Influence of iron/aluminum oxides and aggregates on plant available water with different amendments in red soils

Abstract

The red soils (Ultisol) in a subtropical climate have poor soil aggregation that limits plant available water (PAW) and crop growth. Building the soil aggregate through amendments may alleviate this problem. The objectives of this study were to determine the effect of gypsum, lime, fertilizer, and biochar on soil oxides, aggregates, and PAW on a red soil. A laboratory incubation and field study were conducted on red soils treated with these four types of amendments for 21, 60, 180, and 365 days to demonstrate their effect on electrical conductivity, pH, soil iron/aluminum (Fe/Al) oxides, aggregates, and PAW. The amendments decreased the content of free Fe/Al oxides and induced them to transform into chelated and amorphous oxides with an increase of soil pH. A high content of amorphous Fe/Al oxides (especially Fe oxides) had a positive effect on proportion of soil macroaggregates and the mean weight diameter (MWD), while free oxides were not significantly related with MWD. The MWD was similar under calcium-carbonate (CaCO₃) and calcium-magnesium phosphate (CaMgP), and was greater than biochar and calcium-sulfate (CaSO₄) in the lab and in the field. Due to the change of oxides and aggregates, the PAW was significantly increased after amendments over the control, and it generally reached the highest value after 180 days. The highest PAW occurred with CaMgP. Therefore, considering the CaMgP effect on the reduction in acidity, increase in MWD, and as high an increase in PAW as in treatment with CaCO₃, the CaMgP fertilizer is suggested for use in management. This can help alleviate drought stress and increase productivity in Ultisols in south China.