Phosphorus and Potassium Fertilizer Effects on Alfalfa and Soil in a Non‐Limited Soil

Fertilization strategies for high‐yielding alfalfa ( Medicago sativa L.) should take in account the increase in soil nutritional status that occurred during the last decades in areas with intensive agricultural use. A field study was conducted at the University of Padova, northeastern Italy, to determine the response of alfalfa yield and nutritive value to various combinations of P and K rates in a soil lacking nutrient deficiency. Alfalfa cultivar Delta was seeded in March 2005 on a silt loam soil having 38 mg kg –1 available P and 178 mg kg –1 exchangeable K. Nine treatments deriving from the combination of three P fertilization rates (0, 100, and 200 kg ha –1 P 2 O 5 ) and three K rates (0, 300, and 600 kg ha –1 K 2 O) were compared in a randomized complete block design. Plots were harvested at bud stage during three growing seasons (2005–2007) and dry matter (DM) yield, forage nutritive value, P and K contents, canopy height, and stem density were measured at each harvest. Soil samples were collected at the end of the research period for determination of available P and exchangeable K. The results demonstrated that P application had no impact on yield and did not interact with K in determining productivity, while K had a positive effect on yield. However, the 300 kg ha –1 K 2 O rate appeared sufficient to maximize yield, without adverse effects on the forage nutritive value. Data from soil analyses showed that alfalfa has a high K uptake even when it is fertilized at high rates.