Deformasi dan Perubahan Lengas Tanah pada Sistem Lorong Pengatus Dangkal di Tanah Sawah Jenuh

SITI SUHARYATUN, Prof.Dr.Ir. Bambang Purwantana, M.Agr.; Dr.Ir. Abdul Rozaq, DAA.; Prof.Dr.Ir. Muhjidin Mawardi, M.Eng.

2015 | Disertasi | S3 Ilmu Teknik Pertanian

Lorong pengatus merupakan salah satu alternatif guna mengatasi lamanya masa tunggu tanam kedelai di lahan sawah pada akhir musim penghujan akibat kadar lengas tanah yang terlalu tinggi. Lorong pengatus dibuat untuk mempercepat laju penurunan kadar lengas tanah sehingga kondisi yang sesuai untuk pertumbuhan awal tanaman kedelai lebih cepat tercapai. Penelitian ini bertujuan mendeskripsikan karakteristik deformasi tanah dalam kaitannya dengan gerakan lengas pada tanah sawah setelah pembentukan lorong pengatus, serta mengetahui pengaruh lorong pengatus terhadap peluang percepatan jadwal tanam kedelai dan potensinya dalam menyimpan dan menyediakan lengas tanah. Penelitian dilakukan dalam skala laboratorium dengan menggunakan soil bin, model bajak lorong, dan tanah di dalam kotak yang dijaga homogenitasnya. Penelitian dilaksanakan di Laboratorium Energi dan Mesin Pertanian, Jurusan Teknik Pertanian, Fakultas Teknologi Pertanian, Universitas Gadjah Mada. Lorong pengatus dibuat pada 3 jenis tanah sawah dengan kadar lempung yang berbeda, yaitu 13,12% (tanah geluh, TA), 41,17% (tanah lempung, TB) dan 53,36% (tanah lempung, TC). Penelitian dilakukan dengan menganalisis geometri lorong, deformasi tanah yang terjadi akibat pembentukan lorong pengatus, dan mengukur kadar lengas secara periodik menggunakan gypsum block. Pembuatan lorong pada ketiga jenis tanah menghasilkan lorong yang stabil, kategori baik dengan sedikit patahan (kategori B). Berdasarkan retakan tanah dan celah alur yang terbentuk, deformasi tanah terbesar terjadi pada tanah TC dan terkecil pada tanah TA. Secara fungsional, lorong pada tanah TA berbeda dengan tanah TB dan TC. Lorong pengatus pada tanah TA tidak berpengaruh terhadap laju penurunan lengas tanah, sedangkan pada tanah TB dan TC berpengaruh. Pembentukan lorong pengatus pada tanah TB dan TC dapat mempercepat waktu yang dibutuhkan untuk mencapai kapasitas lapangan. Hasil prediksi menunjukkan bahwa tanah TB membutuhkan waktu untuk mencapai kapasitas lapangan 2 hari lebih cepat dibanding kontrol. Tanah TC membutuhkan waktu 6 hari lebih cepat dibanding kontrol untuk mencapai kapasitas lapangan. Menurunnya waktu yang dibutuhkan untuk mencapai kapasitas lapangan menunjukkan bahwa pembentukan lorong pengatus di tanah TB dan TC berpotensi untuk mempercepat jadwal tanam kedelai.

Mole drainage is one way to shorten the waiting period for planting soybean in paddy fields with high soil moisture content at the end of the rainy season. With mole drainage system, soil moisture content can be lowered increasingly to such a condition that is suitable for early growth of soybean. This study aimed (1) to describe the characteristics of soil deformation in relation to the soil moisture movement in paddy soil after the mole drain formation, and (2) to explore the mole drains effect on the chances to plant soybean early and potential in storing and providing soil moisture. The study was conducted in a laboratory scale using a soil bin, a model of mole plow, and soil which was kept homogeneous in the boxes. The experiment was conducted at the Laboratory for Energy and Agricultural Machinery, Agricultural Engineering Department, Faculty of Agricultural Technology, Universitas Gadjah Mada, Yogyakarta. Mole drain was installed in paddy soils with three different clay content, namely; 13.12% (soil TA; loam soil), 41.17% (soil TB; clay soil) and 53.36% (soil TC; clay soil). The study was conducted by analyzing the geometry of the mole drains, observing the soil deformation wich occured due to the formation of the mole drains, and measuring periodically the moisture content using gypsum blocks.The study showed that perfect mole drains were installed in all of those three types of soil. The mole drains were quite good and stable with little cracks. Based on the soil cracks and fissuring of soil, the largest deformation has occured in soil TC with the highest clay content and the smallest one in soil TA with the lowest clay content. Related to its effect on the rate of lowering soil moisture content, each of the mole drains installed in those three types of soil had different effect. The formation of the mole drains in soil TA had no effect for the rate of lowering soil moisture content but the formation of the mole drains in soil TB and TC influenced the rate of lowering soil moisture content. The mole drains formed in soil TB have caused the soil rapidly reach field capacity. The result predicted that time needed by soil TB to reach the field capacity was 2-day faster than the control and that of soil TC was 6-day faster than control to reach the field capacity. The lesser time required to reach field capacity indicated that mole drains installed in soil TB and TC was potential to plant soybean early in paddy soil. The mole drains potential to store and provide soil moisture has not been described in this study.

Kata Kunci : lorong pengatus, deformasi tanah, lengas tanah, tanah sawah, kedelai / mole drains, soil deformation, soil moisture, paddy soil, soybean