Phosphorus status of intensively cropped soils of the St. Lawrence lowlands

The P saturation degree of agricultural, neutral to slightly alkaline soils in areas where P fertilization is mainly inorganic is relatively unknown. The objective of this study was to determine the P status of the whole prof ile of 27 intensively cropped soils from an area receiving mainly inorganic fertilizers, the St Lawrence lowlands in the province of Quebec, Canada, a nd to compare their P status to soils from a very different agrosystem with high animal density in the Appalachians, The A, B, and C horizons of nine soil series (three sites per soil series) showing a gradient in clay conten t were collected and analyzed fur their Mehlich-III extractable P (M3P), wa ter-soluble P (P-i), organic P (P-w) and total P (P-i) contents. The P sorp tion index (P-si) and the P saturation degree (P-ox/(Al-ox+Fe-ox)) were als o determined. Nine of the 27 studied fields exceeded 112 mg M3P kg(-1), the level considered as excessive for corn (Zea mays L,) and soybean [Glycine max (L.) Merrill], The results suggest that the impact of agricultural prac tices on the soil P status in the lowlands was mainly limited to the A hori zon as few sites had elevated labile P contents or saturation degree in the ir subsoils, Compared to long-term manured, acidic soils from a high animal density watershed, neutral to slightly alkaline soils from the lowlands ha d much lower P-i contents and the proportion of P-i as P-o was also two to three times tower. However, given their low to medium P sorption capacity, P saturation degree of the A horizons was comparable to soils front the hig h animal density watershed.