Soil pH Effects on Nitrification of Fall-Applied Anhydrous Ammonia

November or before April because soils either are frozen or too wet. Soil temperature at the time of application has been the primary It has been recommended that fall applications of N factor used to predict rates of nitrification and assess the risks associbe delayed until soil temperature has decreased to 10 C ated with losses of N applied in the fall as anhydrous ammonia in the at a depth of 10 to 15 cm (Nelson and Hansen, 1968; Corn Belt. We report studies assessing the importance of soil pH as a factor affecting nitrification rates and losses of this N before corn McVickar and Walker, 1978; Follett et al., 1981). The (Zea Mays L.) begins rapid growth in June. Data were collected in underlying assumption is that nitrification is strongly a series of field studies conducted during 4 yr. Anhydrous ammonia inhibited by low temperatures during winter months, so was applied in the fall after soils had cooled to 8 C, and soils were fertilization in late fall instead of early spring does not sampled before corn plants emerged in the spring. Soil pH ranged significantly increase the potential for losses of the fertilfrom 6.0 to 7.5. Significant relationships between soil pH and izer N by leaching or denitrification of NO 3 when excess percentage nitrification were observed each year. Means of measurewater is present during the fall-through-spring period. ments made in mid-April (when planting begins) indicated 89% nitrifiA review by Schmidt (1982), for example, indicates that cation of fertilizer N in soils having pH 7.5 and 39% nitrification nitrifying organisms are essentially inactive in cold soils of this N in soils having pH 6.0. The finding that soil pH influenced and that such limitations prevail until temperatures inwhen nitrification occurred helps to explain why the effects of nitrification inhibitors have been variable in this region. Significant relationcrease to 4 or 5 C. However, nitrification has been obships between soil pH and recovery of fertilizer N as exchangeable served in frozen soils (Nyborg and Malhi, 1979), and NH4 and NO3 were observed in years with above-average rainfall there is evidence that the cumulative effect of relatively before samples were collected in April. The effects of soil pH on slow nitrification in cold soils could be important during nitrification, therefore, influenced the amounts of NO3 lost by denitrithe fall-to-spring period (Frederick, 1956; Sabey et al., fication or leaching during spring rainfall. The observed effects of 1959; Anderson and Boswell, 1964; Frederick and pH on nitrification rates suggest that economic and environmental Broadbent, 1966; Sabey, 1969; Campbell et al., 1973; benefits of delaying application of fertilizer N may be greater in Gomes and Loynachan, 1984; Haynes, 1986). More inhigher-pH soils than in lower-pH soils. formation concerning rates of nitrification during the fall-through-spring period is needed because Balkcom et al. (2003) showed that March-through-May rainfall A ammonia is the most widely used form is a major factor affecting losses of N from fertilized of fertilizer N in the Corn Belt and is applied to cornfields to rivers before plants begin rapid growth many fields in October or November for corn to be in June. planted in April or May. Fertilizing some fields in the Soil pH usually is not considered to be an important fall and others in the spring creates a wider period for factor affecting rates of nitrification of fall-applied N fertilizer application and enables more efficient use of because agricultural soils of the Corn Belt usually have the relatively expensive facilities and equipment needed pH values between 5.5 and 8. Reviews relevant to fertilto store this gaseous form of N, transport it to farms, ized agricultural soils (Pesek et al., 1971; Russell, 1973; and inject it into soils (Kurtz and Smith, 1966; Pesek et Schmidt, 1982) report that nitrification rates in soils are al., 1971; Aldrich, 1980; Black, 1984; Peterson and Voss, little affected by soil pH within this range. Reviews 1984; Randall et al., 1985). Fall applications also reduce relevant to nitrification in nonfertilized soils (Grant, the number of field operations that must be done within 1994; Stark and Firestone, 1996; Norton, 2000) do not a short period in the spring and thereby make it practical discuss the effects of pH on nitrification as an indepenfor farmers to manage more hectares planted to corn. dent step in the overall process of N mineralization. It is Ample time is available for fall applications because well established, however, that nitrification is relatively corn usually is preceded by soybean [Glycine max (L.) slow at pH values 5.5 (Alexander, 1965; Sahrawat, Merr.], which normally is harvested by mid-October. 1982; Schmidt, 1982). Anhydrous ammonia usually cannot be applied after There is some evidence that soil pH in the range of 6 to 8 could influence nitrification rates in soils. The pH of artificial growth media is known to strongly influP.M. Kyveryga and A.M. Blackmer, Dep. of Agronomy, Iowa State ence the activity of nitrifying microorganisms, with maxUniv., Ames, IA 50011; J.W. Ellsworth, Univ. of Idaho, Twin Falls Research and Extension Center, Twin Falls, ID 83303-1827; R. Isla, imum growth at pH values in the range of 7.5 to 8.2 Agricultural Research Service, Government of Aragón, Zaragoza, (Waksman and Starkey, 1931; Alexander, 1965; Norton, Spain. This journal paper of the Iowa Agric. and Home Econ. Exp. 2000). Effects of soil pH on nitrification in the range of Stn., Ames, IA, Project no. 4003, was supported by Hatch Act and 6 to 8 have been observed (Eagle, 1961; Morrill and State of Iowa Funds. Received 31 Mar. 2003. *Corresponding author ([email protected]). Dawson, 1967; Dancer et al., 1973; Focht and Verstraete, 1977; Kissel et al., 1985; Tlustos and Blackmer, 1992). Published in Soil Sci. Soc. Am. J. 68:545–551 (2004). Part of this effect may be due to the presence of carbon Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA ates found at the higher pH values (Schmidt, 1982; Kin-