Improving chemical properties of a highly weathered soil using finely ground basalt rocks

• Basalt application decreased PZC and PZNC soil charge characteristics.
• Basalt significantly increased pH, Ca, Mg, Si, K and Na on a soil solid phase.
• Basalt is able to generate new negative charge on soil surfaces.
• Basalt significantly increased Ca, Mg, Si, K and Na of soil solution.
• Basalt was able to suppress toxic elements (Al and Mn) of an Oxisol.

Chemical property degradation of Oxisols (highly weathered soils) is revealed by very low cation exchange capacity and base cations but high Al saturation. The objective of this study was to increase cation exchange capacity and base cations and to alleviate Al toxicity of a highly weathered soil using finely ground basalt rocks. The topsoil and subsoil representing the natural and severely eroded conditions, respectively, were incubated with various rates (up to 80 t ha− 1) of finely ground basalt (< 50 μm) under ambient laboratory conditions for 24 months. The soils and solution were sampled and analyzed periodically. Changes in soil surface charges were assessed by measuring point of zero charge (PZC) to account for variable charge generation and the point of zero net charge (PZNC) to account for all charge generations (variable and permanent charges). The soil solution was sampled using soil moisture samplers to observe cations released from basalt during each incubation period. Results showed that incubation of an Oxisol with finely ground basalt decreased PZC from 3.9 to 3.5 for the topsoil and from 3.9 to 3.7 for the subsoil. Corresponding values for PZNC measurements decreased from 3.05 to 2.52 for the topsoil and from 3.60 to 2.55 for subsoil. The decrease in PZC and PZNC values showed that basalt application was able to increase soil surface negative charge, while in turn increasing soil cation exchange capacity. At a given similar equilibrium pH value, increasing basalt rates showed an increased net negative charge (NetC) from 0.0–6.3 to 3.2–8.7 cmolc kg− 1, depending on basalt rates. This indicates that each increment of basalt rates generated “new negative sites” on soil surfaces to retain cations. At natural soil pH representing field conditions, the NetC sharply increased from 1.5 to 10.1 cmolc kg− 1 after basalt application. The higher NetC values for the natural condition than the given equilibrium pH values were owing to the higher soil pH values in the former. Interestingly, the total negative charge (CECT) values were comparable to the negative charge occupied by base cations (CECB) at basalt incubation rates of ≥ 10 t ha− 1, indicating that base cations released from basalt successfully displaced acidic cations (e.g. toxic Al) on the soil exchange complexes. Cations released from basalt were revealed by the significant increases in Ca, Mg, K and Na both in the forms of exchangeable cations (measured from the solid phase) and soluble cations (measured from the soil solution), with concomitant reduction in Al and Mn contents. Hence, finely ground basalt is a promising natural material that can be used to restore negative charge and base cations and suppress Al and Mn contents of highly weathered soils, which in turn, have a great impact on preventing cation leaching, increasing soil nutrient availability and reducing elemental toxicities.