0.25 g granules at four points instead of one 1 g granule at one point. ... placement of approximately 30 granules of 0.30g size instead of 9 granules of 1.00g size.
Fertilizer Research 15:193-201 (1988) O Kluwer Academic Publishers, Dordrecht - Printed in the Netherlands
Effect of granule size and the placement geometry on the efficiency of urea supergranules for wetland rice grown on a permeable soil J. C. KATYAL, 1 BIJAY-SINGH, 2 & P.L.G. VLEK 3 ~International Fertilizer Development Center (IFDC), P.O Box 2040, Muscle Shoals, Alabama 35662, USA; 2All India Coordinated Scheme of Micronutrients in Soils and Plants, Punjab Agricultrual University, Ludhiana 141004, India; 3International Fertilizer Development Center-Africa, B.P. 4483, Lorhe, Togo Accepted 9 December 1987
Key words: urea supergranules, placement geometry, N use efficiency, permeable soils, lowland rice Abstract. In a laboratory experiment 5 cm depth of water was allowed to percolate daily down through a 15 cm thick soil (Typic Ustipsamment) layer. It was observed that leaching losses of urea supergranules (USG)-N could be decreased by about 20% by the placement of four 0.25 g granules at four points instead of one 1 g granule at one point. In field microplots, the placement of approximately 30 granules of 0.30g size instead of 9 granules of 1.00g size resulted in reduced leaching of USG-N and, in turn, increased rice yield. In a follow-up field study, the advantage of more frequently placed USG was confirmed. As compared with 1 g USG placed in the usual manner in the center of four rice hills, increasing the density of placement in soil produced 15% more rice grain. Further increase in rice yield could be obtained by increasing the number of USG placed in the soil and decreasing the size of the granule from 1.00g to 0.70 or 0.35g. With USG of 0.35 and 0.70g yields were equal or sometimes even slightly higher than with split application of prilled urea on a heavily percolating, low-CEC, light-textured soil.
Introduction Poor efficiency of fertilizer nitrogen (N) for lowland rice is a worldwide concern. To minimize N losses from rice paddies, some modifications have been made in the major N fertilizer - urea. Supergranules of urea (USG), each weighing 1 to 3 g, represent one such development. These granules are placed at a soil depth of 5 to 10cm in the center of four rice hills. Urea supergranules performed better than split application of prilled urea at a majority of the locations in the rice-growing regions of Asia (IRRI 1979). Tejeda et al. (1980), however, screened the site factors affecting the response of U S G and showed that U S G outperformed urea only in soils
194 with high cation exchange capacity (CEC). Katyal et al. (1985a) confirmed that USG was definitely a poor choice over prilled or ordinary urea on a low-CEC Typic Ustipsamment with a high percolation rate. In their studies, only 9% of the USG-N (15N-labelled) was utilized by rice in comparison with 25% from prilled urea. Greenhouse experiments conducted by Vlek et al. (1980) and field microplot studies made by Katyal et al. (1985b) established that the inferior performance of USG in highly percolating, light-textured soils with low CEC was due tO the susceptibility of N to leaching as unhydrolyzed urea during the first few days after the application. The percolating water (around 10cm/day) facilitates downward movement of USG, resulting in high leaching losses of N. High urea concentrations at the placement site may slow down the hydrolysis by soil urease (Vlek and Carter 1983). Katyal et al. (1985b) recovered a large proportion of unhydrolyzed urea in leachates 2 days after application of USG. In view of these findings, it is logical to believe that for soils with low CEC reducing the point concentration of urea-N by increasing the number of placement points of USG should reduce the losses of applied N through leaching. This hypothesis was tested in laboratory and field experiments. The number of placement points of USG (referred to as frequency of placement in this paper) was manipulated by applying 1-g supergranules at more points and by decreasing the size of the supergranules from 1.00 g to 0.70 or 0.35 g.
Materials and methods
The experiments were conducted on a Fatehpur loamy sand (Typic Ustipsamment) at Punjab Agricultural Univeristy, Ludhiana, India. The soil (0-15cm) of the experimental site tested pH 7.72 (1:2 soil solution ratio), electrical conductivity 0.27 mmhos/cm, organic carbon 0.21%, sand 91%, clay 7%, CEC 7.2meq/100g soil, and total N 0.034%. An average water percolation rate of 10.9 cm/day was recorded at the experimental site. In all, three experiments were conducted.
Laboratory experiments Porcelain pots (20 cm ID × 20 cm) were fitted on the side near the bootom with a leaching outlet (0.5cm ID × 25cm) made of glass tubing. The opening of the glass tubing lying inside the pot was covered by a thick mat of glasswool to facilitate filtration of the soil solution. On the other end of
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Fig. 1. Placement patterns of urea supergranules of different sizes at different rates of application. the glass tubing, protruding out of the pot, a polyethylene tube was fixed. A 6 kg portion of the 0-I 5 cm soil sample was placed in each pot (approximate depth 15 cm). After the soil was puddled to a depth of 7-8 cm, the leaching rate was adjusted by a screw-type pinchcock fixed on the polyethylene tubing so that a 5 cm standing column of water could leach down in 9-10 h. In the field, too, it was seen that irrigation water of 5 cm depth disappeared in about the same time. Supergranules of urea weighing 0.25, 0.50, and 1.00 g were placed at four, two, and one points, respectively, at a 5 cm depth in a symmetrical pattern. The soil was leached every day with 5 cm of water. Leachates were collected in glass bottles. After the volume was measured, samples of the leachates were analyzed for urea-N (Douglas and Bremner 1970) and NHa-N (Bremner and Edwards 1965).
Field microplot experiment In 120 x 120cm field microplots, 36 hills of P R 106 rice (Oryza sativa L.) - a medium-duration dwarf variety- were transplanted with a 20 × 20 cm spacing. Urea supergranules (1.00g) were placed at nine points in the microplots following the usual application pattern in the field (Fig. la). This pattern was compared with a 30-point placement pattern (by placing a
196 granule in the center of four hills without leaving any interrow or intrarow gaps) in which one supergranule was placed for each hill (Fig. lc). To achieve this placement pattern, the size of the supergranule was reduced proportionately. The comparisons of the two placement patterns were made at 29, 58, and 116kgN/ha. A no-N control and prilled urea at 58kgN/ha applied in two splits (applied two-thirds basal and one-third topdressed 50 days after transplanting) were also included. At maturity, the central 16 hills were harvested. Grain and straw yields were recorded at oven-dry weight basis. Plant samples were ground to pass through a 0.50 mm (30 mesh) screen and digested with H 2 S O 4 in the presence of a catalyst mixture for estimation of total N (Buresh et al. 1982).
Field experiment In 1982, a field experiment was conducted to compare urea supergranules, each weighing 0.35 or 0.70 g, with split application of ordinary urea (applied two-thirds basal and one-third topdressed 50 days after transplanting) and 1 g supergranules. As in other experiments, 1 g USG were placed, depending upon the N rate, either one or more than one at a point in the center of four rice hills (Fig. la). Except for the supergranules of 0.70-g size, which were not tested at 58 kg N/ha because of insufficient material, all the sources were applied at 29, 58, and 116 kg N/ha. The plots were replicated four times in a randomized block design. The supergranules were applied 7 days after transplanting (DAT) at a depth of 7-8 cm and in geometrical arrangements (Fig. lb through lg) such that each rice hill at a particular N rate had equal probability of absorbing N from the supergranules. Regardless of the geometrical arrangement or level of N application, only 1 USG was placed at one point. An additional placement pattern with 1 g USG was also included so that depending upon the N rate either one or more than one granule was placed at a point in the center of four rice hills (Fig. la). The purpose of this additional treatment was to follow the method of USG application used in other research. Before the last puddling, all treatments received a uniform application of P (22 kgP/ha), K (41.5 kg K/ha), and Zn (10 kgZn/ha) through superphosphate, muriate of potash, and zinc sulfate, respectively, Forty-day-old seedlings of PR 106 were transplanted at a spacing of 20 x 20 cm. Irrigation water (approximately 5 cm depth) was added daily by flooding for the first 30 days after transplanting and on alternate days thereafter. It was observed that after irrigation the water used disappeared within 8-10 h at least during the first 30 days. At maximum tillering (42 DAT), flowering (72 DAT), and maturity (108
197 DAT), a net area of 1.6 m 2 (40 hills) was harvested from each treatment. Dry matter yields up to flowering and grain and straw yields at maturity were recorded. Plant samples (both straw and grain) were analyzed for total N (Buresh et al. 1982) to determine apparent N recovery by the crop at various growth stages. Results and discussion
The placement of 0.25 g granules at four points instead of a 1 g granule at one point reduced the total leaching loss of N as urea in 6 days by about 20% (Table 1). As mentioned previously, rapid hydrolysis of urea from small but more frequently placed supergranules might be one of the factors that decreased the leaching losses of N as urea. Malhi and Nyborg (1979), working in an upland situation, showed that the larger the urea granule, the slower its hydrolysis was. Leaching losses of N as NH~- were insignificant whether 1 g or 0.25 g granules were placed in the reduced soil zone, which indicated that urea was adsorbed by the soil if converted to N H + . In line with the findings of the laboratory experiment, the grain yield of rice in the field microplot experiment was higher with the 30-point placement pattern than with the 9-point placement pattern (Fig. 2), irrespective of the rate of N application. The availability of U S G - N to the rice was considerably improved by decreasing the size of supergranules and distributing them at more points around rice hills. Yield response followed the same pattern as N uptake. Although the results from the preceding field microplot experiment provided enough evidence in favor of decreasing the size of supergranules to improve their efficiency, the optimum size for maximum N use efficiency remained to be worked out. The 0.30 g granules were too small to be hand placed without a mechanical device. On the other hand 1 g granules were less efficient because they were more susceptible to leaching. Therefore, a size Table 1.
Effect of size and placement of urea supergranules on leaching losses of fertilizer N
as urea. Size of supergranule
Number of placement points
Amount of urea-N lost in 6 days
(g) 1.00 0.50 0.25
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