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SOIL CHEMISTRY

Effects of Tillage on Phosphorus Release Potential in a Spanish Vertisol

Dep. de Ciencias Agroforestales, EUITA, Univ. de Sevilla, Ctra. Utrera Km 1, 41013 Sevilla, Spain

^* Corresponding author (adelgado{at}us.es)

No-till (NT) practices usually increase the P content in soil surfaces. The main purpose of this work was to study the long-term effects of the soil tillage system on the P release potential in the surface layer of a Vertisol from southwest Spain and to correlate it with changes in P content and forms. The three tillage treatments investigated were: (i) conventional tillage (CT) with a moldboard plow and field cultivator; (ii) minimum tillage (MT) with a field cultivator; and (iii) NT, which involved direct sowing into crop residue from the previous year. After 21 yr, NT resulted in enhanced organic matter, Olsen P, organic P, and total P contents relative to CT and MT in the top 5 cm of soil. The ratio of labile inorganic P fractions to P related to sparingly soluble pedogenic Ca phosphates in the surface soil layer was much greater under NT (0.8) than under CT (0.57) or MT (0.55). This was ascribed to a decreased precipitation of sparingly soluble Ca phosphate because of the enrichment in organic matter under NT, and accounted in part for the higher portion of inorganic P related to labile fractions in NT (0.19 vs. 0.09 in CT and 0.10 in MT). The increased P release potential of surface soil as estimated by using P sinks indicated that an increased dissolved P concentration in runoff and a greater amount of potentially releasable P per unit mass of surface soil lost through erosive processes can be expected with NT than with MT or CT.