Nutrient composition of diverse organic residues and their long-term effects on available nutrients in a tropical sandy soil.

Intensive use of sandy soils for agriculture leads to significant land degradation. The application of locally available organic residues can improve soil fertility, particularly in the context of organic farming practices. This research examined nutrient concentrations in locally available organic residues with different biochemical compositions/qualities and investigated the effects of long-term application of these residues on available nutrients, such as P, K, Ca, Mg, Fe, Mn, and Zn, as well as on total organic carbon (TOC) accumulation in tropical sandy soil. A field experiment was conducted in Northeast Thailand, where four local organic residues, groundnut stover (GN), tamarind leaf litter (TM), dipterocarp leaf litter (DP), and rice straw (RS), had been applied annually for 22 years. These organic residues were acidic (pH 3.7-5.8). The macronutrients N, P, and K were present at elevated levels in the high-quality organic residue GN, whereas medium-quality TM and low-quality RS and DP were dominated by the macronutrients Ca and Mg and the micronutrients Fe, Mn, and Zn. The incorporation of organic residues, particularly TM, resulted in the accumulation of TOC. Furthermore, long-term incorporation of TM increased soil pH, whereas incorporation of GN, DP, and RS did not. The higher increase in the soil pH of TM soil is likely because TM contains higher levels of ash alkalinity compared to other residues. The application of medium-quality TM increased the soil available P, Ca, and Mg, whereas low-quality organic residue RS and DP applications increased the concentrations of soil micronutrients (e.g., Mn and Zn). However, long-term applications of local organic residues did not increase available K in the sandy soil.