AFRIKAANS: Verskille in loging tussen Stikstofbronne
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 utilized 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 en 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 case study 2010/2011).
Figure 4A: Vertical and lateral movement of nitrogen (N).
Figure 4B: Compare the vertical and lateral movement of nitrogen (N) in A, B was planted eight days later than A adjacent to A on the same field. A received 110 mm and 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 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 A where the maize was yellow and stunted but 179 kg/ha where the maize was much more prolific and also greener. N analysis between the rows where N was not applied was 32 kg N/ha in the top 60 cm soil for both 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 incorrectly ascribed to poor fertilizer quality or uneven applications.
What could we do to reduce the risk of N-leaching?
Before planting
- The practice 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 utilize 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 neutralization 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 for 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 50 mm 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 utilization 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 also 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 utilize 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.
PLEASE NOTE: 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.
