Field Estimation of Phosphate Retention by Andepts

In an attempt to develop a field method for soil surveyors and field agronomists to rapidly evaluate whether or not a particular site would retain a large amount of applied phosphate, 59 samples of Andepts from Costa Rica and Guatemala were incubated with rates of P, then analyzed. Phosphate retention ranged from 25 to 99% and correlated quite well with ammonium oxalate extractable Al concentration ( R 2 = 0.56) in a linear model. In samples with <2.4% ammonium oxalate extractable Al, the amount of applied phosphate retained was related even better to this measurement, while at > 2.4% oxalate extractable Al, retention was not influenced appreciably by oxalate extractable Al concentration. Phosphate retention was also correlated with pH in NaF. By the methods used, >90% phosphate retention was predicted by an oxalate extractable Al concentration of >2.4% or a NaF pH of >10.7. A quick test was devised to assess soils with extremely high phosphate retention. The method consisted of adding a few drops of 0.1 mol NaF L −1 to approximately one gram of soil in a spot plate and, after 10 min, checking the pH with two drops of 0.04% thymolphythalein indicator. This technique was found to correctly identify 79% of the samples as having either ≷90% phosphate retention.