| Sumario: | Under greenhouse conditions, it was evaluated by effect of four levels, and three sources of P2O5 on oat production and on the soil P fractions, as well as the interactions of these factions with the absorption of P by the plant. The soil used was a latosol from the Pacific coasts of Nariño, Colombia.
The levels of P2O5 linearly and significantly increased dry matter production, P absorption, non-apatitic calcium phosphates and aluminum phosphates. Their effect on the apatitic calcium phosphates was quadratic and on organic-P was cubic. Levels of P205 did not affect iron phosphates.
Basic slag and triple superphosphate were equally efficient in supplying P to the plant, largely surpassing phosphoric rock whose effectiveness was nil. The interaction effect of level due to source showed that the precipitation of the added-P into the different P fractions was independent from the source used.
The contribution of P forms to P absorption by the plant of operated in the following priority order: P-Al (r = 0.70**) > P-Fe (r = 0.59*) ≥ non apat: P-Ca (r = 0.50) > P-org (r = 0.43) > apat. P-Ca and non-apatitic P-Ca, in the three added sources, was in the following intensity sequence:
P-Al.
Basic slag (r = 0.75**) > Triple sup. phosp (r = 0.55) > Phosp rock (r = 0.30).
P-Fe.
Triple sup. phos (r = 0.70**) > Basic slag (r = 0.34 > Phosp. rock (r = 0.30)
Non apat P-Ca.
Basic slag (r = 0.72**) > Triple sup. phosphate (r = 0.67*) > Phosp. rock (r = 0.28).
Results indicated that the greater part of the P added to the soil as fertilizer was precipitated into the different phosphate forms but being afterwards released to be available to the plant.
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