Effect of alkali residue and humic acid on aggregate structure of saline-alkali soil

Humic acid (HA) and alkali residue (AR) exert an essential impact on soil aggregate structure, but how HA and AR promote the formation of macroaggregates jointly is not clear. The aim of this research was to explore influence of alkali residue and humic acid (AHA) on the structure and stability of soil aggregates, with saline-alkali soil in North China as an example. The morphology and structure of the soil with AHA added were detected by a scanning electron microscope, Fourier transform infrared spectroscopy, and X-ray diffraction. Soil aggregate structure was analyzed by mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (D), and the trend of soil colloid forming macroaggregates was evaluated by zeta potential and surface charge. The results showed that under dry and wet sieves, MWD and GMD both surged initially and then decreased with the rise of AHA content, while D had the opposite trend. With the addition of 3%, AHA, MWD, and GMD reached their peak which proves that the proportion of soil macroaggregates was the highest. In the meantime, absolute value of zeta potential was low, which illustrates that the soil colloid system was in an unstable state of condensation and microaggregates tend to transform into macroaggregates. A large number of ions such as Ca2+ and Mg2+ in AHA promoted variable charge of saline-alkali soil, and the binding frequency of Ca2+ with negatively charged humus and soil colloid particles was increased. AHA holds great promise for improving the aggregate structure of saline-alkali soil. The combined effect of alkali residue and humic acid (AHA) can reduce soil fractal dimension and increase permeability. The surface charge and variable negative charge of the clay particles increased after the addition of 3% AHA. Ca2+ tends to construct bridges between mineral particles and humus, eventually forming macroaggregates. The addition of AHA can increase the particle size of soil aggregates significantly. AHA reduces zeta potential of soil colloids, causing colloidal system to tend to coalesce into large particles.