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 profile of 27 intensively cropped soils from an area receiving mainly inorganic fertilizers, the St Lawrence lowlands in the province of Quebec, Canada, and 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 content were collected and analyzed fur their Mehlich-III extractable P (M3P), water-soluble P (P-i), organic P (P-w) and total P (P-i) contents. The P sorption index (P-si) and the P saturation degree (P-ox/(Al-ox+Fe-ox)) were also 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 practices on the soil P status in the lowlands was mainly limited to the A horizon as few sites had elevated labile P contents or saturation degree in their subsoils, Compared to long-term manured, acidic soils from a high animal density watershed, neutral to slightly alkaline soils from the lowlands had 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 high animal density watershed.