Phosphorus Retention on Soil Surface of Tilled and No-tilled Soils

Decreasing P buffer capacity of the soil surface in no-tilled soils has been attributed to enrichment of both P and OM (OM). As soil surface enrichment with P and OM usually occurs simultaneously in no-tilled soils, it is not clearly established whether the increase of soil OM affects the capacity of soils to retain P. A study was conducted to assess whether the variations in soil OM (total and particulate) at soil surface affects P retention capacity. Thirty five soils from the Rolling Pampa (17 tilled and 18 no-tilled) were selected. All soils had medium to low levels of available P (PBray1 < 20 mg kg-1). Soil samples were taken from the top 5 cm, and several soil characteristics determined: available and total P, P sorption index, OM, particulate OM (POM) ( >53 μm), particle size distribution, and pH. Tilled and no-tilled soils did not differ in soil texture, pH, total and available P content. On the contrary, no-tilled soils had more OM (+14%) and POM (+56%) than tilled soils. The capacity of soils to retain added P was not different in both groups of soils. Phosphorus sorption index was not related to soil content of total OM or POM. Variation in P sorption index was only related to soil clay content (r2 = 0.44). This study provides evidence contradicting the long held assumption that the reported decrease in P sorption at the surface of no-tilled soils was caused in part by OM enrichment.