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Abstract: The impact of the Rht dwarfing genes on P utilization efficiency (PUTE = grain dry matter per kg P in above-ground biomass), total P uptake (Pt) and related traits was studied in the varietal backgrounds of two tall wheat cultivars, Maringa and Nainari 60. Four sets of near-isogenic lines carrying different combinations of the alleles Rht-B1b, Rht-D1b and Rht-B1c for gibberellin-insensitive dwarfism in the hexaploid wheat (Triticum aestivum L.) were compared with tall controls in two field trials under conditions of adequate nutrient supply and irrigation in Northwest Mexico. The yield-increasing effect of the dwarfing genes Rht-D1b and Rht-B1b led to improved PUTE in Maringa and total P uptake in both cultivars. Also, the double dwarf line of Maringa had larger grain yields and P uptake compared to the tall control. The Rht-B1c genotypes showed low PUTE, thick roots and high P concentration in vegetative biomass indicating a surplus of assimilates and P, which could not be translocated into the grains. A similar problem could be observed in Nainari 60 with Rht-B1b and Rht-D1b, which produced the largest grain dry matter with the lowest P concentrations in grains although they showed high P accumulation in straw. Most of the net P uptake occurred before anthesis. P absorption after anthesis was more critical for the dwarf genotypes. For double dwarfs and Rht-B1c, respectively, only 3% and 21% of the total accumulated P at maturity was absorbed at post-anthesis. The grain P of the dwarf lines came more from P accumulated at pre-anthesis and translocated from the vegetative biomass into the grain. The pre-anthesis P accumulation was positively correlated with spikes per m2 (r = 0.91), whereas post-anthesis P accumulation correlated better with grains per spike(r = 0.72), and thousand kernel weight (r = 0.51). P uptake efficiency played a secondary role under these non-P-limiting conditions, and differences in root length density were only slightly affected by Rht-genes.

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)2SO4was limited, whereas loss of (NH4)2CO3was 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.

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−1to 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 CaCO3kg−1) Mollisols. On the other hand, NH3losses from a slightly calcareous (20 g CaCO3kg−1) Vertisols were 10.5% and 3.1%, and from a slightly calcareous Entisols (2.6 g CaCO3kg−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 CaCO3soils (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 CaCO3merits further testing.