Differences in N-sources to be used after planting- SASOL


Nitrogen (N) is the element which is required in the largest quantities for crop production. N-demand increases with vegetative growth but will start decreasing after pollination. Certain N-sources can be toxic when placed in close proximity of plant roots.

At the same time N is also very mobile resulting in losses through volatilisation and leaching. Normal efficacy differences between N-sources should be considered together with risks associated with extreme weather conditions, certain soil conditions and cultivation practices.
Ammonium Nitrate Solution (ANS)
ANS (21% N) is the most effective (generally available) N-source in solution which could be applied after planting. This product consists of 50% ammonium-N and 50% nitrate-N which are close to the ideal ratio for most crops. These forms of N are immediately available for quick uptake. Ammonium-N will not leach while nitrate-N may leach if not taken up immediately. Ammonia volatilisation losses from ANS are very low and for this reason ANS can be broadcast without incorporating into the soil. Plant residues will not have any significant effect on ammonia volatilisation from ANS. ANS can be recommended as a knifed-in side dressing 15 to 20 cm from rows due to very low toxicity effects. ANS will only result in topsoil acidification. The quantity of pure lime required to neutralise one kg N from ANS is 1.78 kg.
Limestone Ammonium Nitrate (LAN)
LAN (28% N) is the most effective (generally available) N-source in granular form which could be applied after planting. As with ANS this product contains 50% ammonium-N and 50% nitrate-N which are immediately available for uptake. Ammonia volatilisation, leaching and toxicity effects are also very low. No difference in N-efficacy between ANS and LAN is therefore expected. LAN is also suitable for broadcast surface applications and as a side dressing. As with ANS plant residues will not have any significant effect on ammonia volatilisation from LAN. LAN contains 20% dolomitic lime and for this reason close to, 4.1% calcium (Ca) and 2.1% magnesium (Mg) which is beneficial when soils are depleted of these elements. LAN will only cause topsoil acidification and the lime contained in LAN will help to ameliorate this acidity in the specific zone of application. LAN is less acidifying than ANS due to the lime contained in it (1.12 kg lime/kg N).
Urea (46% N) can be applied in granular form or dissolved in water. Urea is normally cheaper per unit of N but much more difficult to manage than ANS or LAN. Urea-N is less effective than ANS or LAN due to low availability for uptake, high leachability, high volatilisation effects and severe toxicity.
Urea-N is first converted to ammonium-N and then to nitrate-N. These processes can take a few days or several weeks when cold and wet. Urea is therefore less suitable for quick N-responses under severe deficiencies.
Urea-N has no charge and will therefore leach easily. This characteristic can beneficially be applied to wash urea into the soil but excess water will leach urea through the soil profile without being utilised.
Ammonia volatilisation of surface applied urea is much higher than with ANS or LAN or ammonium sulphate especially with high temperatures, when the soil pH is high and with an abundance of plant residues. Surface applications of urea can only be justified when soil incorporated or washed in immediately after application.
Urea, applied as a side dressing 15 to 20 cm from rows can be very toxic at high concentrations, when followed by a dry spell and should for this reason only be considered with irrigation.
In contrast to ANO and LAN, urea will result in both top and subsoil acidification. Urea is also more acidifying (3.57 kg lime/kg N) than ANS or LAN. Subsoil acidity is more difficult and more expensive to rectify than topsoil acidity.
The use of urea under no-till or minimum-till practises can result in severe volatilisation losses due to plant residues on the soil surface but also to subsoil acidy which could ultimately lead to the termination of these practises.
Urease-inhibitors will reduce urea volatilisation and toxicity but at the same time will delay the availability for uptake and will enhance leaching. Depending on climatic conditions urease inhibitors can therefore increase yields but may also carry the risk of decreasing yields.
Ammonium Sulphate (AS)
AS (21% N, 24% S) is available in granular or crystal forms which can also be dissolved in water. N is only available in the ammonium form which means that although not in the correct ratio with nitrate-N it can be taken up immediately. The fact that ammonium-N does not leach significantly, not even in well-drained soils, is a great advantage. Volatilisation will also be increased with high temperatures and high pH. Volatilisation of AS will be higher than with LAN but remains considerably less than with urea. AS is suitable for broadcast applications on the soil surface, regardless of plant residues. AS can be applied together with LAN as a side dressing but then not more than 40 kg N/ha as AS. AS is the most acidifying N-source (7.14 kg lime/kg N) which is a disadvantage under acid conditions but beneficial under alkaline conditions. Under strongly alkaline conditions the acidifying effect of AS will increase the availability of micro-nutrients for uptake. The sulphate-S will displace sodium (Na) and an excess of other elements such as magnesium (Mg) and calcium (Ca) (which can impair plant growth) to the subsoil.
Urea Ammonium Nitrate (UAN)
UAN (32% N) is normally a 50/50 blend of urea and ANS in solution. Pros and cons of these products are therefore also divided in a 50/50 ratio. Efficacy of UAN will be in the middle between urea and LAN. UAN is suitable for broadcast applications on the soil surface through centre pivot irrigation but can be toxic when band placed at high concentrations.
Calcium Nitrate (15.5% N, 19% Ca), Potassium Nitrate (13% N, 38% K) and Magnesium Nitrate (10.5% N, 9.4% Mg)
These products can either be applied in granular form or dissolved in water. The optimum nitrate-N/ammonium-N ratio for most crops is close to 75/25 and therefore these products can help to optimise this ratio. Applications at later growth stages will be more effective due to better developed root systems which will quickly take up nitrate-N, minimizing leaching in the process. Volatilisation in the forms of atmospheric N (N2 and N2O) will only be significant when soils are waterlogged for prolonged periods of several days. Volatilisations as ammonia gas and soil acidification are not applicable to these products.

Dr Erik Adriaanse, Manager Product Development and Technical Support, Sasol.

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Please consult a qualified agronomist for more locality specific applications.




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