Ameliorating subsoil acidity by surface application of calcium fulvates derived from common organic materials

Subsoil acidity is a serious constraint to crop production, and is difficult to correct by conventional liming practices. Thus, a different approach to ameliorating acid subsoils was evaluated. Subsoil material of an acid Ultisol (pH 4.4) was packed into 50-cm long columns, then leached with solutions of CaCl₂, CaCO₃ (suspension) or Ca fulvates prepared from chicken manure, cowpea green manure, or sewage sludge. The total water applied was 30.26 cm (or 800 ml) in 2 days. Thereafter, the columns were dismantled and cut into 5-cm segments for chemical analysis. The results indicated that only 2% of the added Ca from CaCO₃ moved past the 15-cm depth, compared to 68% from CaCl₂ and 35–75% from Ca fulvates. Correspondingly, CaCO₃ precipitated all KCl-extractable Al in the top 5 cm, but had no effect beyond the 10-cm depth. The CaCl₂ displaced a small but significant portion of extractable Al from the top 15 cm and redeposited some of that Al in lower depths. Similar to CaCO₃, Ca fulvates from chicken manure and green mamure only decreased extractable Al significantly in the top 10-cm layers, but had little effect beyond that depth. By contrast, the Ca fulvate from sewage sludge decreased Al down to the 45-cm depth. In terms of reducing Al saturation as a percentage of total extractable cations (effective cation exchange capacity), the Ca fulvates were as effective as CaCO₃ in the 0-to 5-cm layer, and more effective than CaCl₂ in any soil layer because of the increased exchangeable Ca and/or decreased Al. In general, surface application of common organic material-derived Ca fulvates can increase subsoil Ca and decrease the Al saturation percentage. However, Mg depletion and enrichment of unwanted metals (e.g., Na or heavy metals) may be a problem when leaching with these organic sources.