Identifikasi Mineral Fraksi Pasir pada Tanah yang Terdampak Material Erupsi Gunung Semeru Berdasarkan Topografi dan Hubungannya terhadap Potensi Kesuburan

Abstract

The eruption of Mount Semeru on 4 December 2022 produced volcanic materials, including ash, sand, and lapilli, which became the parent material for soil formation in Candipuro District, Lumajang Regency. The soil sand fraction plays an important role as a reservoir of primary minerals, which serve as a long-term nutrient reserve for plants, so that its composition can be used as an indicator of soil fertility potential. This study aimed to determine the composition of sand fraction minerals, the relationship between individual mineral composition and soil chemical properties, and soil fertility potential at three topographic positions: the upper, middle, and lower slopes. Sampling points were determined using purposive sampling with three replications at each slope position. Identification of sand fraction minerals was carried out using a polarizing microscope. At the same time, soil chemical properties analyzed included pH, cation exchange capacity (CEC), exchangeable base cations (Ca, Mg, K, Na), and available Fe. The results showed that soils at all topographic positions were dominated by Easily Weathered Minerals (EWM) at a very high proportion (89.50–93.33%), comprising rock fragments, volcanic glass, plagioclase, hornblende, augite, hypersthene, biotite, and olivine, while Resistant Minerals (RM) only ranged from 6.67–11.00%. Correlation analysis showed that most individual minerals — including albite with available Na, hornblende with available Mg, Ca, and Fe, and orthoclase with available K — had very low and non-significant correlations with soil chemical properties (r = 0.104–0.149), except for hornblende, which showed a moderate correlation with available Fe (r = 0.453). Fertility potential assessment combining mineral composition and soil chemical property data showed that the middle slope had the highest fertility potential with a score of 36 (High category), supported by a base saturation of 74.29%, exchangeable Ca of 11.43 cmol(+)/kg, and available Fe of 13.38 ppm, while the upper and lower slopes each obtained a score of 32 (Medium category).