**6. Effects of humic+fulvic acid on symbiotic nitrogen fixation**

Organic matter is one of the most important issues of agriculture and it contains three very important components: humic acids, fulvic acids and humin. Plants and microorganisms in soil benefit from applications of humic acid in several ways. Humic acid stimulate root growth, increase carbohydrate production, have a hormone-like affect within the plant, and increase soil microorganisms (Lawn Care Academy, 2010). The incorporation of humic acid fractions in media designed for the enumeration of soil micro-organisms belonging to specific physiological groups was found to result for some groups in appreciably higher counts. It is suggested that by influencing the enzyme systems of certain micro-organisms, humic compounds may affect the range of substrates which they can utilize. The effect could have implications on the activity of organisms in environments in which humic substances are normally present, such as soils and natural waters (Visser, 1984).

Coskan et al. (2010) carried out a pot experiment to represent effects of humic + fulvic acid (HFA) applications on biological nitrogen fixation under soybean vegetation. Humic + fulvic acid application realized by either incorporate to soil or admixing by irrigation water. Seeds are inoculated by appropriate Bradyrhizobium japonicum strain, before sowing. In flowering stage, roots are removed from soil and the number of nodule determined (Fig 5).

Fig. 5. Effects of humic + fulvic acid application on nodule number of soybean (-Inoc: noninoculated)

Due to the fields where soil was taken is not previously introduced with the rhizobia that match with soybean; no nodule occurrence was observed in the pot which non-inoculated control variant. A few nodules observed in non-HFA applied pots however HFA application increased nodule occurrence considerably. Both "incorporation to soil" and "admixing by irrigation" applications were effective on formed nodule number; however because of the dilution effect in incorporation to soil application, admixing by irrigation application seems to be more effective than incorporation to soil. (dilution effect). Increasing doses of HFA increased the number of nodule, thus findings expressing the considerable positive effects of HFA on biological nitrogen fixation.
