Diffusive movement and reactions of phosphate and associated cations in an acid soil and in aluminum-rich soil-like systems
Phosphate availability and movement is very low in acid soil due to fixation reactions of aluminum with phosphate. This research proposed that P fixation reactions are controlled, among other factors, by hydrogen ion production/consumption. A series of experiments were designed to demonstrate the proposed research model through the study of one-dimensional diffusive movement and precipitation reactions of applied P fertilizer (and associated cations). The effects of exchangeable Al, gibbsite, lignosulphonate, and type of P fertilizer ($\rm KH\sb2PO\sb4,\ K\sb2HPO\sb4$ and $\rm K\sb3PO\sb4$) were investigated in a simulated soil consisting of acid-washed sand and various levels of Al-saturated cation exchange resin. The diffusive transport of P and associated cations were also studied in a limed and unlimed gibbsite-rich Oxisol. When $\rm KH\sb2PO\sb4$ was applied to the surface of columns with different levels of exchangeable Al, gibbsite, and lignosulphonate and incubated for various times, the soluble amount and movement of P decreased as levels of exchangeable Al increased. Gibbsite alone did not reduce movement of P, but the combination of gibbsite with exchangeable Al greatly reduced the movement of P. Lignosulphonate decreased the soluble amount and movement of P in columns with exchangeable Al, but the opposite effect occurred in columns with both exchangeable Al and gibbsite, especially after four weeks. Studies using $\rm KH\sb2PO\sb4$ in unlimed and limed gibbsite-rich Oxisol showed that P movement was very limited in both; liming had very little effect. Among the three P fertilizers tested, P movement was greatest with $\rm KH\sb2PO\sb4$ and least with $\rm K\sb3PO\sb4$ in columns with exchangeable Al without gibbsite. But there was little difference among the three P fertilizers in columns with both exchangeable Al and gibbsite. However, P concentration was the highest for $\rm K\sb3PO\sb4$ as time increased. Tripotassium phosphate and $\rm K\sb2HPO\sb4$ were also very effective in increasing pH. The pH values either increased or decreased depending on the initial pH and the presence or absence of H$\sp+$ ion producing/consuming materials in all experiments. Data obtained in these studies are consistent with the proposed research model showing that H$\sp+$ ion production/consumption is crucial to P fixation and movement.