Mineralization of Urea-Formaldehyde Fertilizer and Its Availability to Oil Palm Seedling under the Tropical Environment

Easily dissolved fertilizers release nutrient in excess amount to be assimilated by plant roots. Some portions these fertilizers leach out from the root zone and adversely impact the environment. Controlled-release fertilizers are more favorable to reduce fertilizer loss, labor cost, and environmental impact. Urea-formaldehyde (UF) was synthesized by polymerization of urea and 40% formaldehyde solution using H3PO4 as a catalyst. Three mole ratios of urea:formaldehyde, namely, 1.0:1.0, 1.5:1.0 and 2.0:1.0 were synthesized. Mineralization of the UF was conducted using eight different mixtures, four different moisture, and four incubation periods. The experiment included soil alone, soil with compost, soil with UF (1.0:1.0), soil with UF (1.5:1.0), soil with UF (2.0:1.0), soil with UF (1.0:1.0) and compost, soil with UF (1.5:1.0) and compost, and soil with UF (2.0:1.0) and compost. Moisture of the mixtures was adjusted to 20%, 40% 60% and 80% water holding capacity (WHC) of the soil. The mixtures were incubated at room temperature for 1, 2, 4 and 8 weeks, the released NH4 and NO3 were extracted by 1 M KCl and analyzed via a distillation method. Rates of mineralization increased with mole ratio of urea and moisture content of the soil. N loss increased with the moisture content. The best performance for the compromised condition was a mole ratio less than 1.5:1.0 (urea:formaldehyde) at 60% WHC. Availability of UF and serpentine-phosphate for oil palm seedlings was conducted using 10 treatments. The experiment consisted of soil without amendment; soil with Multicote; soil with cow manure; soil with cow manure and urea; soil with cow manure and UF (1.0:1.0); soil with cow manure and UF (1.5:1.0); soil with cow manure, MgHPO4 and UF (1.0:1.0); soil with cow manure, MgHPO4 and UF (1.5:1.0); soil with cow manure, serpentine-phosphate and UF (1.0:1.0); and soil with cow manure, serpentine-phosphate and UF (1.5:1.0). All amended soils increased vegetative growth of oil palm seedlings compared with the non-amended soils. Urea and UF increased the N content in seedling leaves, while Multicote, cow manure, MgHPO4, and serpentine-phosphate increased the Mg content. The best performance was found in the combination of cow manure, serpentine-phosphate and the UF with mole ratio of 1.0:1.0.


Introduction
Urea is the most commonly applied for N-source fertilizers. However, urea is substantially lost through leaching, runoff and volatilization (Yan & Zheng-yin, 2007;Jantalia, 2012;Yamamoto et al., 2016). The efficiency of N application for cereal production worldwide was estimated to be only 33% (Raun & Johnson, 1999). The N losses from agricultural lands contribute to ground water contamination, eutrophication and emission of greenhouse effect gases (Ju et al., 2009;Jantalia et al., 2012;Geng et al., 2015). Loss of urea has been manipulated by various methods such as incorporated with nitrification inhibitors or urease inhibitors, coating with synthetic or natural polymers, and chemically modified urea molecules (Trenkel, 2010;Azeem et al., 2014;Rychter et al., 2016).
Controlled-release fertilizers (CRF) are beneficial to reduce environmental pollution, cost of fertilizer application and improve the efficiency of fertilizer application. Geng et al. (2015) found that application of controlled-release urea at a rate of 70% of urea produced equal yields of rice and rape seed compared with those of the urea. Single application of controlled-release urea as a basal fertilizer could achieve higher yield than split application of urea. Yen and Zheng-yin (2007) found that the N-use efficiency, N-agronomy efficiency and N-physiological efficiency increased by 11.4%, 8.32 kg kg -1 and 5.17 kg kg -1 respectively, for rice when a CRF was applied. Wada, Aragones and Ando (1991) reported that the application of controlled-release urea with only half of ammonium sulfate can produce better yield and increase N in rice to almost an equal level. Oil palm (Elaeis guineensis) is an economic crop of Southeast Asian countries. It has an average potential yield of 33.2 tons fresh fruit bunches (FFB) ton -1 during productive phase, and having the maximum of 40.4 tons FFB ton -1 at 10 years after transplanting (Enler, Hoffmann, Fathoni, & Schwarze, 2016). However, average actual yields stagnated at around 17 to 19 tons FFB ton -1 . Enler, Hoffmann, Fathoni and Schwarze (2016) found that fertilization during productive phase has strongly affected on yield performance. Donough et al. (2006) reported that oil palm yields in Malaysia increased rapidly after nutrient status in the soil was improved. Therefore, fertilization is a key factor to reduce yield gap of the oil palm. General recommendation of fertilizer rate is larger than 1.2 tons ha -1 to achieve high yield for oil palm having age more than 10 years after transplanting. A large amount of fertilizer loss can be expected in tropical climate, hence CRF should be introduced. Oil palm seedlings have to be cultivated in a nursery around 1 year, and fertilizers have to be applied twice a month. The CRF is also a good alternative for saving labor cost and reducing fertilizer loss. However, information on using of CRFs especially Urea-formaldehyde polymer (UF) for oil palm is still scarce.
UF has been used as a controlled release fertilizer for many decades. However, this polymer is not widely used in Southeast Asia. UF is synthesized by reacting urea (NH 2 CONH 2 ) with formaldehyde solution by using H 3 PO 4 or phosphate compounds as catalyst. Paraformaldehyde (short-chain polymerized formaldehyde) may be used instead of formaldehyde solution for reduction of water content in the reaction and product (Yamamoto et al., 2016). Typical UF products contain N between 37% and 40% (Alexander & Helm, 1990). Mineralization of UF depends on mole ratio of urea and formaldehyde, soil pH, soil moisture, temperature and microbial activities. The rate of nitrate production from UF is higher in acidic soil than in neutral soil. The released N may not sufficient to plants if the soil temperature is below 15 C (Basaraba, 1963;Jahns & Kaltwasser, 2000;Guo, Liu, Liang & Niu, 2006). UF supplies only N, therefore other nutrients have to be supplied separately. Similar to the UF, serpentine (Mg 3 Si 2 O 5 (OH) 4 ) has long been used for Mg source. However, Mg availability of this mineral is very low. The availability can be improved by an acidulation process. Reacting of Serpentine mineral with H 3 PO 4 produces Serpentine-phosphate, which can be used as a source of both Mg and P (Nakamura, Yamazoe, & Kishimoto, 1956;Hanly, Loganathan, & Currie, 2005).
The objectives of this study were to examine mineralization of the UF under different mole ratios, soil moisture, and supplement with other sources of nutrients, and respond of oil palm seedling to these combinations.

Pot Experiment
A soil sample was taken from the same place of the previous experiment (section 2.1). A completely randomized design (CRD) with 10 treatments and 5 replications was employed. A total of 8 kg of the air-dried soil was placed into a plastic pot with a diameter of 25 cm. Deionized water was added to increase the soil moisture to 65% of WHC. Fertilizers and cow manure were mixed with the soil separately to prepare 10 treatments: soil; soil with Multicote (19-10-13+1.5MgO, Haifa CRF fertilizer); soil with cow manure; soil with cow manure and urea; soil with cow manure and UF01; soil with cow manure and UF02; soil with cow manure, UF01 and MgHPO 4 ; soil with cow manure, UF02 and MgHPO 4 ; soil with cow manure, UF01 and Serpentine-phosphate; soil with cow manure, UF02 and Serpentine-phosphate (Table 2). Oil palm seedlings (90 days after germination) were transplanted into treatment pots (one seedling per pot). The experiment was carried out in a plastic-sheet covered house for 38 weeks. At the end of experiment; plant height was measured, number of fully expanded leaves were counted, and upper-ground of the seedlings were cut and their dry weight were measured. The leaf samples were collected, gently wiped with a clean cloth, and packed into marked paper bags. The samples were dried at 70 C for 72 hours, ground and passed through a 1 mm sieve and then kept in plastic container to analyze the macronutrient and micronutrient elements. Prior to the analysis, the samples were re-dried at 70 C for 2 hours and cooled down in a desiccator. Total-N was analyzed using the Kjeldahl method (Jones, 2001), A portion of the sample was digested by HNO 3 :HClO 4 (2:1) mixture. The P concentration was analyzed using the Vanadomolybdate method, the K concentration using flame emission spectrophotometry, and Ca, Mg concentrations using atomic absorption spectrophotometry (Jones, 2001). Statistical analysis was performed using Duncan's Multiple Range test. Soil (8 kg) + cow manure (1 kg) + UF01 (5.6 g) T6 Soil (8 kg) + cow manure (1 kg) + UF02 (5.6 g) T7 Soil (8 kg) + cow manure (1 kg) + UF01 (5.6 g) + MgHPO 4 (4.6 g) T8 Soil (8 kg) + cow manure (1 kg) + UF02 (5.6 g) + MgHPO 4 (4.6 g) T9 Soil (8 kg) + cow manure (1 kg) + UF01 (5.6 g) + Serpentine-P (4.6 g) T10 Soil (8 kg) + cow manure (1 kg) + UF02 (5.6 g) + Serpentine-P (4.6 g)

Soil Chemical Properties
The chemical properties of the soil sample are listed in Table 3. The soil was acid and non-saline. The soil also contained low organic matter and low concentrations of P, K, Ca, Mg, S and Cu. In addition, extractable Fe and Mn were high, and Zn was in a medium range (Jones, 2001(Jones, , 2003. The properties indicated that acidity and salinity of the soil do not inhibit microbial activities. However, multiple nutrients have to be amended to improve microbial activities and plant growth.

Minera
The soil sa contained The soil n and NO 3 i mg-N kg -1 afterward. 60%, 88% (Figure 1).     Vol. 12, No. 11;2020 recommended. A low fertile soil amended with cow manure, UF01 and serpentine-phosphate showed the best combination as media of oil palm seedling during nursery phase.