Management of Urea Supergranule (USG) Application to Improve Growth and Yield of Rice (Oryza Sativa L) In Some Paddy Soils of Togo and Ghana

Abstract

Nitrogen (N) is known to be a major limiting crop nutrient which is required in large amounts as compared with other major nutrients. In rice (Oryza sativa L.) cropping, urea is the main source of N which is applied to the crop because of its relatively low cost and its high N concentration (46%). However, low N use efficiency is always associated with urea application under paddy fields because of its high N loss, mostly through ammonia (NH3) volatilization. The split application of prilled urea (PU) has for a very long time being the common strategy for reducing urea-N losses but its efficiency has been of much concern of late. The present study addresses the increase in rice yield and nitrogen use efficiency (NUE) using the urea supergranules (USG) deep placement. The study comprised of (i) a greenhouse experiment carried out in the Sinna Garden, University of Ghana to evaluate the effect of USG application at different depths on ammonia volatilization, rice yield and NUE in some paddy soils, (ii) a field trial conducted in 2017 in the irrigated scheme of Zio valley in Togo to determine the efficiency of USG and its optimum rate to improve rice yield and NUE in three paddy soils and (iii) an open field pot experiment to determine the effect of seedling age and time of application of USG on rice yield and NUE. The first experiment comprised four paddy soils: Canne and Voudou series that belong to Oxisols (USDA) were sampled in Togo, and Akuse and Bumbi series that belong to Vertisols (USDA) were sampled in Ghana. Six modes of urea application were formulated as treatments: prilled urea (PU, 1.8 g pot-1) applied at soil surface and urea supergranule (USG 1.8 g) applied at soil surface (0 cm) 4, 8, 12 and 16 cm depths and a control without N application. Rice variety IR-841 was planted and a closed chamber device method was used to trap NH3. The results indicated significant decrease in ammonia loss with deep placement of USG over surface split application of PU in the different paddy soils. Regardless of the depth of USG application, the type of paddy soil significantly affected the cumulative ammonia loss which varied from 8% of N applied in Canne series to 14% in Bumbi series. Ammonia loss decreased drastically with depth of USG application (37% of applied N at the soil surface to 0% at 16 cm depth). The USG deep placement significantly increased rice yield and agronomic use efficiency (AE) over PU in the different paddy soils. The optimum depth of USG application to reduce ammonia loss and to achieve the highest yield and AUE varied according to the soil type: 4 cm in Voudou and Bumbi series, 8-12 cm in Canne and 8 cm in Akuse series. The field experiment was conducted at three different sites in Togo (Ablotsri, Hahome and Kouto). Three factors were studied: rice variety (IR-841 and TGR-405), the type of urea (PU and USG) and the N rate (0, 52, 78 and 104 kg ha-1). Results indicated that the efficiency of USG deep placement (UDP) varied significantly with the rice cropping and site. USG significantly increased rice growth parameters and yield over PU. Rice yield increased by 17-23% on the clay soil of Ablotsri and sandy-clay-loam soils of Hahome. In the sandy-loam soil of Kouto, USG increased grain yield only by 4% in the first season while in the second, USG and PU gave similar yields. The TGR-405 rice variety increased grain yield over IR-841 by 5-7% but there was no significant interaction effect between the type of urea and rice variety. The USG increased nitrogen uptake by 34-47 kg ha-1, agronomic use efficiency (AE) by 13-16 kg kg-1 and recovery efficiency (RE) by 13-16 over PU at Ablotsri and Hahome sites while at Kouto site, no significant increment in the parameters was obtained with USG when compared to PU. Rice yield increased with increasing USG rates. However, application of USG rate of 78 kg ha-1 at Ablotsri site, and 104 kg ha-1 at Hahome site were more lucrative, while at Kouto site, 104 kg ha-1of PU gave the highest income. The third experiment consisted of four different ages of seedling (10, 14, 21 and 28-days old) and four different USG application times (0, 7, 14, and 21 days after transplanting (DAT). Results indicated that the younger the seedlings, the better were rice performances. The highest rice yield, NU, AE and RE were obtained with 10 and 14-day old seedlings, while the poorest performances were observed for 28-day old seedlings. Application of USG at 7 or 14 DAT gave the highest rice parameters. The interactions between seedling age and time of USG application showed the highest rice performances when USG was applied at 7 or 14 DAT to rice seedlings of 10 to 14-day old. The overall conclusion from the research was that the USG deep placement significantly reduced ammonia loss in paddy soils over the PU. The optimum depth and rate of USG application to improve rice growth yield and NUE are soil specific. Young seedlings and early USG application (7-14 DAT) should be considered for best rice cropping. .