Assessing of the influence of organic and inorganic amendments on the physical-chemical properties of a red soil (Ultisol) quality

Many highly weathered soils in subtropical environments have low organic matter contents that limit the aggregation, plant available water (PAW) and have acidity issues, increasing the risk of erosion and limiting plant growth. The objective of this study was to determine the influence of gypsum, lime, fused calcium-magnesium phosphate (fertilizer) (CaMgP), and biochar on soil physical and chemical properties in two types of red soil (Ultisol). In a laboratory study, two types of red soil aggregate stability, PAW, the saturated hydraulic conductivity (Ksat), pH, and Fe/Al oxides were measured after 21, 60, 180, and 365 days application. The soil amendments increased soil pH and the amount of amorphous Fe/Al oxides for two soils. The clayey red soil amorphous Fe/Al oxides (ammonium oxalate extracted oxides) were significantly (P < 0.05) positively correlated with the small macroaggregate fraction (0.25–2 mm) and hence increased the mean weight diameter (MWD). Consistent with the increased clayey red soil aggregation stability compared to control, the PAW was significantly increased after amendments where CaCO3 and CaMgP resulted in greater PAW than biochar followed by CaSO4. Ksat was not obviously changed after all amendments except for biochar. However, for granite red soil, the oxides and aggregates exhibited no change overtime under each amendment except for Al oxides compared to clayey red soil, resulting in a slight increase in PAW but a significant increase in Ksat. Application time for two red soils was also significant (P < 0.05), displaying substantial change 60 days after application. These findings suggest that amendments are helpful to red soil quality through their positive increase in aggregate stability, PAW and Ksat over time as influenced by the pH and soil amorphous oxides, probably beneficial for future crop yields.