Agronomic Insights

ENTEC in Horticulture - Nitrogen stabilisers in the soil

Horticulture crops need nitrogen (N) to consistently produce high yielding and exceptional quality crops.

The challenge is to select a nitrogen fertiliser product which will reduce risk, improve nutrient use efficiency, optimise production, and benefit quality.

Not all fertilisers perform equally. This is where ENTEC^®, an ammonium stabiliser and enhanced efficiency fertiliser, has a great fit. 

Fertiliser product choice starts with understanding the N cycle and potential losses (Figure 1). Nitrogen in fertiliser, crop residues, manure or compost is transformed by soil microorganisms either into an organic form (soil organic matter) or into a plant-available form (nitrate). 

Over time, any nitrogen immobilised into organic matter can become plant-available later. This is only a temporary loss. Two other losses – denitrification and leaching – are more costly.

 

 

 

Figure 1: The Nitrogen cycle. 

Nitrogen Sources

Soil N supply from organic matter
Mineralisation of soil organic matter is a significant contributor to crop N supply. Mineralisation involves soil microorganisms transforming the ‘non-plant available’ organically-bound N into ‘plant available’ mineral N.  Any nitrogen associated with the likes of crop residues, fallow crops or composts, also enters the pool of organically-bound N. 

Soil N supply from fertilisers
Soil testing is important in horticulture. Testing tells the difference between what ‘soil fertility’ can provide crops naturally, compared to the amount of nutrients crops require to flourish. The type and amount of nutrient differences between these ‘crop required’ & ‘soil fertility’ levels can be applied as fertilisers.   

Nitrogen Losses

Immobilisation 
Just as N is subject to biological transformation to be made ‘plant-available’ under certain soil conditions, a similar biological transformation can also tie-up nutrients. This is known as immobilisation, and it is considered to be a short to medium term N loss pathway. Over time, this N may become plant available again.

Nitrogen conversion chemistry - nitrification and leaching
During the process of nitrification, nitrogen is converted by Nitrosomonas soil bacteria from Ammonium (NH₄₊) into Nitrite (NO_2-) and then into Nitrate (NO_3-) by another group of soil bacteria, Nitrobacter: 

NITRIFICATION
Ammonium (NH4+) ------------------  Nitrite (NO2-) --------------- Nitrate (NO3-)
                             Nitrosomonas                          Nitrobacter

In terms of loss pathways, the issue with NO₃- is that it can be lost via both leaching and denitrification. Leaching takes nitrate through the soil & out of the root zone. Denitrification turns nitrate into gasses which are then lost from the soil to the atmosphere.

The reason NO₃- leaching occurs is due to its negative charge, i.e. soil colloids and soil organic matter possess a negative charge, hence two negatives cannot attract. So as water moves through lighter soils, NO₃- moves with it. There is a fine balance at play here – between water & NO₃-. We need to regulary irrigate, to keep the crops productive. Then rainfall events occur also. However, every time water moves through the soil, NO₃-  is also moving with it. If nitrate moves beyond the root zone, crops can not access it to grow. Leaching is common in soils with a cation exchange capacity (CEC) of <4 cmol(+)/kg.

Denitrification occurs due to the microbes in soil, as they start running out of oxygen in wet to saturated conditions (i.e. 60% or greater water filled pore space). This can be worsened by poorly structured or compacted soils, where high soil organic matter levels create high soil microbe populations, with over irrigation or when experiencing significant rainfall events. Under these conditions, the microbes use NO_3- as an oxygen supply. The conversion is as follows: 

           NO₃-      to   NO₂-    to      NO    to      N₂O   to N₂ 
         (Nitrate)         (Nitrite)     (Nitric Oxide)     (Nitrous oxide*)    (Di-nitrogen)
* A potent green house gas, 298 time worse than carbon dioxide (CO2).

Why ENTEC^®?

Entec is an ammonium stabiliser – it slows the conversion rate of ammonium to nitrate. It works by inhibiting the Nitrosomonas group of soil bacteria which keeps N in the top soil in the ammonium form (NH₄₊)  for longer. As ammonium it has a positive charge, and therefore, will be not readily leach through the profile and has a zero denitrification loss potential. Crops can still take up N when it is in the ammonium form.

ENTEC also reduces nitrous oxide emissions. In Victorian trials, ENTEC Nitrophoska was shown to reduce the average nitrous oxide N₂O emission flux by 69 to 100% compared to traditional fertilisers (Melville et al. 2012).

This means ENTEC keeps more nitrogen where you want it most – in the soil and available to the crop.

In a horticulture situation, seedlings or transplants do not use a lot of nitrogen during early establishment. Using ENTEC with the base fertiliser helps to reduce these nitrogen losses by keeping the N in the more stable and still plant-available ammonium form. As the crop becomes established, there is N available in the root zone for crop uptake.

ENTEC has been shown to be effective in the soil from 4 – 10 weeks. This has been demonstrated in laboratory results in a moist alkaline vertosol soil at 250C, 97% of applied fertiliser had converted to nitrate after 14 days without ENTEC protection. When ENTEC was used, over 70% of the nitrogen was stabilised as ammonium after 50 days (Suter et al. 2008).

This is very important for crops grown under irrigation. For example, for an irrigated broccoli crop where there is crop residue, wet conditions (due to overhead irrigation), compacted soil and warm temperatures, the ENTEC blend improved yield (Table 1). 

Figure 2: ENTEC stabilises nitrogen as ammonium for longer than conventional ‘untreated’ fertiliser.

Using NH4+ stabilisers ensures flexibility regarding timing of application. Fertiliser application can be done early in crop development, without compromising yield N loss potentials. Potentially fewer fertiliser applications can be required when using an NH4+ stabiliser, with more N being protected and ‘banked’ in the topsoil. Melville et al. (2012) demonstrated that one fertiliser application could be dropped in Victoria due to enhanced N use efficiency. 

Crop

Location and Year

Product/s

ENTEC Blend Yield

Conventional Blend Yield

Percentage Yield Increase Over Conventional

Statistics

Potato

Atherton Tablelands, 2011

Entec CK-800 v CK-800 (each applied at 1850 kg/ha) + Calcium Nitrate 250 kg/ha (3 wks post full emergence)

39.25 t/ha

34.95 t/ha

12.3%

LSD 3.79 P = 0.05

Potato

Atherton Tablelands, 2011

Entec Nitrophoska v Nitrophoska (each applied at 1250 kg/ha) + Calcium Nitrate 250 kg/ha (3 wks post full emergence)

36.55 t/ha

32.10 t/ha

13.9%

LSD 3.79 P = 0.05

Potato

Pinnaroo, 2016

Entec Nitrophoska v Nitrophoska (each applied at 835 kg/ha)

73.05 t/ha

59.65 t/ha

22.5%

LSD 10.55 P = 0.05

Broccoli

Boneo, 2012

Entec Nitrophoska v Nitrophoska (each applied at 400 kg/ha) + Calcium Nitrate with boron at 40 kg/ha (budding)

4.98 kg/plot

3.23 kg/plot

58.6%

LSD 1.29 P = 0.001

Broccoli

Boneo, 2012

Entec Nitrophoska v Nitrophoska (each applied at 600 kg/ha) + Calcium Nitrate with boron at 40 kg/ha (budding)

5.55 kg/plot

3.14 kg/plot

76.8%

LSD 1.29 P = 0.001

Table 1: ENTEC® - Horticulture Crop Responses (statistically significant), Australia.

Figure 3: Ammonium fertilisation with nitrification inhibitor.

There is more to ENTEC than just reducing leaching & denitrification:

Crop physiological benefits

Plants may spend less energy on NH4+ uptake than on NO3– uptake (Ullrich 1992; Wang et al. 1994), especially at a high rate of NH4+ supply (Glass et al. 1997).

Ammonium is directly incorporated into amino acids and proteins. The energy saved in comparison to conventional nitrate nutrition can be used for other metabolic processes. Trenkle, 2010).

Ammonium can be directly used for N adsorption (Pasda et al. 2001), whereas nitrate has to undergo reduction at a high energy cost to the plant prior to N uptake.

Ammonium will be taken up by the plants rather than Nitrates when NH4+  is supplied to a crop together with NH4+  stabilisers Pasda et al. (2001). 

Ammonium has been established in various studies that it has a positive effect on the synthesis of polyamines, cytokinins and gibberellins. Klein et al. (1979), as well as Gerendás and Sattelmacher (1995), found higher concentrations of the polyamines putrescine and spermine when plants were supplied with NH4+ instead of NO3–. It is known that polyamines function as secondary messengers for flower induction. Better tillering and growth of tillers in cereals, and a higher number of fruits in fruit-bearing crops as the result of NH4+ nutrition, are seen in connection with the increased cytokinin metabolism. Smiciklas and Below (1992) and Wang and Below (1996) detected higher cytokinin contents in maize and wheat, when the plants received NH4+ instead of NO3-. 

Significantly higher concentrations of gibberellin have been found in xylem sap with NH4+  and mixed NH4+/ NO3- nutrition, compared to NO3- nutrition only (Ali et al. 1998).

 Ammonium stabilisers can decrease the soil pH in the rhizosphere (Trolldenier 1981; Thomson et al. 1993). The localised lower pH will improve the availability of other nutrients, especially micronutrients, for plant uptake.

Application of NH4+ combined with P can significantly improve maize growth and nutrient use efficiency during early growth stages by stimulating root proliferation and rhizosphere acidification (Jing et al. 2010). 

On farm, this could be achieved by using Nitrophoska/ENTEC or blends containing MAP/ENTEC or DAP/eNpower. 

Partial NH4+ nutrition can decrease the NO3- concentration of the crop (Slangen and Kerkhoff 1984; Venter 1984; Müller and Wedler 1987; Burghardt and Ellering 1988; Hähndel et al. 1994), but not its N uptake (Buerkert et al. 1995). As a result, trials have shown that plants are darker in colour and more upright when using ammonium stabiliser treated fertilisers.

Advantages of Ammonium nutrition

Low energy consumption for nutrient uptake, increased N uptake even when low root zone temperature is experienced;

Low energy consumption for N assimilation in the plant, because Ammonium is already reduced & available for use by the crop;

 Increased synthesis of the phytohormones like cytokinin, and larger content of polyamines. 

For more information on making the most of your fertiliser programs, please contact us on conrad.leeks@incitecpivot.com.au / 0466 664 026 or rob.dwyer@incitecpivot.com.au / 0428 111 471.

 

References:

• Hähndel R, Lang H, Hermann P (1994) Wirkung von Ammoniumstabilisierten Düngern auf Ertrag und Qualität von Gemüse. Agribiol Res 47:101–108.
• Buerkert B, Horlacher D, Marschner H (1995) Time course of nitrogen in soil solution and nitrogen uptake in maize plants as affected by form of application and application time of fertilizer nitrogen. J Agron Crop Sci 174:325–336.
• Burghardt H, Ellering K (1988) Beeinflussung des Nitratgehaltes von Spinat durch unterschiedliche Kulturbedingungen. Gartenbauwissenschaft 53:201–205.
• Müller H, Wedler A (1987) Ein Beitrag zum Einsatz des Nitrifkationshemmstoff Dicyandiamid (DCD) zur Verminderung des Nitratgehaltes in Gemüse. Landwirtsch Forsch 40:78–87.
• Venter F (1984) Nitrate contents of head lettuce as influenced by nitrificides. Acta Hortic 163:231–236.
• Slangen JHG, Kerkhoff P (1984) Nitrification inhibitors in agriculture and horticulture: a literature review. Fert Res 5:1–76.
• Ali IA, Kafkafi U, Ymaguchi I, Sugimoto Y, Inanaga S (1998) Response of oilseed rape plant to low root temperature and nitrate: ammonium ratios. J Plant Nutr 21:1463–1481.
• Gerendás J, Sattelmacher B (1995) Einfluß des Ammoniumangebotes auf Wachstum, Mineralstoff- und Polyamingehalt.
  Junger Maispflanzen. Z Pflanzenernaehr Bodenkd 158:299–305.
• Klein, H, Priebe, A., & Jager, H. (1979) Putrescine and Spermidine in Peas: Effects on Nitrogen Source and Potassium Supply. Physiologia Plantarum, 45: 497-499.
• Melville et al. (2012) Carbon and Sustainability. A demonstration on vegetable properties across Australia.
• Pasda, G., Handel, R. and Zerulla, W (2001) Effects of fertilizers with the new nitrogen inhibitor DMPP (3,4-dimethlypyrazole phosphate) on yield and quality of agricultural and horticultural crops. Biological Fertility Soils, 34, 85-97
• Jing et al. (2010) Localized application of phosphorus and ammonium improves growth of maize seedlings by stimulating root proliferation and rhizosphere acidification. Field Crops Research. 119. 355-364.
• Ulrich, W. (1992) Transport of nitrate and ammonium through plant membranes. In: Mengel K. Pilbeam D. (eds) Nitrogen metabolism of plants. Oxford Science, Clarendon press, Oxford, pp 121-137.
• Smiciklas KD, Below FE (1992) Role of cytokinin in enhanced productivity of maize supplied with NH4+ and NO3-. Plant Soil. 142:307–313.
• ‘Ammonium uptake by rice roots’ Wang, M., Glass, A., Shaff, J. and Kochian, L. (1994) III. Ectrophysiology. Plant Physiology, 104, 899-906.
• Wang X, Below FE (1996) Cytokinins enhanced growth and tillering of wheat induced by mixed nitrogen source. Crop Sci. 36:121–126.
• Trolldenier G (1981) Influence of soil moisture, soil acidity and nitrogen source on take-all of wheat. Phytopathol Z 102: 163–177.