Plant mixtures differ to a large extent in nitrogen (N) composition. Nitrate-N: ammonium-N ratios vary according to raw material contents. The optimum nitrate: ammonium-ratio is close to 3:1 while 100% ammonium-N could impair plant growth and yield (Adriaanse 1990). Some companies use urea-N as the primary N source in plant mixtures. Urea in close proximity to developing seedlings could impair or terminate growth. The band placement of certain N sources away from the plant mixture could also reduce yield. (Adriaanse 2012). Furthermore yield could be reduced by an overall excess of N in the soil. (Adriaanse and Schmidt, 2003). This article focuses on potential negative effects of band placed LAN and urea on germination, emergence, and production of maize and wheat.
Plant Population Reductions
The application of urea and ammonium nitrate at the same relatively high N rate may result in high seedling mortalities for urea compared to no mortalities for ammonium nitrate (Figure 1). These symptoms are often mistaken for genetically associated poor germination or poor seedling vigour. In addition these symptoms are often wrongly ascribed to damage caused by soil insects or meerkats.
The band placement of 100 kg N/ha, 50 to 100 mm directly below maize seed at row widths of 1.5 m, resulted in a plant population loss of 4800 plants/ha with LAN compared to a loss of 13300 plants/ha when urea was used (Figure 2). Reductions in plant population were significantly more with urea compared to LAN at both 75 and 100 kg N/ha (Figure 2). This research illustrates relative differences in toxicity between urea and LAN under very specific conditions but does not imply that band placement of either of these N sources directly below the seed at low N rates is an acceptable practice under all conditions.
Reductions in plant population due to the application of high N rates, is an indication of very severe N toxicity effects. Impairment of plant growth and yield loss could occur at much lower N concentrations. In another study where urea and LAN were band placed at planting, at a distance of 10 to 15 cm from the row, at a depth of 10 cm, the yield loss for urea compared to LAN was 20% at 100 kg/ha and 44% at 175 kg/ha (Adriaanse, 2012). Row widths were 1.5 m. Most farmers would probably not have been aware of the fact that urea toxicity had occurred. The yield at 100 kg N/ha in the form of urea was 5 ton/ha which is in line with the yield potential of the area. No toxicity symptoms were observed on the plants, however a yield improvement of 20% at 100 kg N/ha could have been achieved had LAN been used under the same circumstances.
Toxicity symptoms associated with high rates of both urea and LAN applications would probably result in yield loss due to high N concentrations in the soil. In contrast, scorching of leaves associated with spreading of N sources over leaf canopies would probably not result in yield loss. Typical symptoms resulting from high N concentrations in the soil are necrotic leaf tips and edges on the youngest leaves. (Figure 3 and Figure 4). Scorching of leaves due to spreading of fertilisers over leaves show some similarities but lesions in areas where fertilisers were in direct contact with leaves would also be visible. Scorching symptoms resulting from urea spreading (Figure 5) is often less prominent than symptoms associated with LAN spreading (Figure 6) but neither of the two, as was previously stated, is expected to result in yield losses.
Conclusions and recommendations
- The risk of urea containing plant mixtures impairing germination and seedlings development is much greater compared to LAN based plant mixtures. Reductions in plant populations and retarded growth could result in significant yield losses. Although toxicity effects are strongly affected by climatic conditions it is recommended that urea should rather not be used in plant mixtures intended for band placement.
- The band placement of urea at planting away from the plant mixture could also result in significant yield loss at relatively high N rates while this effect was not observed with LAN. For this reason it is recommended that urea should rather not be band placed close to plant rows at planting.
- Farmers should look out for toxicity symptoms such as necrotic leaf tips and leaf edges surrounding leaf tips. These symptoms may result from high urea-N concentrations or from general oversupply of N to the soil. Significant yield losses could be expected to follow these symptoms. In contrast the scorching of leaves due to the spreading of either LAN or urea over leaves is not expected to result in yield losses.
NB. Consult a qualified agronomist for locality specific applications. The results referred to in this article were obtained under specific conditions and are therefore not generally applicable under all conditions.
ADRIAANSE F.G., 1990. Effects of nitrate: ammonium ratios, times of application and prolificacy on nitrogen response of Zea mays L. Ph. D. University of the Free State.
ADRIAANSE F. G. 1991. Unpublished research report on nitrogen placement, ARC-GCI, Potchefstroom.
ADRIAANSE, F. G., 2012. KAN of ureum: voor-plant, met-plant of na-plant. S A Graan/Grain 05/12.
ADRIAANSE, F. G. & SCHMIDT. 2003. N-aanbevelings volgens N-ontledings in
die grond – die enigste antwoord. S A Graan/Grain. 7/03.