Chemical characterization of leonardite and its potential use as a soil conditioner for plant growth enhancement

摘要

Humic substances which consist of natural organic matter (NOM) play a major role in plant growth through their impact on soil fertility (physical, chemical, and biological soil properties) and plant development. Concerns about soil and environmental degradation caused by intensive high-input agriculture stimulated substantial efforts to increase organic matter, particularly humic substances, in soils. Leonardite is NOM that contains high amounts of humic substances, especially humic acid. Leonardite from various deposits in Thailand was examined as a potential NOM for soil improvement. The chemical composition of leonardite varied among the numerous deposits. Leonardite from the Lee Mine contained the highest amount of humic acid (39.19 to 85.05%). Plant nutrients contained in all the leonardite samples were quite high (N, K, S, Ca, Mg, Fe, Zn and Mn) except for phosphorus. Characterization of leonardite samples using X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF) showed that silica (Si) was the major element in these materials. High contents of plant nutrients and humic acid in leonardite samples indicated its high potential use to improve organic matter, humic acid and some plant nutrient levels in the soils. However, its very low pH values (1.84 to 2.55) and low P content (28.6 to 211.2 mg kg-1) of most leonardite samples should be increased before use. The information obtained from our study is useful for determining the most appropriate use of leonardite in agriculture. Additionally, with its layered structure, high humic acid and nutrient contents, leonardite could be used as microbial carrier as well as peat. Further investigation should be performed to improve the benefit of using this natural organic material.