No abstract

No abstract

Ammonia volatilization from urea is a considerable problem if fertilizer is surface applied, whether by choice (minimum tillage) or by lack of implements, which is common in the tropics. Urea phosphate [CO(NH2)2·H3PO4] was tested in several experiments as a means of reducing ammonia volatilization losses. Incubation experiments using 15N‐labeled urea were conducted to evaluate ammonia loss from urea, urea phosphate, and urea‐urea phosphate mixtures (N content = 460, 190, 370, and 350 g kg−1, respectively), applied at rates up to 100 kg N ha−1 to a variety of soils using a range of initial moisture contents. Evolved ammonia was measured by collection in an external acid trap. Ammonia volatilization losses from surface‐applied urea ranged from 10% to 30% of that applied, dependent on soil type. Initial soil moisture affected the timing of N loss, but not the total amount. Addition of urea with phosphate (190 g N kg−1) slightly decreased ammonia loss from 17.0% to 12.2% of the applied N on a highly calcareous (225 g CaCO3 kg−1) Mollisols. On the other hand, NH3 losses from a slightly calcareous (20 g CaCO3 kg−1) Vertisols were 10.5% and 3.1%, and from a slightly calcareous Entisols (2.6 g CaCO3 kg−1) were 14.4% and 4.4% of the applied N for urea and urea phospate, respectively. The relative savings of urea‐N were much higher in low CaCO3 soils (approximately 70%) than in the highly calcareous soil (approximately 30%). Mixtures of urea and urea phosphate were substantially less effective in reducing ammonia volatilization with relative reductions ranging from 0% to 20%. Detailed studies of the mobility and reactions of urea phosphate in soils revealed that rapid precipitation of calcium phosphates near the placement site and the diffusion of urea away from the placement site were responsible for the poor performance of granular urea phosphate in highly calcareous soil. It is concluded that urea‐urea phosphate mixtures should probably not be recommended as a means of reducing ammonia volatilization on highly calcareous soils, though its use on noncalcareous soils and soils low in CaCO3 merits further testing.

Sustainability in agriculture is a rising concern in the tropics where fertilizers are introduced to meet increasing food demands. The adverse effects of long‐term fertilizer use were monitored through changes in the soil exchange complex in packed columns of an Oxic Paleustalf, a Typic Paleudult, and a Tropeptic Haplustox. Following each application of N at rates of 0, 50, or 100 kg ha−1 as urea, ammonium sulfate (AS), or calcium ammonium nitrate (CAN), the soils were flushed with distilled water, representing the water draining from these soils in their respective tropical environments during a growing season. The procedure was repeated 30 times during 2 yr with 6‐wk rest periods after each five cycles. The rate of acidification reflected the rate of application and acid‐producing capacity of the N source. In the Oxisol and Ultisol, further acidification resulted in an increase in exchangeable Al and Mn, which were eventually found in the effluent, suggesting mineral dissolution. In the Alfisol, acidification resulted in a loss of cation‐exchange sites and the increasing predominance of Al and Mn. This acid‐consuming process caused the development of a descending acidification front, in which soil pH dropped from 6.7 to 5.0. Leachates remained neutral until the acidification front reached the bottom of the soil column. Applied N was quantitatively recovered in the leachates of all soils. Although plant growth would moderate these processes, the results show that indiscriminate N use on such fragile soils may eventually cause soil degradation and groundwater pollution.

Laboratory studies were conducted to explain the wide variation in reported estimates of ammonia volatilization losses from N‐fertilized paddy fields. The ammonia volatilization capacity of a system was found to be equivalent to its alkalinity [NH3(aq), HCO32‐]. In solutions lacking alkalinity, loss of (NH4)2SO4 was limited, whereas loss of (NH4)2CO3 was essentially complete.Ammonia loss from solution is best described as a consecutive reaction with opposing step. Ammonia volatilization per se followed first‐order reaction kinetics. The rate of ammonia volatilization is thus directly related to the concentration of aqueous ammonia and therefore to the concentration of ammoniacal N and pH. The rate of ammonia volatilization was severely restricted by limiting the movement of air above the water, as is often the case in the laboratory and field studies reported to date. Ammonia volatilization was enhanced by water turbulence and increased exponentially with temperature from almost nil at 0°C to approximately 20 mg N/100 cm2/5 hours at 46°C.