Ameliorasi Kalium Humat sebagai Ligan Penukar Fraksi-fraksi P Terjerap dan Keterkaitannya dengan Sifat-sifat Kimia Andisol Pagar Alam, Sumatera Selatan

Abstract

Andisols merupakan salah satu ordo tanah produktif untuk budidaya tanaman pangan, hortikultura dan perkebunan di Indonesia. Namun, dominansi klei XRD-amorf seperti Alofan pada Andisols, yang memiliki gugus fungsional Al-OH (aluminol) dengan kapasitas jerapan spesifik yang sangat tinggi terhadap fosfat, menyebabkan kadar P-tersedianya menjadi sangat rendah meskipun totalnya tinggi. Jerapan spesifik P terjadi melalui proses pertukaran ligan. Fosfor tanah dapat dibedakan atas fraksi organik (Po) dan anorganik (Pi), yang ketersediaannya dapat dievaluasi melalui fraksionasi untuk memahami distribusi fraksi Po dan Pi serta mengendalikan transformasi P tanah, meningkatkan pemanfaatan P oleh tanaman, dan menyusun rekomendasi pemupukan yang optimal. Salah satu upaya prospektif untuk menurunkan kapasitas jerapan P pada Andisols adalah melalui ameliorasi kalium humat yang diharapkan dapat berfungsi sebagai ligan penukar fosfat pada tapak jerapan koloid agar P tanah didominasi oleh fraksi yang lebih tersedia bagi tanaman. Penelitian ini bertujuan mengevaluasi pengaruh ameliorasi kalium humat terhadap dinamika fraksi P dan keterkaitannya dengan sifat-sifat kimia tanah pada Andisol Pagar Alam, Sumatera Selatan. Percobaan lapang dilaksanakan pada Mei sampai November 2023 di lahan tanpa ditanami milik petani Desa Gunung Agung Pauh, Kecamatan Dempo Utara, Kota Pagar Alam, Sumatera Selatan. Analisis tanah dilakukan di Laboratorium Departemen Ilmu Tanah dan Sumberdaya Lahan, Fakultas Pertanian, IPB. Contoh tanah diambil secara komposit dari 5 subcontoh lapisan topsoil (0-30 cm) dan subsoil (30-60 cm) pada setiap petak perlakuan. Aplikasi perlakuan dilakukan menurut Rancangan Acak Kelompok yang terdiri atas 4 taraf ameliorasi kalium humat (kg.ha-1), yaitu P0 (kontrol) = 0, P1 = 20, P2 = 40, dan P3 = 60, dengan 3 ulangan. Analisis tanah dilakukan terhadap tekstur (3 fraksi, Pippete), sifat-sifat kimia yang meliputi pH NaF dan pH H2O (pH-meter), Aldd (N KCl, titrasi HCl), C-organik (Walkley & Black), N-total (Kjeldahl), P-total (HCl 25%, Spektrofotometer), Kdd, Nadd, Cadd, Mgdd, dan KTK (N NH4OAc pH 7,0, AAS), KB (nisbah Kdd+Nadd+Cadd +Mgdd terhadap KTK), dan Kejenuhan Al (nisbah Aldd terhadap KTK) pada tanah awal dan setelah aplikasi perlakuan, serta fraksionasi P yang hanya dilakukan setelah aplikasi perlakuan kalium humat (modifikasi Tiessen dan Moir, 1993). Hubungan dan keterkaitan antar dosis perlakuan kalium humat, sifat-sifat kimia, dan fraksi P tanah disajikan secara grafis dan deskriptif. Siginifikansi pengaruh dosis perlakuan sebagai peubah bebas (X) terhadap fraksi-fraksi P tanah sebagai peubah terikat (Yi) didasarkan atas hasil analisis regresi linier sederhana (Yi = ai+biX), sedangkan antar fraksi P (Yi) yang terpengaruh signifikan oleh perlakuan kalium humat dengan sifat-sifat kimia yang mempengaruhinya secara signifikan, yaitu pH H2O, Aldd, C-organik, P-total, KTK, dan KB tanah (Xi-n) didasarkan atas hasil analisis regresi linier berganda (Yi = a + b1X1 + b2X2 +…+ bnXn) menggunakan prosedur backward stepwise. Andisol di lokasi penelitian, baik pada topsoil maupun subsoil, memiliki kelas tekstur klei, reaksi tanah (pH H2O) agak masam dengan kejenuhan Al rendah, dan pH NaF >11 yang menunjukkan keberadaan Alofan. Kadar C-organik sangat rendah di kedua lapisan tanah dengan P-total dan KTK sangat tinggi, sedangkan kadar kation basa-basa dapat ditukar terkategori rendah sampai sedang yang menyebabkan nilai KB sangat rendah. Pada subsoil, aplikasi kalium humat berpengaruh sangat nyata (p<0,01) negatif terhadap fraksi PH2O, PNaHCO3-Po, dan PNaOH-Po serta positif terhadap fraksi PNaHCO3-Pi, PNaOH-Pi, dan PHCl. Hasil penelitian ini juga menunjukkan bahwa ketersediaan P tanah bagi tanaman merupakan resultante dari fraksi P anorganik dan P lambat tersedia (PHCl) di topsoil serta fraksi P sangat cepat tersedia (PH2O) dan P organik di subsoil. Dinamika N-total dan PResidual tanah tidak dibahas karena keduanya tidak terpengaruh secara signifikan oleh perlakuan kalium humat. Berdasarkan hasil analisis regresi linier berganda, sifat-sifat kimia tanah yang berpengaruh nyata terhadap dinamika fraksi P di topsoil adalah pH H2O, KB, Aldd, dan P-total, dengan p-value terendah (paling nyata) pada keterkaitan antara KB dan P-total terhadap fraksi P organik (Po = 96,051 – 0,811 KB + 0,028 P-total, R2 = 0,55; p = 0,027). Sifat kimia subsoil yang berpengaruh paling nyata terhadap dinamika fraksi P tanah adalah KB pada keterkaitannya dengan fraksi P organik (Po = 50,293 – 0,705 KB, R2 = 0,41; p = 0,025). Selain menyebabkan alterasi terhadap keseimbangan anion-kation di tapak reaktif koloid tanah, hal ini juga menunjukkan bahwa penurunan KB meningkatkan afinitas jerapan tanah terhadap fosfat, terutama fraksi Pi. Secara umum, pemanfaatan kalium humat hingga dosis 60 kg.ha-1 sebagai penukar ligan terbukti efektif dalam meningkatkan ketersediaan P Andisol yang diteliti dalam bentuk fraksi Pi di topsoil dan Po di subsoil.

Andisol is one of the productive soil orders for cultivating food, horticulture, and plantation crops in Indonesia. However, the dominance of XRD-amorphous clays such as Allophane in Andisols, which is characterized by aluminol (Al-OH) functional group that poses a vey high specific adsorption capacity to phosphates, causes the soils’ available P levels become tremendously low even though their total-P status are high. Specific soil P adsorption occurs through ligand exchange processes. Soil P can be divided into organic (Po) and inorganic (Pi) fractions, the availability of which can be evaluated through fractionation to understanding the soil Po and Pi fractions distribution and controlling the P transformations, increasing P utilization by plants, and developing an optimal fertilizer recommendation. One prospective effort that necessitates being assessed for its effectiveness in decreasing P adsorption capacity in Andisols is by applying potassium humate, which is expected to function as ligand exchangers to phosphates at the soil colloidal adsorption sites, hence soil P will be dominated by labile fractions that are more available to plant. This study aims to evaluate the effects of potassium humate amelioration on the dynamics of soil P fractions and their relationships with the chemical properties of Andisol of Pagar Alam, South Sumatra. A field experiment was done from May to November 2023 on the unplanted farmers’ land at Gunung Agung Pauh Village, North Dempo District, Pagar Alam City, South Sumatra. Soil analysis was conducted at the Soil Laboratory of the Department of Soil Science and Land Resources, Faculty of Agriculture, IPB University. Soil samples were taken compositely from five subsamples of topsoil (0-30 cm) and subsoil (30-60 cm) layers in each treatment plots. The application of potassium humate was according to a Randomized Block Design, consisted of 4 levels of potassium humate amelioration (kg.ha-1), P0 = 0 (control), P1 = 20, P2 = 40, and P3 = 60, each with 3 replications. Soil analyses were carried out on soil texture (3 fractions, Pipette), chemical properties including pH NaF and pH H2O (pH-meter), exchangeable Al (N KCl, HCl titration), organic-C (Walkley & Black), total-N (Kjeldahl), total-P (HCl 25%, Spectrophotometer), exchangeable-K, -Na, -Ca, -Mg, and cation exchange capacity, CEC (N NH4OAc pH 7.0, AAS), base saturation, BS (ratio of exchangeable –K + –Na + –Ca + –Mg to CEC), and Al saturation (ratio of exchangeable-Al to CEC) of the initial soil samples and those after the application of the potassium humate treatment, and P fractionation that only carried out on soil samples taken after the application of potassium humate treatment (modified Tiessen and Moir, 1993). The relationships and correlations among the potassium humate treatment doses, soil chemical properties, and soil P fractions were presented graphically and descriptively. The significance levels of the treatment dose effects as dependent variables (X) on the soil P fractions as independent variables (Y) were based on the results of simple linear regression analysis (Yi = ai + biX), whilst among the P fractions (Yi) that significantly affected by the treatment with the soil chemical properties that significantly affected them, pH H2O, Aldd, organic-C, total-P, CEC and BS (Xi-n), were based on the results of multiple linear regression analysis (Y = a + b1X1 + b2X2 + … + bnXn) using a backward stepwise procedure. Andisol at the study site has a clayey soil textural class, slightly acidic soil pH H2O with low Al saturation level, and soil pH NaF of >11 that showed the occurrence of Allophane in both topsoil and subsoil. The soil organic-C levels were very low with very high total-P and CEC and low to medium exchangeable bases that resulted in a very low soil BS . In subsoil, the application of potassium humate was very significantly (p<0.01) and negatively affected soil PH2O, PNaHCO3-Po, and PNaOH-Po fractions, but positively on soil PNaHCO3-Pi, PNaOH-Pi, and PHCl fractions. The results of this study also showed that the soil P availability was mainly attributed to the resultant of the inorganic P (Pi) and the slowly available P (PHCl) fractions in topsoil as well as the quickly available P (PH2O) and the organic P (Po) fractions in subsoil. Dynamics on soil total-N and PResidual were not discussed because they were not significantly affected by the potassium humate treatment. The soil chemical properties that performed significant effects on the P fraction dynamics in topsoil were soil pH H2O, exchangeable Al, total-P, and BS, with the lowest p-values (most significant) found in the relationships between soil BS and total-P with the organic P fractions (YPo = 96.051 – 0.811 BS + 0.028 total-P, R2 = 0.55; p = 0.027). The chemical properties of subsoil with the most significant effects (performed the lowest p-value) on the dynamics of soil P fractions was soil BS in its relationships with the organic P fractions (YPo = 50.293 – 0.705 BS, R2 = 0.41; p = 0.025). Apart from causing alterations on the anion-cation equilibrium in the soil colloidal reactive sites, this result also revealed that a decrease in soil BS increased soil adsorption affinity for phosphate, particularly the Pi fractions. In general, the use of potassium humate as ligand exchangers up to 60 kg.ha-1 was effective in increasing the availability of soil P in the studied Andisol, in the form of Pi in topsoil and Po fractions in subsoil.