Impact of Seaweed Extract and Phosphorus Application on Productivity of Dahlia Plants

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A pot experiment was conducted for two successive seasons on a sandy soil and Dahlia pinnata plants grown on it. The randomized block design was used with two factors, the first was seaweed extracts (SWE) 0.0, 0.5, 1 and 1.5 % and the second was phosphorus (P) element treatment through soil application technique at four levels 0.0, 100, 200 and 300 ppm. The objective of this study was to determine the effect of different treatments of seaweed extract on chemical behavior and uptake of phosphorus at different levels in the sandy soil and on plant growth, flowering and chemical composition of growing plants and how the net negative surface charge and phosphate adsorption processes in a sandy soil were affected by seaweed and inorganic fertilizer applications. Soil samples ware collected from a field experiment where seaweed (SWE) and triple superphosphate (TSP) were applied annually on an equivalent plant-available P basis. The results showed that adding seaweed extract (Oligo- x) at 1% and P at 200 ppm revealed significant increase in the studied parameters in both seasons. Significant increases were in the stem length, stem dry weight, number of leaves and their dry weight, leaf area as well as the flower diameter, duration and dry weight while a significant increase in number of days needed to reach flowering was recorded with all fertilizer treatments in both seasons. Tuberous root fresh weights and their number/Plant were obviously stimulated by the used treatments. Meanwhile the content of the leaves of N, P and K (%) as well as the total content of chlorophyll and carbohydrates were significantly stimulated in both seasons compared to the control. After 2 years, SWE-soils had 37% more negative surface charge in soil suspensions from pH 4 to 7 than TSP-fertilized soils. Phosphate adsorption was 23% lower in soils received SEW than TSP. The phosphate adsorption data (adsorption of added phosphate + native adsorbed phosphate) was modeled with the Langmuir 2-surface equation, which revealed that the binding strength for phosphate, including native adsorbed phosphate, was about 50% less in the SWE-fertilized soils than the TSP-fertilized soils. These results suggest that SWE applications increased the net negative surface charge also reduces the soil phosphate adsorption capacity. Phosphate adsorption isotherms showed that phosphate was weakly bound in SWE-fertilized soils, compared with TSP-fertilized soils. Data showed that seaweed not only decreased phosphate retention by the soil but also increased phosphate release into the soil solution. It could be recommended to grow dahlia bulbs in sandy soil by adding of seaweed extract (SWE) at rate (1%) with supplying phosphorus at a level of 200 ppm.