A soil spectral library of New Zealand

Abstract

Diffuse reflectance spectroscopy, both in the visible and near-infrared (vis-NIR: 350–2500 nm) and the mid-infrared (MIR: 2500–25,000 nm) ranges, has been increasingly employed as an alternative to obtaining a multitude of soil information and data non-destructively, rapidly, and cost-effectively. Soil spectral libraries (SSLs) have been developed across the world to calibrate prediction models for rapid and non-destructive assessment of soils at local, regional, national and global scales. Several continental or national libraries exist, for example, vis-NIR SSL in Europe, Australia, Czech Republic, Denmark, France, China and Brazil, and MIR SSL in the U.S., Switzerland and Brazil. In this study, we document the development of a vis-NIR and MIR SSL of New Zealand based on legacy and modern samples that have been sieved to 2 mm and air-dried, and explore the application of such libraries for predicting a wide range of soil properties, including soil carbon (total carbon or organic carbon), total nitrogen, pH, soil texture (sand, silt and clay), phosphate retention and available water capacity (field capacity and permanent wilting point). The vis-NIR and MIR spectral features were compared between New Zealand Soil Classification (NZSC) soil orders and the performances of both vis-NIR and MIR SSLs were compared with other national SSLs. Results of the study indicate that both vis-NIR and MIR SSL demonstrated reliable modelling performance using the PLSR model. However, MIR outperforms vis-NIR for all soil attributes. Furthermore, the study found that both vis-NIR and MIR have the potential to distinguish different soil orders within the NZSC.