Effects of Limited Irrigation Regimes and Chemical and Biological Phosphorous Fertilizers on Quantitative and Qualitative Characteristics of Turnip (Brassica rapa L.)

To evaluate the effects of phosphate solubilizing bacteria and chemical phosphorous fertilizer on turnip (Brassica rapa) forage yield under limited irrigation regimes, a field experiment was conducted in Research Farm of College of Agriculture and Natural Resources, University of Tehran, in Karaj/Iran during 2009 growing season. The experimental treatments were arranged as split plots. Five levels of irrigation consisted of: IR0 (no irrigation), IR1 (Irrigation at sowing time), IR2 (Irrigation at sowing time + commencement of tube formation), IR3 (Irrigation at sowing time + commencement of tube formation + commencement of flowering) and IRN (normal irrigation) were assigned to the main plots. Four levels of fertilizing systems viz: F0 (no phosphorous fertilizer), 100% Fch (100% chemical phosphorous fertilizer according to soil test), FBI (seed inoculation by pseudomonas putida bacteria strains 41 and 168), 50%Fch+FBI (50% chemical phosphorous fertilizer + seed inoculation by pseudomonas putida strains 41 and 168) were assigned to the subplots. A randomized complete block design with four replications was employed to analyze the data. The results showed that turnip is very sensitive to water stress in germination and emergence stage of growth. Irrigation at sowing time was the most efficient irrigation in turnip forage production. However, irrigation at stem elongation stage did not affect forage yield. The results also revealed that application of chemical and biological phosphorous fertilizer could moderate negative effect of water stress on turnip forage yield. As the severety of drought stress increased, dry matter digestibility and metabolizable energy of turnip forage followed a decreasing trend, while acid detergent fiber, water soluble carbohydrates and crude protein content followed an increasing trend. Based on the results of the experiment, integrated fertilizer could be a good substitute for chemical fertilizer. Total forage yield of turnip reduced by 8% in integrated fertilizer application compared to chemical fertilizer treatment. However, in integrated fertilizer treatment 50% reduction in chemical fertilizer utilization was achieved. Also integrated fertilizer could moderate the adverse drough stress effects on turnip growth and forage yield.