaCl2-TEA) extractable acidity increased. In all cases, significant changes were observed only when N application exceeded plant demand. Discontinuation of fertilization for 20 yr resulted in a significant, but incomplete, trend of the exchange properties toward values of the unfertilized soils. Thus, the acidification resulting from NH4 application, which occurred at even low application rates, was reversed somewhat after fertilization ceased.
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