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American Journal of Agricultural and Biological Sciences

Year: 2009  |  Volume: 4  |  Issue: 1  |  Page No.: 18 - 23

Reduction of Ammonia Loss by Mixing Urea with Liquid Humic and Fulvic Acids Isolated from Tropical Peat Soil

Susilawati Kasim, Osumanu Haruna Ahmed, Nik Muhamad Ab. Majid, Mohd Khanif Yusop and Mohamadu Boyie Jalloh


Problem statement: Fertilizer N use efficiency is reduced by ammonia volatilization. Under low soil CEC and high pH, N from soil solution is released to the atmosphere. Ammonia loss due to low worldwide N use efficiency (33%) has been implicated in global warming. Thus, the objectives of this laboratory study were to evaluate the effectiveness of liquid humic and fulvic acids, isolated from tropical peat soils in reducing N loss from urea fertilizer as well as to investigate the ability of these acids to retain NH4+ and NO3¯ or reduce soil pH.
Approach: Formulated liquid N fertilizers consisting of urea and different types of humic molecules (HA or FA or mixture of both), solid and liquid urea were surface applied to 250 g of soil. A closed dynamic air flow system was used to trap NH3 loss in boric acid after which samples were titrated with 0.01 M HCl to estimate NH3 loss. After 30 days of incubation, the soil was air dried and analysed for pH, exchangeable NH4+, available NO3¯ and exchangeable cations. The results were analysed using SAS and treatments means were compared using Duncan’s New Multiple Range Test (DNMRT).
The use of humic molecules reduced NH3 loss and increased exchangeable NH4+. The high CEC of Humic Acids (HA) made the LHA treatment the best in reducing N loss after surface application. The presence of HA and Fulvic Acids (FA) increased NH4+ recovery. Even though, the soil pH of all the treatments were high, significant reduction of N loss was observed for humic molecules treatments.
The use of liquid organic N fertilizer has the ability to reduce NH3 volatilization in acid soil. The use of both humic and fulvic acids could be effective in promoting NH4+ retention. Thus, it can be concluding that, humic substances, in general, have great ability in controlling NH3 loss and retaining NH4+ in acid soils. It could be a cheapest, practical and easiest way to control N loss.

Table 1). The soil contained 0.26% N, 2.99% total organic carbon and 5.16% organic matter. Low exchangeable cations (K, Ca and Mg) was recorded (Table 1). The CEC was 24.5 cmol kg-1. These information are consistent with those reported by Paramananthan[16].

Table 1: Selected chemical properties of nyalau series

Fig. 1: Soil pHw after 30 days of incubation. (Different letter indicate significant difference between means using Duncan’s New Multiple Range Test (DNMRT) at p = 0.05)

The pH (water and KCl) recorded for treated soils were significantly different between treatments (Fig. 1 and 2). Soil treated with T1 or T2 gave higher pHw value as compared to other treatments; the highest was noted for T2. T5 was the third highest. Statistically it was not different from T1. As compared to T0 and T7, the pHw of the treated soil with urea (T1 and T2) and organically based liquid N fertilizers (T3 to T6) was significantly higher. Interestingly, T3 gave almost similar pHw values with T6; and these two treatments only showed their differences from T2, T0 and T7. Even though T3 gave the highest pH value (9.04) during liquid organic fertilizer formulation (Table 2), its effect was temporary. After 30 days of incubation, the pH recorded for T3 was among the lowest, among organically based liquid N fertilizers treatments. The lowest pHw was recorded for T7 treatment followed by T0 and T4. In considering the effect of humic substances in reducing pHw, treated with urea, LFA was better among liquid organic N fertilizer treatments.

In KCl solution, the treatments gave different results. The highest pHKCl was recorded for T5, followed by T1, T2 and T4 (Fig. 2). These four treatments were not significantly different. However, they were significantly different from T6 and T3.

Table 2: Average pH values of formulated liquid fertilizers

Fig. 2: Soil pHKCl after 30 days of incubation. (Different letter indicate significant difference between means using Duncan’s New Multiple Range Test (DNMRT) at p = 0.05)

Fig. 3: Daily loss of ammonia for 30 days of incubation

As compared to T0 and T7, all the treatments with liquid organic based N fertilizers showed higher and significant results.

Daily NH3 loss from T1 and T2 was maximum at day 3 (7.89%) and day 5 (8.12%), respectively (Fig. 3). T4 and T5 recorded similar peaks of NH3 loss (6 days after treatments application). However, T6 and T3 showed a different trend.

T4 delayed N loss by 1-2 days as compared to T2 and T1, respectively. Similar result was noted for T5 treatment. T6 had no effect in delaying the N loss. In the case of T3 treatment, N loss occurred a day earlier compared to T2 and 2 days earlier compared to T1.

Table 3: Cummulative NH3 loss for 30 days of incubation
Different letter indicate significant difference between means using Duncan’s New Multiple Range Test (DNMRT) at p = 0.05; *nd = Not Determine

Table 4: Soil exchangeable NH4+ and available NO3¯ contents for 30 days of incubation
Different letter indicate significant difference between means using Duncan’s New Multiple Range Test (DNMRT) at p = 0.05

The minimum period for maintaining significant N loss (1% from added N, as urea) was different for all the treatments. Generally effects of organically based N fertilizers were different, vis a vis organic molecules type used in fertilizer formulation. In this study, presence of LHA (T3) required a minimum of 10 days to reach 1% N loss, whilst T6 required 12 days. Thirteen and 15 days were recorded for T1 and T2, respectively.

In general, the used of organic additives (e.g., LHA or LFA separately or their mixture) significantly reduced N loss (Table 3). Significant reduction of cumulative N loss, of 29.21% was recorded for T3 followed by T4 (Table 3). Other liquid organic N fertilizers treatments gave no significant effect as compared to T1.

Exchangeable NH4+ and NO3¯ content in the soil were significantly different among treatments. The highest NH4+ recovery was recorded for T7 treatment (Table 4). It was followed by T4 and T5. Lowest NH4+ recovery was recorded for T1 and T2 followed by T3. Other organic additives did not result in any significant difference as compared to T1 and T2. The effect of T0, T1, T2, T3, T4, T5 and T6 on soil available NO3¯ was not significantly different from T7 (Table 4). The NO3¯ was generally low in the soil after 30 days of incubation (Table 4).

Table 5: Soil exchangeable K, Ca, Mg and Na for 30 days of incubation
Different letter indicate significant difference between means using Duncan’s New Multiple Range Test (DNMRT) at p = 0.05

Increase of exchangeable K was recorded for all of the liquid organic N fertilizers (Table 5). As compared to T1, T4, T5 and T6 gave a range of 76 to 95% increase in exchangeable K. Other treatments [including (NH4)2SO4] gave no significant results, as compared to T0 or T1 and T2. Exchangeable Ca and Mg from T1, T2, T3, T4, T5 and T6 treatments were lower compared to T0 and T7. They did not show any statistical difference among them. The highest content of exchangeable Na was noted for T7 treatment. It was significantly different from those of T0, urea (T1 and T2) and all organically based liquid N fertilizers (T3 to T6). The Na contents of the other treatments were only significantly different from that of T0.


As expected, solid form of urea resulted in higher pHw value compared to other treatments. This was probably due to hydrolysis processes that occurred just after application of urea fertilizer. Hydrogen released mainly by hydrolysis of Al, non-exchangeable hydroxyl Al and Fe and dissolution of organic matter, could be used during urea hydrolysis. This process generates OH¯ which accumulate not only at the microsite but completely in the soil [17]. This may have contributed to the increase of pH. Even though some of the fertilizer mixtures [e.g; T5 (1.60), T4 (1.64)] (Table 2), used in this study were acidic, the pH increased sharply after 30 days of incubation. Higher quantity of H+ (due to HCl during fractionation stage) in these mixtures could have facilitated urea hydrolysis and promoted the production of OH-.

Ammonium sulphate was the effective fertilizer in reducing the soil pH because N fertilizer, in the form of NH4+ has the ability to react during nitrification in soils (equations 1 and 2) to produce acidic residues in the form of H+, NO3¯ and even SO4. According to Foth and Ellis[18], almost 12 kg of CaCO3 will be needed in recovering the acidity by (NH4)2SO4 in 1 kg soil:

(NH4)2SO4+4O2 → 4H++2NO3+SO4+2H2O

(NH4)2SO4 + 4O2 → 2HNO3 + H2SO4 + 2H2O

The CEC provided by HA, which ranged between 417-583 cmol kg-1 (based on our previous study) may have contributed to ammonia loss reduction. The negative sites due to ionization of carboxylic (COOH) and phenolic (OH) might have improved NH4+ retention hence reduction in N loss[19]. These negative charges could develop with the level of salt and pH, that occurred in soil[20,21]. More salt will produce more negative charge in soil. A similar situation will occur at high pH. Thus, the presence of KOH (residues by extraction procedure), as a source of salt, could enhance HA charges and indirectly reducing the N loss.

The high pH recorded for water and KCl could be another factor for the efficient use of humic molecules in controlling N loss, due to high pH recorded and could increase the CEC[21,22]. Humic molecules have the ability to retain cations at their surfaces. The retention of NH4+ might be a factor for the low available NO3¯ in soil. Reduction conversion of NH4+ to NO3¯ would then occur due to lack of NH4+ in soil solution. A similar observation has been reported by Ahmed et al.[23].

The information on exchangeable Ca and Mg supports NH3 loss reduction. The increase in these two cations suggests that there was some replacement and retention of NH4+ in the soil treated with liquid organic fertilizers[23]. Even though, no statistical difference was observed, the trend of NH3 loss provides early information on the relationship between NH4+ retention, reduction of N loss and exchangeable cations.

T7 caused the highest exchangeable Ca, Mg and Na in this study. This was expected for NH4+-N fertilized soils. Since the exchange sites of soils saturated with cations such as Ca, Mg, Na and K in the order of Ca > Mg > K > Na, more NH4+ in particular is expected to be in soil solution for T7[18].

Higher exchangeable K recorded in some of the treatments was probably due to higher content of K associated with them. Based on our analysis, the content of K in fulvic acid solution was high, almost 2000 ppm in 50 mL. This value was 20 times higher as compared to LHA [previous paper, almost 100 ppm in 50 mL (after 1st washing)]. In the case of T4, splitting LHA and LFA reduced the K content in LFA solution and this could be the reason why LFA gave lower exchangeable K as compared to T5 and T6 treatments.


Liquid organic N fertilizer has the ability to reduce NH3 volatilization in an acid soil. Even though, the amount used was small as compared to the quantity of soil used, the effect on NH3 loss cannot be ignored. HA was the most effective material that effectively controlled NH3 loss. The use of both humic and fulvic acids is effective in promoting NH4+ retention. Thus, it can be concluded that, humic substances, in general, have a great ability in controlling NH3 loss and retaining NH4+ in an acid soil. It could be one of the cheapest, easiest and practical ways in controlling N loss. However, the rate or amount that is most effective together with its possibility to be used as foliar fertilizer needs to be investigated in detail in future research.


The researchers acknowledge the financial support of this research by the Ministry of Higher Education Malaysia.

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