EVALUATION OF AVAILABLE P WITH DIFFERENT EXTRACTANTS ON GUATEMALAN SOILS

The high P fixation capacity of soils rich in x-ray amorphous minerals has been well documented. Some Andisols in central Guatemala which ha ve received additions of volcanic ash have extraordinarily high extrac table P levels with acidic extractant. These ash materials have low fi xing capacities but they weather rapidly. This study examined the rela tionship between different soil P tests on soils from Guatemala. The s tudy area has a strong volcanic influence. Some volcanos are still act ive, from which the ash being deposited is rich in apatite. Age and co mposition of the parent materials exert the strongest influence on soi l properties. Thirty-two soils from the southern part of Guatemala wer e divided according to their classification and soil properties. Chemi cal methods (Bray I and Mehlich I) were compared to two other soil tes t techniques (Pi strip and Resin-strip). Phosphate retention character istics were also measured. In general, very good correlations were fou nd between all methods on soils that do not have any recent ash influe nce. On soils with ash deposition containing apatite, Bray I and Mehli ch I readily dissolve this Ca-P compound and gave exaggeratedly high P values. The Pi strip and Resin-strip methods were more appropriate in estimating available P in all soils irrespective of their properties, suggesting that either of these two methods could be adopted for the purpose of P fertilizer recommendations. The thickness and weathering status of the recent ash deposits vary with the catenary sequence down the volcano and determine soil P fertility management. Phosphorus fix ation was lower on soils being rejuvenated by ash depositions, but wat er retention properties were reduced. On soils with a buried Andisol c overed by fresh material on the surface, specific soil management stra tegies should be adopted. The presence of apatite in volcanic ash infl uencing soil P availability has not been described in the literature.