Irrigated alfalfa response to phosphorus and potassium in a calcareous soil

The response of alfalfa (Medicago sativa L.) to phosphorus (P) and potassium (K) fertilizer often varies with soil type, the initial soil test P and K levels, irrigation and harvest management, and yield level. The objectives of this study were to determine the response of irrigated alfalfa to P and K fertilizer on a Wetherill loam soil in southwestern Colorado and to assess the impact on soil test P and K levels. Four P (0, 37, 74, and 148 kg ha(-1)) by three K (0, 93, and 186 kg ha(-1)) rates were applied to newly seeded alfalfa in the fall of 1995. An additional 37 kg P ha(-1) was applied annually to half of each plot in the spring of 1997, 1998, and 1999. Potassium fertilizer increased alfalfa dry matter (DM) yield significantly in 1996 only. Alfalfa K uptake averaged 311 kg ha(-1) yr(-1) in 1997-1999. Soil test K level at the 0- to 30-cm depth was not impacted by K fertilization and remained above the critical level throughout the duration of the study. On the basis of these and other research results, a response to K fertilization is unlikely on most alfalfa-producing soils in Colorado because of their high K-buffering capacity. The initial application of 37-148 kg P ha(-1) did not affect alfalfa DM production significantly in 1996, 1997, 1998, and in total from 1996 through 1999. Alfalfa DM increased by a total of 3.2 Mg ha(-1) in 1997 through 1999, with 37 kg P ha(-1) yr(-1) when no P was applied initially. Both the initial and annual P fertilizer applications increased alfalfa P concentration significantly in 1997 through 1999. Alfalfa P uptake averaged 32.6 kg ha(-1) yr(-1). Similarly, the initial and/or annual P application increased soil ammonium bicarbonate-diethylenetriaminepentaacetic acid (AB-DTPA) extractable P significantly in 1998 through 2000. When no P was applied, soil test P level at the 0- to 15-cm depth dropped from 8.3 mg kg(-1) at the initiation of the study in August 1995 to less than 1.0 mg kg(-1) at study termination in April 2000. It took a total of approximately 150 kg P ha(-1) to raise soil test P to its initial level after 4 years of production. More than 260 kg P ha(-1) would have been required to raise soil test P level to where a response to P fertilization would not be expected. Alfalfa yield response to P fertilizer was small even when soil test P level dropped to well below the critical level. No yield advantage was observed to maintaining soil test P above the critical level. Although not conclusive, our results indicate that annual P fertilizer applications are superior to one-time applications on our calcareous soils. More research is needed to refine P fertilizer recommendations and soil test correlation relationships for irrigated alfalfa hay production in southwestern Colorado.