Improved phosphorus recommendations using modified Morgan phosphorus and aluminum soil tests

The need for reliable recommendations for phosphorus (P) application on cropland has become increasingly important in recent years with the need to optimize economic inputs and minimize adverse water quality impacts. We used results from a series of recent laboratory and field experiments with Vermont and New York soils, combined with those of earlier work, to revise state recommendations for P application on field crops. Based on results of field and greenhouse studies, we established a critical value of 4 mg kg(-1) (optimum range of 4.0 to 7.0) using modified Morgan extractant [1.25 M ammonium acetate (NH4OAc), pH 4.8] as a measure of plant-available P. Aluminum (Al) in the same extractant ("reactive" Al) correlated well with the amount of added P needed to increase soil test P. Consequently, P rate recommended for soils below 4 mg kg(-1) is based on a combination of extractable P and Al. Within the "Optimum" range only a low rate of P is recommended. Modified Morgan P also correlated well with P desorbed into water or calcium chloride (CaCl2) solution, suggesting it is a good indicator of the potential for a soil to release P into runoff. These research results support use of modified Morgan-extractable P and Al for P fertilizer recommendations for crop production and water quality purposes in the New York-New England region of the United States.