IMPROVED PHOSPHORUS RECOMMENDATIONS USING MODIFIED MORGAN PHOSPHORUS AND ALUMINUM SOIL TESTS

The need for reliable recommendations for phosphorus (P) application o n cropland has become increasingly important in recent years with the need to optimize economic inputs and minimize adverse water quality im pacts. We used results from a series of recent laboratory and field ex periments with Vermont and New York soils, combined with those of earl ier work, to revise state recommendations for P application on field c rops. 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 m odified Morgan extractant [1.25 M ammonium acetate (NH4OAc), pH 4.8] a s a measure of plant-available P. Aluminum (Al) in the same extractant (''reactive'' Al) correlated well with the amount of added P needed t o increase soil test P. Consequently, P rate recommended for soils bel ow 4 mg kg(-1) is based on a combination of extractable P and Al. With in the ''Optimum'' range only a low rate of P is recommended. Modified Morgan P also correlated well with P desorbed into water or calcium c hloride (CaCl2) solution, suggesting it is a good indicator of the pot ential for a soil to release P into runoff. These research results sup port use of modified Morgan-extractable P and Al for P fertilizer reco mmendations for crop production and water quality purposes in the New York-New England region of the United States.