Effect of Mixing Urea with Humic Acid and Acid Sulphate Soil on Ammonia Loss, Exchangeable Ammonium and Available Nitrate

Problem statement: Ammonia loss due to rapid hydrolysis of urea in so il following surface application can be substantial. Ammonia loss in agr iculture is estimated to be 1 to 60%. This laboratory study compared the effect of three diffe rent urea-humic acid-acid sulphate soil mixtures on NH3 loss and soil ammonium and nitrate contents, with loss from surface applied urea without additives (urea alone). Approach: Humic acid, acid sulfate soil and soil use in the i ncubation study was analyzed for selected soil physical-chemical pr ope ties. The fertilizers mixture and ammonia loss was done using standard methods. The treatments wer e evaluated in Randomized Complete block Design with 3 replications. The data obtained at th e end of the study on total ammonia loss, soil pH, exchangeable ammonium and available nitrate were an alyzed using analysis of variance and the means were compared using Duncan’s test using Statistical An lysis System (SAS) version 9.2. Results: The soil used to test treatments was a sandy clay loam Typic Paleudults (Nyalau series). The mixture significantly reduced NH3 loss by between 31.5 and 36.7% compare with urea a lone with larger reductions with higher rate of humic acid (0.75 and 1 g Kg of soil) and acid sulphate soil (0.75 g Kg −1 of soil). The impact of the treatment also showed s ignificant effect on the soil pH and ammonium content at the end of study. Conclusion: Amending urea with acid sulphate soil and humic ac id can reduce ammonia loss in acid soils by improving ammo nium retention. This may in effect improve urea-N use efficiency as well as reducing environme tal pollution in agriculture.


INTRODUCTION
Urea is noted for being the most widely used Nitrogen (N) fertilizer in agriculture, because of its high N and low price. Ammonia loss due to rapid hydrolysis of urea in soil following surface application can be substantial [1,2] . An approach to solve this problem involves control of the dissolution and hydrolysis of urea-N availability by reducing the microsite pH with acid materials such as acid phosphates and phosphoric acid [3][4][5] . The cost of amendments, however, prohibits their use and effective low-cost, readily available amendments would encourage their application. Urea phosphate, which is a mixture of urea and phosphoric acid, has been reported to reduce ammonia (NH 3 ) volatilization from urea and reduces seedling damage [6,7,8] . However, these mixtures are corrosive and require special precautions in handling and storage.
Considering the low pH (usually less than 3.5) and low cost of acid sulphate soil (ASS) and humic acid (HA) from peat soils, a paradigm approach could be the use of little amount of these soils to amend urea before soil application. This could be of a great benefit to Malaysia and elsewhere because an alternative such as this encourages the usage of acid sulphate soils and peat soils in the country where these soils have estimated to be 0.5 million ha [9] and 2.5 million ha [10] , respectively. At the moment, Malaysia imports HA based fertilizers from China and Australia at a high cost.
The new approach of reducing ammonia loss at the same time increasing N use efficiency in agriculture is worth investigating because the cultivation of crops such as oil palm, coconut, rice and so on and so forth has not be successful on acid sulphate and peat soils in Malaysia as desired [9] .
Thus in this study, the effects of mixing urea with HA and ASS on ammonia loss, exchangeable ammonium (NH 4 ) and available nitrate (NO 3 ) under laboratory condition were investigated.

MATERIALS AND METHODS
Urea, HA and ASS were the materials used to produce Urea-N fertilizer mixtures using the method described by Ahmed et al. [3] with some modification where the materials were weighed separately based on the treatments below before mixing them in a plastic vial by using reciprocal mechanical shaker (200 rpm).
The HA was isolated from tropical peat soil by the method described by [11,12] with 4 h extraction and fractionation periods. The oven dried yield of HA was express as percentage (%) of the weight of soil used. Functional group analysis was conducted by the method described by Inbar et al. [13] . Level of humification of HA was determined by E 4 /E 6 method using spectroscopy [14] . The model of the spectrometer used was Lambda 25 UV/VIS (Shelton, CT, USA).
The ASS was collected from Kuching, Sarawak (Telaga Air mangrove and Rempagi) at 0-15 cm depth. The soil was air dried, meshed and sieved to pass a 2 mm sieve before it was characterized for pH [15] , Cation Exchange Capacity (CEC) by leaching with 1 N ammonium acetate (adjusted to pH 7) followed with steam distillation technique [16,19] , total N by the Microkjedhal method [17] , exchangeable cations (K, Ca, Mg, Na) by atomic absorption spectrophotometry (A Analyst 800, Perkin Elmer Instruments, Norwalk, CT) [18] and inorganic N (NO 3 and NH 4 ) [19] . Both HA and ASS were meshed again to pass sieve less than 1 mm after which they were used to mix urea.
The soil used for the incubation study on daily ammonia loss was Nyalau series (Typic Paleudults) and its texture was sandy clay loam. This soil was collected from UPM Bintulu Sarawak campus at 0-15 cm depth, air dried, meshed and sieve to pass a 2 mm sieve. This soil was analyzed for field capacity, bulk density, soil pH, total N, available NO 3 and exchangeable NH 4 , Ca, Mg, Fe, Cu and K.
Analysis of variance was used to test for treatments effects and means were compared using Duncan's test [20] .

RESULTS
The selected chemical properties of the soil ( Table 1) were typical of Nyalau series and were consistent with those reported by Paramananthan [21] . pH and CEC of the ASS were similar to those reported by Shamsuddin [9] who also give the properties of tropical ASS. The carbon, phenolic, carboxylic and total acidity of HA were comparable with those reported by Schnitzer [22] and Tan [23] .
The daily loss of NH 3 is shown in Fig. 1. The treatments with urea additives (T3, T4 and T5) significantly reduced NH 3 loss compared to urea alone (T2).    Table 2 is shows the total amount of NH 3 loss for the 22 days of incubation. Even though the pH for T3, T4 and T5 were significantly higher than that for T2 at the end of the incubations the former treatments (T3, T4 and T5) significantly reduced NH 3 loss compared to the latter (T2).
The soil exchangeable NH 4 and available NO 3 accumulation at the end of the study is presented in Table 3. There was no significant effect of T2, T3, T4 and T5 on available nitrate. In terms of exchangeable ammonium, those of T3 and T4 were significantly lower than those of T2and T5.

DISCUSSION
The reduction of ammonia loss for all treatments with HA and ASS (T3, T4 and T5) compared treatments with urea alone (T2) was because of the temporary acidic condition at the urea micro-site during urea hydrolysis. This observation was consistent with the work of Ahmed et al. [3] and Siva et al. [5] who also found a reduction in NH 3 loss when urea was mixed with HA or TSP. This may have effectively increased the volume of soil with which urea was mixed with and also increased the time required for complete hydrolysis [4] .
The significant increase in soil pH for T2, T3, T4 and T5 treatments shows that the urea hydrolysis occurred but this process was significantly for T3, T4 and T5. This suggests that the more hydrogen ions were consumed under these treatments. The sharp increase in soil pH could be due to the soil texture (Table 1).
There was significant accumulation of exchangeable NH 4 of mixtures with HA and ASS (T3 and T4) compared with mixture of 1 g HA (T5) as high amount of HA (phenolic and carboxylic group) can help to retain more NH 4 nevertheless the available nitrate did not have any significant effect. The insignificant effect of available nitrate for all treatments is respected to the similar effect of ammonia loss where the N from urea has been volatilized. This observation was consistent with the research of [3][4] .

CONCLUSION
Amending urea with ASS and HA can reduce ammonia loss in acid soils by improving ammonium retention. This may in effect improve urea N use efficiency as well as reducing environmental pollution in agriculture.

ACKNOWLEDGMENT
The researchers acknowledge the financial support (Fundamental Research Grant Scheme) received from the Ministry of Higher Education, Malaysia via University Putra Malaysia.