Improving the Quantitative, Qualitative and Agronomic Phosphorus Efficiency of Soybean (Glycine max L.)

 One of the most important issues in improving the growth and increasing the yield of oil plants is proper nutrition and providing the nutrients needed by the plant during the growing season. Today, the use of biofertilizers in agriculture is considered as an effective way to reduce the consumption of chemical inputs to increase the quantitative and qualitative yield of plants, using beneficial soil microorganisms. Among these organisms, phosphate-solubilizing bacteria can be mentioned, which can increase the phosphorus uptake efficiency of plants. According to reports, humic acid is a stimulant of plant growth mainly by changing the root structure and growth dynamics, increases the root size, branching and its density.

A field experiment was conducted at the agricultural research station, Bu-Ali Sina University in 2020 growing season. The employed soybean cultivar was Habit. This factorial experiment was performed based on a randomized complete blocks design with three replications. Experiment factors were two levels of phosphorus fertilizer including application and non-application, biophosphate including inoculated and non-inoculated and foliar application of humic acid including 0, 2 and 4 g/l (during two stages of 15 and 30 days after emergence at the rate of 250 liters per hectare). In the present study, the interaction effect of humic acid and phosphate solubilizing biofertilizer was evaluated on phosphorus agronomic efficiency, water use efficiency, yield, yield components and quality characteristics of soybean. In this study, after checking the residual normality of the data, SAS software (Ver. 9.1) was used to analyze the variance of the data. Also to compare the means, Duncan's multiple range test at the level of 5% probability was used.

    The results showed that with phosphorus fertilizer application, simultaneous use of biophosphate and humic acid increased plant height (19.2%), biological yield (28.8%) and number of pods per plant (20.4%) compared to the lack of biophosphate and humic acid conditions. For the treatment level of non-use of phosphorus fertilizer and use of biophosphate, foliar application of 4 g/l of humic acid compared to 2 g/l increased the biological yield of soybean by 13.9%. 1000-grain weight of soybeans in phosphorus fertilizer application conditions did not show a significant difference between humic acid levels, but in non-phosphorus fertilizer conditions, foliar application of 4 g/l of humic acid caused a significant increase in 1000-grain weight compared to non-humic acid treatment. It seems that the application of humic acid has strengthened the relationship of the plant with phosphate solubilizing bacteria. Also the number of grains per pod of soybean increased by 14.6% with the use of biophosphate and 16.8% with the foliar application of 4 g/l of humic acid compared to the conditions of no-application of any of them. Applicatin of 4 g/l of humic acid and application of biophosphate increased grain yield by 27.3% and 26.4%, respectively. The highest percentage of seed oil (23.21) was obtained in the absence of phosphorus fertilizer and simultaneous application of 4 g/l of humic acid and biophosphate, while the highest percentage of grain protein (51.3) was obtained in the presence of phosphorus fertilizer, 4 g/l of humic acid and no biophosphate. In the absence of available phosphorus in the soil, the application of biophosphate takes precedence over foliar application of humic acid. This is evident as, without biophosphate application, there was no significant difference in the percentage of seed oil across various levels of humic acid. When biophosphate and humic acid were used in combination, compared to the sole use of biophosphate, the individual application of 4 g/l and 2 g/l of humic acid increased phosphorus agronomic efficiency by 21.1%, 35.7%, and 48.3%, respectively. The results indicate that the combined use of biophosphate and foliar application of 4 g/l of humic acid, in comparison to their absence, improved the water use efficiency of soybean by 25% and 26%, respectively.

Therefore, simultaneous application of 4 g/l of humic acid and biophosphate can improve growth, yield and increase soybean phosphorus agronomic efficiency and water use efficiency. However, to achieve a higher percentage of seed oil, the use of 4 g/l of humic acid alone is recommended.