Judul: Dynamics of Nitrogen Release by Four Types of Urea in Flooded Conditions
Abstrak: Nitrogen (N) merupakan salah satu unsur hara esensial bagi tanaman. Salah satu penyebab rendahnya ketersediaan N adalah pencucian. Pupuk urea merupakan sumber hara N yang paling banyak digunakan di Indonesia, termasuk pada lahan sawah. Urea merupakan pupuk N yang larut air sehingga penggunaan urea di sawah perlu dikaji dalam hubungannya dengan pelepasan pupuk. Di dalam penelitian ini digunakan urea dengan formulasi berbeda yaitu urea kujang dan pusri yang merupakan urea konvensional dan urea prill dan granul yang merupakan urea modifikasi yang mengandung formaldehida masing-masing 0,10 dan 0,50 %. Metode yang digunakan adalah inkubasi dan pencucian. Inkubasi dilakukan selama 7, 14, 28, 45, 60, dan 90 hari dan dilakukan pencucian pada setiap akhir masa inkubasi. Hasil pencucian ditetapkan kandungan amonium dan nitrat. Hasil penelitian menunjukkan bahwa pelepasan N dalam bentuk amonium dan nitrat antar perlakuan pupuk urea tidak berbeda nyata. Hasil total N tercuci menunjukkan nilai paling tinggi diperoleh oleh urea granul sebesar 78,0 mg tabung-1, kemudian disusul oleh urea prill 75,2 mg tabung-1, urea kujang sebesar 71,5 mg tabung-1, dan urea pusri sebesar 68,4 mg tabung-1. Berdasarkan hasil persamaan first order kinetic, diperoleh nilai pelepasan maksimum dan konstanta kecepatan. Nilai pelepasan maksimum tertinggi dimiliki oleh urea granul sebesar 94,6 mg tabung-1, kemudian urea prill sebesar 88,3 mg tabung-1, urea pusri sebesar 82,3 mg tabung-1, dan urea kujang sebesar 79,2 mg tabung-1. Sementara untuk nilai konstanta kecepatan, konstanta kecepatan tertinggi diperoleh pada urea kujang sebesar 0,0349 hari-1, kemudian dilanjutkan oleh urea prill dan pusri sebesar 0,0256 hari-1, dan urea granul sebesar 0,0253 hari-1. Hasil penelitian menunjukkan bahwa urea yang diperlakukan dengan formaldehida terutama dalam bentuk granul cenderung memiliki kemampuan untuk memperlambat proses pelepasan N yang ditunjukkan dengan nilai konstanta kecepatan yang paling rendah., Nitrogen (N) is one of the essential nutrients for plants. One of the causes of the low availability of N is leaching. Urea fertilizer is the most widely used source of N in Indonesia, including in paddy fields. Urea is a water-soluble N fertilizer, so the use of urea in paddy fields needs to be studied in relation to the release of fertilizer. In this study, urea with different formulations was used those were urea kujang and pusri as conventional urea, while urea prill and granule as modified urea containing 0,10 and 0,50 % formaldehyde, respectively. The method in this study was the incubation and washing method. Incubation was carried out for 7, 14, 28, 45, 60, and 90 days and washing was conducted at the end of each incubation period. The results of washing were determined for the content of ammonium and nitrate. The results showed that the release of N in the form of ammonium and nitrate among urea fertilizer types were not significantly different. The results of total leached N showed that the highest value was obtained by granul urea at 78,0 mg tube-1, then followed by prill urea at 75,2 mg tube-1, kujang urea at 71,5 mg tube-1, and pusri urea at 68,4 mg tube-1. Based on the results of the first-order kinetic equation, it was revealed the maximum release and constant rate. The highest value
Keyword: anaerobic, conventional, granul, Latosol

Judul: Comparison of Leached Nitrogen in Four Types of Urea
Abstrak: Penggunaan pupuk urea yang tidak tepat menyebabkan rendahnya efisiensi nitrogen oleh tanaman. Modifikasi kimia molekul urea dengan senyawa formaldehid dapat dilakukan untuk mengontrol pelepasan NH4+ dan NO3− dari urea. Uji inkubasi dan pencucian tanah dilakukan untuk mengetahui kelarutan N urea termodifikasi formaldehid (urea granular dan prill), dan urea konvensional (urea kujang dan urea pusri) sebagai perlakuan pembanding. Campuran Latosol dan pasir kuarsa dengan perbandingan (1:1) diinkubasi, kemudian dicuci pada hari ke 7, 14, 28, 45, 60 dan 90 dengan melewatkan air di kolom tanah. Hasil pengujian menunjukkan bahwa transformasi urea dalam tanah dimulai dari pelarutan menjadi NH4+, kemudian teroksidasi menjadi NO3-. Proses tersebut berlaku sama untuk semua jenis urea yang diuji. Urea yang dimodifikasi formaldehid melepaskan nitrogen dalam periode waktu yang lebih lama daripada urea konvensional. Kadar formaldehid mempengaruhi laju pelepasan N. Urea granular (0,5% formaldehid) menghasilkan N tercuci lebih rendah daripada urea prill (0,1% formaldehid) tetapi tidak berbeda nyata. Kadar N-NO3- tecuci kumulatif dari urea granular dan urea prill tidak berbeda dengan urea konvensional. Urea granul memiliki kandungan N-NH4+ tercuci kumulatif yang jauh lebih rendah daripada urea konvensional. Sedangkan kadar N-NH4+ tercuci kumulatif pada perlakuan urea prill tidak berbeda nyata dengan perlakuan urea konvensional (Kujang dan Pusri urea)., Improper use of urea fertilizer causes low nitrogen efficiency by plants. Chemicalmodification of urea molecules with formaldehyde compounds can be carried out to control the release of NH4+ and NO − from urea. Incubation and soil washing tests were carried out to determine the solubility of formaldehydemodified N urea (granular and prill urea), and conventional urea (kujang urea and pusri urea) as comparison treatments.The mixture of latosol and quartz sand in the ratio (1:1) was incubated, then washed on days 7, 14, 28, 45, 60 and 90 by passing water in the soil column. The test results show that the transformation of urea in the soil starts from dissolving into NH4+, then oxidizes to NO3-. The process applies equally to all types of urea tested. Formaldehyde-modified urea releases nitrogen over a longer period of time than conventional urea. The level of formaldehyde affects the rate of N release. Granular urea (0.5% formaldehyde) produceslower leached N than prill urea (0.1% formaldehyde) but not significantly different. Thecumulative leached N-NO3- levels of granular urea and prill urea were not different fromthat of conventional urea. Granulated urea has a significantly lower cumulative leached N-NH4+ content than conventional urea. Meanwhile, the cumulative levels of N-NH4+ leachated in the prill urea treatment were not significantly different from the conventional urea treatment (Kujang and Pusri urea).
Keyword: formaldehid, inkubasi, nitrogen, pencucian, urea

Judul: Penentuan Nilai Eigen dan Vektor Eigen dari Matriks Tridiagonal 2-Toeplitz dengan Pendekatan Polinomial Chebyshev
Abstrak: The eigenvalues and eigenvectors of a matrix can be determined by finding its characteristic polynomials. The characteristic polynomials of a tridiagonal 2-Toeplitz matrix is shown to be closely connected to polynomials which satisfy the Chebyshev recurrence relationship. If the order of the matrix is odd, then the eigenvalues are found explicitly in terms of the Chebyshev zeros and the eigenvectors are found in terms of the polynomials satisfying the recurrence relationship. For even ordered matrices, the situation is more complicated. The problem in these cases is that although the Chebyshev recurrence formula is still applied, its initial values are not generating Chebyshev polynomials
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