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Inorganic nitrogen (N) dissolved in groundwater could be lost for crop production through downward and sideway movements of groundwater, resulting in lower yields and profit margins above costs. Differences in leaching between N sources can effectively be utilised to reduce the risk of N leaching. N management practices such as application methods and timing could also contribute significantly to reductions in leaching losses. Basic scientific principles and case studies associated with severe losses in revenue were used to develop guidelines for combatting N leaching losses.


The application of different N-sources results in one or a combination of nitrate-N, ammonium-N and urea-N dissolved in groundwater. The vertical movement of these forms of inorganic N in groundwater are displayed for a Sandy Loam soil in Figures 1 and for a Clay Soil in Figure 2. Ammonium-N resulted in very little leaching but large portions of the applied Nitrate-N and Urea-N moved with the groundwater to the level of water penetration. A little bit more Ammonium-N moved into the Sandy Loam soil compared to the Clay Soil but these amounts were insignificant for both soils. Larger portions of the applied Urea-N and Nitrate-N moved with the groundwater to the level of water penetration in the Sandy Loam soil compared to the Clay soil. Half of the LAN will show a similar response to Ammonium Sulphate and the other half similar to Calcium Nitrate since LAN consists of 50% Ammonium-N and 50% Nitrate-N.
According to Figures 1 and 2 the immediate leaching potential of LAN is about 50% less than that of Urea. Ammonium-N could however over time be converted to leachable nitrate-N through the process of nitrification. The effect of LAN which was applied shortly before planting and at planting, followed by heavy downpours, resulting in severe leaching are presented in Figure 3. Severe N deficiencies in leaves and in the soil up to a depth of 60 cm have been confirmed with this case study. Yield loss as a result of N leaching was estimated between 7 and 8 ton/ha. Although risks of N-leaching are much less with LAN compared to Urea it is recommended that neither LAN nor urea be applied before planting on well drained soils.

Figure 1. Leaching of nitrogen sources on a Sandy Loam Soil. (Redrawn from Broadbent et al. 1958, Gardner & Roth, 1984)

Figure 2. Leaching of nitrogen sources on a Clay Soil. (Redrawn from Broadbent et al. 1958, Gardner & Roth, 1984)

 

Figure 3. Leaching of LAN broadcast two weeks before planting at a rate of 63 kg N/ha and band placed at planting as part of a plant mixture at 40 kg N/ha. More than 100 mm rain was recorded shortly after planting on this well drained Sandy Soil. Typical N deficiency symptoms showing a reverse V-Shape on the oldest leaves were visible (Sasol Nitro case study 2010/2011)


Image A

Image B
Figure 4. Vertical and lateral movement of nitrogen (N) in image A and B was planted eight days later than A adjacent to A on the same field. Image A received 110 mm and Image B 14 mm rain from planting to topdressing. N was applied as Ammonium Nitrate at a rate of 29 kg/ha in the plant mixture and in the form of UAN at a rate of 63 kg N/ha as a topdressing. The clay content for the depth increment 0 to 60 cm was 11% for both A and B (Sasol Nitro case study 2010/2011).


The effect of vertical as well as lateral movement of applied N due to excess rain is visible in Figure 4. N analysis in a strip over the rows to a depth of 750 mm was 39 kg/ha for Image A where the maize was yellow and stunted but 179 kg/ha where the maize was much more prolific and greener. N analysis between the rows where N was not applied was 32 kg N/ha in the top 60 cm soil for both Image A and B. Variation in crop growth was therefore directly related to variation in soil N analysis over rows. This effect is often observed under high rainfall conditions on sandy soils, irrespective of time of N application. These symptoms are often wrongly ascribed to poor fertiliser quality or uneven applications.
What could we do to reduce the risk of N-leaching?
Before planting
The practise of pre-plant N applications should be limited when the risk of N leaching is high. Most N should then rather be applied after planting when the crop can utilise applied N effectively.
Considering pre-plant broadcast applications on well drained soils, ammonium sulphate would be most effective, not resulting in any significant leaching. (Adriaanse 2011). Such applications would be beneficial for the mineralization of plant residues, supplementation of S deficits and the neutralisation of alkaline conditions and therefore within limits justifiable.

At planting

Plant mixtures should contain a combination of Nitrate-N and Ammonium-N. The use of LAN / Ammonium Nitrate and Ammonium Sulphate in plant mixtures would ensure such combinations. Nitrate-N can also leach but young seedlings will take up Nitrate–N soon after emergence. Urea is not immediately available for uptake. Heavy downpours may result in leaching while low rainfall conditions may result in urea toxicity effects (Adriaanse 2012b). Urea is therefore not recommended for incorporation in plant mixtures which are intended for band placement. DAP contains only Ammonium-N and for this reason it will not leach immediately but like urea it may be toxic especially when used in plant mixtures at wide rows.
The risk of N-leaching from plant mixtures cannot significantly be compensated by increasing N applications above 40 kg/ha. The availability of both Nitrate-N and Ammonium-N is critically important at this stage. Downpours of more than 50mm should immediately be followed by applications of Ammonium Nitrate based products to address deficiencies resulting from leaching. These timely follow up applications should stimulate growth and result in better utilisation of N from the sub-soil that was previously leached from the root zone. The conversion processes of urea to forms that are readily available for uptake could be very slow under certain conditions. For this reason, and because urea leaches very easily, responses to LAN would be quicker and more effective compared to urea.

After planting
The application of most N after planting is strongly recommended when the risk for leaching is high. Extensive root systems can utilise applied N quickly before it leaches but can also reach deep into the soil profile for the portion of applied N that does leach.
LAN and other Ammonium Nitrate based products such as Ammonium Nitrate solutions will give quicker and more effective responses than urea (Adriaanse 2012a) or UAN which consists of 50% urea. Ammonium Nitrate based products are 100% available for uptake and 50% leachable while urea are 0% available for uptake and 100% leachable. The conversion of urea to Ammonium-N could be within a few days when it is warm but could take several weeks when it is cold (Hoeft et. al. 2000).
Multiple N applications would certainly also help to reduce leaching losses. Under high rainfall conditions two applications are recommended, the one 3 to 4 weeks after planting and the other 4 to 5 weeks after planting. Later tractor applications are usually impractical due to the height of the maize crop at this stage. Later aerial applications should be justified by assessments of inorganic N deficits in the soil. Under irrigation two thirds of the N requirement should be applied between 5 weeks after planting and 2 weeks before pollination. As many as 7 applications should be considered.

Dr Erik Adriaanse, Manager Product Development and Technical Support Sasol Chemicals, Fertiliser Division

References
ADRIAANSE, F. G., 2011 Maak ammoniumsulfaat deel van jou bemestingsprogram. S A Graan/Grain. 7/11
ADRIAANSE, F. G., 2012a. KAN of ureum : voor-plant, met-plant of na-plant S A Graan/Grain. 5/12
ADRIAANSE, F. G., 2012b. Die verskil in toksisiteit tussen KAN en ureum. S A Graan/Grain. 7/12
BROADBENT, F.E., HILL, G.N., & TYLER, 1958. Transformation and movement of urea in soil. Soil Sci. Soc. Am. Proc. 22:303-307.
GARDNER, B. R., & ROTH, R.L., 1984. Applying nitrogen in irrigation waters. In R.D. Hauck (ed.) Nitrogen in Crop Production. pp 493-506. Am. Soc. of Agron., Madison, WI. USA.
HOEFT. R.G., NAFZIGER. E.D., JOHNSON. R.R.& ALDRICH. S.R., 2000. Modern Corn and Soybean Production pp 136. MCSP Publications, 1520, Yorkshire Dr, Champaign, IL, USA.
For more information contact Sasol Nitro Contact Centre at 087 350 2222 or This email address is being protected from spambots. You need JavaScript enabled to view it.
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.

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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 close to developing seedlings could impair or terminate growth. Die 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 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 Meer cats.                  

                                                                                                         

Figure 1. The effect of relatively high concentrations of urea and ammonium nitrate at the same N concentration on germination and emergence of maize seedlings. (Yara, 2008)

Figure 2. Effect of increasing rates of Urea and LAN on the reduction of maize plant population. N Sources were band placed 50 to 100 mm below the seed. Row widths were 1.5 m. Clay content in the top 15 cm soil was 9.1%. (Adriaanse, 1991).

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 these specific conditions, indicating that the toxicity risk associated with the use of urea at higher concentrations is significantly higher than with LAN. The fact that these results did not show toxicity effects at lower concentrations for either urea or LAN 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. Expert advice should always be used when nitrogen is to be applied in close proximity of the seed.

Yield Loss

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 known urea toxicity had occurred. 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

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.

Figure 3. Urea toxicity symptoms on leaf tips of wheat, due to high concentrations in the soil (Yara, 2008)


Figure 4. Urea toxicity symptoms on maize leaf tips and edges due to high concentrations in the soil (Yara 2005)

Figure 5. Urea scorching effects after spreading over the leaf canopy (Yara, 2006)


Figure 6. LAN scorching effects after spreading over the leaf  canopy (Adriaanse 2010)

Conclusions and recommendations:

1. 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.
2. 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.
3. 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.

Dr Erik Adriaanse, Manager: Technology and Special Projects, Sasol Base Chemicals

References:

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.

For more information contact Sasol Base Chemicals Contact Centre at 087 350 2222 or This email address is being protected from spambots. You need JavaScript enabled to view it.

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.


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