Effect of P rates in long-term conservation agriculture trials on the vertical distribution of soil acidity and nutrient availability.

Few studies are published on the long-term impact of phosphorus (P) rates as triple superphosphate (208 g P kg^-1) on the vertical distribution of soil acidity and the availability of macro and micronutrients within conservation systems. This study explores the long-term impacts of increasing P rates on the vertical distribution of soil acidity and Mehlich-3 nutrient availability in long-term trials managed under conservation tillage in North Carolina. In 2022, soil samples were collected from two contrasting long-term trials: one on a sandy, poorly drained Portsmouth soil at Tidewater Research Station (67 years) and another on a well-drained, clay-rich Lloyd soil at Piedmont Research Station (37 years), at depths of 0-5, 5-10, 10-20, and 20-30 cm. In general, the greater crop yield and nutrient removal over the years occurred with higher rates of P, resulting in a decrease in the soil available potassium (K). Historical use of P increased the content of other nutrients that are part of phosphate compounds (i.e., calcium [Ca] and magnesium [Mg]) or are contaminants in these fertilizers (i.e., zinc [Zn] and manganese [Mn]). The distinct trend of sulfur (S) in both locations, where soil available S content increased with P rates at Tidewater and decreased at Piedmont, reflects the complexity of S dynamics in soils with contrasting characteristics. These findings reveal differences between soils and the importance of integrated nutrient management in long-term experiments to avoid bias in the crop response over the years, especially in fields managed under conservation tillage.