In the natural environment, soils undergo wetting and drying (WD) cycles due to precipitation and evapotranspiration. The WD cycles have a profound impact on soil physical, chemical, and biological properties and drive the development of structure in soils. Degraded soils are often lacking structure, and the effect of organic amendments and WD cycles on structure formation of these soils is poorly understood. The aim of this study was to evaluate the role of biotic and abiotic factors on aggregate formation and stabilization of sodic soils after the addition of gypsum and organic amendments (feedlot manure, chicken manure, lucerne pallets, and anionic poly acrylamide). Amended soils were incubated at 25 ^∘C over four WD cycles, with assessment of soil microbial respiration, electrical conductivity, pH, sodium adsorption ratio (SAR), aggregate stability in water (ASWAT), aggregate size distribution, and mean weight diameter. Our results demonstrate that WD cycles can improve aggregate stability after the addition of amendments in sodic Vertisols, but this process depends on the type of organic amendment. Lucerne pellets resulted in highest soil microbial respiration, proportions of large macroaggregates (>2000 µm), and mean weight diameter. In contrast, dispersion was significantly reduced when soils were treated with chicken manure, whilst anionic polyacrylamide only had a transient effect on aggregate stability. When these organic amendments were applied together with gypsum, the stability of aggregates was further enhanced, and dispersion became negligible after the second WD cycle. The formation and stability of small macroaggregates (2000–250 µm) was less dependent on the type of organic amendments and more dependent on WD cycles as the proportion of small macroaggregates also increased in control soils after four WD cycles, highlighting the role of WD cycles as one of the key factors that improves aggregation and stability of sodic Vertisols.

The establishment and early growth of a clonal teak plantation on a leached and acidic alluvial soil on North Queensland, Australia was studied over 18 months. The soil was treated with three lime treatments (0, 1 and 2 Mg ha⁻¹) and three CaCl₂ treatments (0, 50 and 100 kg ha⁻¹). After 18 months, the average tree height ranged from 105 to 227 cm and survival ranged from 39% to 97%. Tree height and survival was not causally affected by the lime or CaCl₂ treatments. Detailed investigation of soil properties in each plot revealed teak growth and survival was positively affected by total soil carbon concentration, water-holding capacity (WHC) and Al saturation, and negatively affected by exchangeable Mg and DTPA-extractable Mn. Results indicate that teak is relatively tolerant of low pH (pH 4.4) and moderately high exchangeable Al concentrations (1 cmol(c) kg⁻¹) in soil. We conclude that organic matter (carbon) plays a critical role at our site by improving the soil physical characteristics such as soil porosity, aggregate stability and WHC. Also, sites with high total carbon had high Al saturation of the soil and high teak growth, suggesting that soil C acts by immobilising Al and Al impairs soil C degradation. Growth may be impaired by Mn toxicity and by Mg affects on soil structure. From a teak plantation management perspective, it is desirable to select soils with high organic matter, whereas pH and Ca status have little effect on early growth.