Effect of long-term phosphorus fertilization on soil phosphorus fractions

A limitation to crop production in the southeast of the Buenos Aires province (Argentina) is the low phosphorus (P) availability. P fertilization is required for high yields. The objective of this work was to quantify the forms of soil P as affected by different P fertilization strategies after 8 years under wheat. The combination of high rate (H; 176 kg P ha-1), low rate (L; 88 kg P ha-1), single application (S; at the beginning of the experiment), and fractionated application (F; annual fertilization of 22 and 11 kg P ha-1) were evaluated. Soil total P, total inorganic P, total organic P, organic (Po) and inorganic (Pi) extractable with NaHCO3 and NaOH, were determined, plus available P (P-Bray 1). Fertilization did not change the total P or the total Po (344 and 412 mg P kg-1, respectively). Fertilized treatment, compared with a non-fertilized one, increased the concentration of Pi-NaHCO3 (14.49 and 7.62 mg P kg-1) and Pi-NaOH (47.13 and 28.37 mg P kg-1). The H rate increased the Pi extracted with NaHCO3 (13.16 mg P kg-1) and with NaOH (53.82 mg P kg-1) compared with the L rate (9.82 and 40.43 mg P kg-1, respectively). The FH rate increased the concentration of Pi-NaOH (59.55 mg P kg-1) compared to SH (48.10 mg P kg-1), while the low rate produced no changes in this fraction. In summary, when the amount of P added was fractioned and exceeded the quantity removed by crop, the excess was converted mainly to Pi-NaOH. A positive and significant correlation (r2 = 0.95; P < 0.001) between the sum of Pi-NaHCO3, Pi- NaOH and P-Bray 1 was established.