Dr FG Adriaanse, for Kimleigh Chemicals SA (Pty) Ltd
Nitrogen-stabilizers include both urease and nitrification inhibitors. Urease-inhibitors reduces the volatilization of nitrogen (N) in the form of ammonium gas from the soil surface and nitrification inhibitors reduces the leaching of nitrate-N through the soil profile.
Urease inhibitors reduces the rate of conversion of urea to ammonia (gas) or ammonium (in solution). The mode of action is to inhibit urease-enzymes which are responsible for this process. On the soil surface ammonia losses will be reduced, but in the soil toxic effects of high concentrations of both ammonia and ammonium near plant roots will be prevented.
Nitrification inhibitors reduces the rate of conversion of ammonium-N to nitrate-N which is facilitated by Nitrosomonas bacteria in the soil. Since ammonium is positively charged, it binds to the negatively charged clay minerals and will therefore not leach. Nitrate is in contrast negatively charged and for this reason, very much leachable.
As early as 1980 several urease and nitrification inhibitors were commercially available. The urease inhibitor which was and probably still is mostly used worldwide is NBPT (N-(n-butyl) thiophosphoric triamide). This product was made available in liquid form for both granular and liquid fertilisers. In 2016 Kimleigh Chemicals SA (PTY) LTD developed a metal complex in the form of a Schiff base to substitute NBPT. The efficacy of this Schiff base was shown to be significantly better than NBPT. The Kimleigh products AZANON® U and THIOCOTE® S both contain the same active ingredient and can be used for granular and liquid fertilisers.
The nitrification inhibitors which have been most widely used in the past are Nitrapyrin in liquid form for liquid fertilizers and Dicyandiamide (DCD) in powder form for granular fertilizers. Kimleigh has replaced these active ingredients with a heterocyclic nitrogen complex contained in a product called InnoX. This product comes in a liquid formulation but can be used for both granular and liquid fertilizers. Kimleigh has also combined the active ingredients of its urease and nitrification inhibitor in one product called DetaiN.
The efficacy of urea treated with Kimleigh nitrogen stabilizers and NBPT were tested in a field trial near Deelkraal during the 2019/20 season. A statistical trial with five replications were arranged in a randomized block design.
Treatments were broadcast 5 weeks after planting on 21st December 2021 directly after 8 mm rain was received. Slightly wet conditions on the soil surfaced therefore favoured volatilization. The precipitation of only 6 mm on the 23rd January was enough to wet the topsoil but not enough to result in significant leaching into the soil.
Treatment effects on Maize Grain Yield are presented in Figure 1. When only urease inhibitors are compared, AZANON® U showed the highest yield followed by THIOCOTE® S and then NBPT. Apart from recognized differences in efficacy between Schiff bases and NBPT, NBPT also decomposed at a faster rate under acidic conditions. The pH(KCl) of this topsoil was in fact 4.5 which would have resulted in relative fast breakdown of NBPT (Engel, Towey & Gravens, 2015).
The nitrification inhibitor, InnoX showed higher yield than AZANON® U, which is an indication that leaching was a bigger problem than volatilization for this season. This statement is supported by the fact that this topsoil contained only 8% clay with no restrictive layer for water movement in any part of the soil profile. Furthermore, rainfall figures of 38, 48, 38 and 24 mm on 8, 11, 25 February and 4 March respectively were certainly enough to result in leaching.
DetaiN, showed higher yield than any other product indicating that conditions were conducive to both volatilization and leaching. The Yield difference between DetaiN treated urea and untreated urea was statistically significant at P = 0.15 indicating an 85% chance of getting the same results when this trial is repeated under similar conditions.
Figure 1. Grain yield response to urea treated with different products compared to untreated urea. Treatments were broadcast 5 weeks after planting at 75 kg N/ha. At planting 35kg N/ha, 23kg P/ha and 23kg K/ha were band placed for all treatments. Figures indicate averages of five replications. LSD (0.15) = 0.431 ton/ha
Kimleigh’s N-stabilizers resulted in yield increases above untreated urea of 127 kg/ha (3.1%), 186 (4.6%), 322 (8.0%) and 439kg/ha (10.8%) for THIOCOTE®S, AZANON®U, InnoX and DetaiN respectively. Gross income, for DetaiN, will therefore be R1 624/ha higher at a maize price of R3 700/ton for treated urea compared to untreated urea. The current cost of DetaiN plus coating costs, amounts to about R1/kg urea at a dosage of 6 litres DetaiN /ton urea. Since 163 kg urea is required for 75 kg N the cost of DetaiN treated urea was R163/ha for this trial. The increase in net income for DetaiN at current prices is therefore R1461/ha for this trial. Yield increases resulting from urea treatments will depend on conditions conducive to N volatilization and leaching. Extreme weather conditions can result in much higher yield differences between treated and untreated urea than reported for this trial. The treatment of urea can therefore also be considered a form of insurance against nitrogen losses associated with unexpected, extreme climatic conditions.
An average yield increase of 7.2% or 680 kg/ha resulting from the use of NBPT under conditions conducive to volatilization was reported in the USA (Bundy, 1992; Gordon, 2008; Schwab & Murdock, 2010). Based on reported results it can be expected that this new generation Schiff base will outperform NBPT under similar conditions. Furthermore, results clearly demonstrated the benefit of combining this urease and nitrification inhibitor into one product.
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