ALFALFA (Medicago sativa L.) FORAGE YIELD RESPONSES TO TRIPLE SUPER PHOSPHATE, PHOSPHATE SOLUBLIZING BACTERIA AND GIBBERLLIC ACID FOLIAR APPLICATION

Phosphorus (P) is an essential element as plant macronutrition. Approximately all the most important biochemical and assimilation processes relate to phosphorus availability. In the old alfalfa stands the phosphorus availability becomes lower and lower and, consequently, alfalfa forage yield decreases extensively. Thus, in order to study the effect of P on alfalfa recovering rate we compared the combined effect of P and plant growth regulator - gibberllic acid (GA) application in a 3-years-old alfalfa stand, cut three times in year. Sources of P were phosphorus solubilizing bacteria (PSB) biofertilizer and triple super phosphate (TSP) fertilizer. Effects on alfalfa were studied after each of the three cuts in a field experiment set up as a split-plot randomized complete block design in four replications in 2011 year in the region of Shazand, Arak, Iran. Phosphorus treatments at 3 levels were applied to the main plots and included P0: control, P1: PSB and P2: TSP. GA levels, applied to the sub-plots, were also in 3 levels; G0: control, G1: 35 ppm of GA and G2: 75 ppm of GA. The results showed that in the third cut, application of TSP had a significant effect on the increase of fresh and dry weight of alfalfa’s stem and leaf. GA applied with biological and chemical fertilizers effectively increased forage yield of alfalfa as well. Application of GA in the G2 concentration significantly increased stem yield up to 5.37 t/ha.