The nitrate-N: ammonium-N ratio effectively resulting from the application of LAN should therefore soon be close to the optimum. In contrast Urea consists of 100 % urea-N which is not available for plant uptake. Urea will firstly be converted to 100% ammonium-N through the process of hydrolysis and then to nitrate through the process of nitrification in the soil. Many soil conditions will influence the rates of the hydrolysis and nitrification processes. Never the less Urea will take much longer than LAN to be readily available for plant uptake and also to reach the optimum nitrate: ammonium ratio under all conditions.
Most nitrate-N and urea-N will leach to the level of water penetration while very small quantities of ammonium-N will leach (Gardner en Roth, 1984). The potential for leaching immediately after application would therefore be 50% for LAN but 100% for Urea. The fact that 100% of the N in LAN is immediately available for crop uptake compared to 0% of the N in Urea further reduces the risk of LAN leaching, but at the same time increases the risk of Urea leaching.
Research done by the ARC-GCI in the Viljoenskroon district was carried out over a three-year period from 1998 to 2000 on the same plots to determine whether the application of LAN and Urea will result in grain yield differences. The soil was from the Avalon form containing a soft plinthic layer, varying in depth from 1.2 m to 1.7 m. The clay content for different depth increments was 9.1 % from 0 to 15 cm, 10.4 % from 15 to 30 cm and 16.0 % from 30 to 60 cm. Under theses specific conditions nitrogen will leach to reach the water in the water table, but it will also move upwards with the water table. Leaching of nitrogen is therefore restricted and movement of water in the sub soil is also much slower due to the higher clay content. Every year the trial was preceded by a rip on row tillage practise to a depth of 0.6 to 0.7 m.
Treatments consisted of five N rates (35 to 175 kg N/ha, three application times (before, at and after planting) and two N sources (LAN and Urea) plus two controls (0 kg N/ha). Treatments were arrangement in a factorial combination and a randomised block design was used which was replicated twice. Applications were either, three weeks before, at or after planting, 10 to 15 cm from the row, at a depth of 10 cm, manually by hand. Except for the control, 20 kg N/ha of each N rate was applied in the form of LAN together with 30 kg P/ha (Triple Superphosphate 20%) and 30 kg K/ha (Potassium Chloride) 5 cm away and 5 cm deeper than the seed with a specialised planter at planting. The maize cultivar PANNAR 6479 was planted at row widths of 1.5 m on 18/12/97, 23/11/98 and 5/12/99 to effective plant densities of 26514, 27083 en 25069 plants/ha respectively for the three seasons.
Yield responses to N rates, applied before planting are presented in Figure 1. Yield did not differ between LAN and Urea at N rates below 110 kg/ha but at higher N rates yield for LAN was as much as 600 kg/ha or 7 % higher than for Urea. The reason for yield depression at high Urea N rates was probably due to Urea toxicity. Nitrification was probably inhibited when more than 90 kg Urea N/ha (110-20 kg N/ha) was band placed before planting. No differences in yield responses between LAN and Urea at low N rates are indicative of conversion processes that were completed, no differences in leaching losses and no differences in availability for uptake under these specific conditions.
Figure 1: Relationship between N rates and maize Grain Yield, over three years, for LAN and Urea, applied three weeks before planting. Treatments were applied 10 to 15 cm from the row at a depth of 10 cm. At planting 20 kg N/ha of each N rate, except the control was applied as LAN in the plant mixture
Yield responses to N rates, applied at planting are presented in Figure 2. Applications of LAN resulted in much higher grain yields compared to Urea. Differences in yield varied from 1 ton/ha (20%) at 100 kg N/ha to more than 2 ton/ha (44%) at 175 kg N/ha. These differences in yield are mainly ascribed to urea toxicity which in effect was due to ammonium toxicity resulting from nitrification inhibition when band placed. Grain Yield responses to LAN rates, for example at 100 and 175 kg N/ha at planting (Figure 2) was much the same than for the same LAN rates before planting (Figure 1) while this was not the case with Urea applications.
Yield responses to N rates, applied after planting are presented in Figure 3. These responses to LAN, was clearly more efficient than to Urea. Grain Yield at 100 kg N/ha was 350 kg/ha or 5.8% more with LAN than with Urea. Viewed from another angle, 140 kg N/ha applied as Urea resulted in the same yield as 100 kg N/ha applied as LAN. LAN was more efficient because the N in LAN was immediately available for uptake which on its own will result in less leaching. Furthermore, the ammonium-N from LAN will not leach much after band placement. The N in Urea in contrast is not readily available for crop uptake while at the same time it is 100% subjected to leaching. Although nitrogen leaching will not be out of the profile, it can be expected that N leachate in deeper soil layers will be dilute for uptake.
Figure 2: Relationship between N rates and maize Grain Yield, over three years, for LAN and Urea, applied at planting. Treatments were applied 10 to 15 cm from the row at a depth of 10 cm. At planting 20 kg N/ha of each N rate, except the control was applied as LAN in the plant mixture
Figure 3: Relationship between N rates and maize Grain Yield, over three years, for LAN and Urea, applied three weeks after planting. Treatments were applied 10 to 15 cm from the row at a depth of 10 cm. At planting 20 kg N/ha of each N rate, except the control was applied as LAN in the plant mixture.
Responses to applications before planting (Figure 1) were compared with responses to applications after planting (Figure 3) in the composited Figure 4. LAN applications after planting were more efficient than LAN applications before planting at N rates lower than 150 kg/ha. LAN applications before planting was more efficient than LAN applications after planting at higher band placed applications than 155 kg N/ha (175-20 kg N/ha). Urea applications after planting were slightly more efficient than Urea applications before planting but these differences were not really significant. LAN applications after planting were however more efficient than Urea applications before or after planting (Figure 4).
Figure 4: Relationship between N rates and maize Grain Yield, over three years, for LAN and Urea, applied three weeks before planting and three weeks after planting. Treatments were applied 10 to 15 cm from the row at a depth of 10 cm. At planting 20 kg N/ha of each N rate, except the control was applied as LAN in the plant mixture
Conclusions and recommendations:
1. LAN and Urea should be equally efficient when band placed before planting up to N rates of 90 kg N/ha at row widths of 1.5 m, 10 to 15 cm from the rows at a depth of 10 cm on water table soils. LAN should be more efficient than Urea before planting at higher N rates than 90 kg N/ha. Pre-plant applications can be recommended on water table soils when post-plant applications are impractical or impossible. Managing large areas may require post-plant applications in some areas while the planting process is still going on in other areas. Water tables are often saturated with water which will delay or rule out the possibility of post-plant applications. NB. There is a risk involved in pre-plant applications of most of the N either as LAN or Urea on well drained soils due to the possibility of N leaching. This risk will be much lower in low rainfall areas but still unpredictable due to unpredictability of rainfall. The risk of N leaching will also be lower on more clayey soils, when applications are made closer to panting and when LAN are used instead of Urea.
2. LAN applications at planting were as efficient as LAN applications before planting when band placed. Urea applications at planting, 10 to 15 cm from 1.5 m rows, at a depth of 10 cm, should not be recommended due to toxicity effects. Urea band placements further than 15 cm from the row at planting may result in unavailability for uptake when rip on row cultivation practises are followed.
3. The application of most N in the form of LAN after planting was most efficient in terms of highest yield obtained with lowest N rate applied. Grain Yield for this treatment was about 6% better in comparison with most other treatments at 100 kg N/ha. Good responses to Urea applications after planting were also demonstrated but these responses were not as efficient as with LAN. Other research showed that after plant applications in dryer areas should be earlier (3 to 5 weeks after planting) than in wetter areas (4 to 9 weeks after planting) (Adriaanse, 1990; Adriaanse en Human 1993).
4. Price differences between Urea and LAN per unit of N applied, the maize price as well as all input costs should carefully be considered together with these results. Calculations for return on input costs will reveal which N source, N rate, time of application will maximise profit above costs. According to economic calculations which were carried out in 2001, LAN was more profitable than Urea when applied after planting (Adriaanse et. al. 2001). Furthermore, the most cost effective practise is not necessarily convenient or possible to manage and might also result in higher risks.
