NOVELTY - Fermented algae liquid fertilizer synergistic agent is prepared by mixing natural algae powder, corn flour, sugar and water to obtain natural algae fermentation material; spraying fermentation effective microorganisms (EM) stock solution onto natural algae fermentation material and fermenting at room temperature for 5-10 days to obtain fermented natural algae material; extracting fermented natural algae material; adding algae extraction solution with sodium octaborate tetrahydrate; and adding disodium EDTA, zinc sulfate solution and manganese sulfate solution into extract.    USE - Method of producing fermented algae liquid fertilizer synergistic agent (claimed) used for promoting crop root growth.    ADVANTAGE - The fertilizer synergistic agent fertilizer can provide slow-releasing effect to supplement boron, zinc and manganese trace elements on soil to increase crop yield. It can increase crop absorption on phosphorus, potassium and microelement and reduce nutrient loss, while avoiding secondary pollution and reducing fertilizer production cost. Further, the synergistic agent has wide raw material source, and is convenient to add in fertilizer production.    DETAILED DESCRIPTION - Production of fermented algae liquid fertilizer synergistic agent comprises:    (A) mixing (pts.wt.) natural algae powder (80-1200), corn flour (10-200) and sugar (30-80), adding water to the mixture to make water content to 30-50 wt.% of the mixture, and then adjusting pH value of the mixture to 6-8 using inorganic acid or inorganic alkali aqueous solution to obtain a natural algae fermentation material;    (B) adding (pts.wt.) EM flora fermentation liquid (0.1-0.2) and sugar (0.1-0.2) with water (0.5-20), uniformly mixing, putting into a sealed container for 20-30 hours to obtain fermentation EM stock solution;    (C) diluting the stock solution with water for 300-500 times to obtain dilute solution, then spraying dilute solution (10-100 pts.wt.) onto natural algae fermentation material (800-1200 pts.wt.), uniformly stirring, and fermenting at room temperature for 5-10 days to obtain fermented natural algae material;    (D) putting (pts.wt.) fermented natural algae material (800-1200) into a reaction kettle, adding 5-25 wt.% sodium hydroxide or sodium carbonate aqueous solution (10-50), stirring and extracting at 70-100 degrees C for 6-12 hours, then filtering, evaporating and concentrating filtrate to obtain algae extraction solution having solids content of 59-20 wt.%;    (E) using algae extraction solution as mother liquor, adding sodium octaborate tetrahydrate to each liter of mother liquor, and dissolving to obtain algae extraction solution II;    (F) respectively preparing 0.1-0.3 mol/L of disodium EDTA, zinc sulfate solution and manganese sulfate solution at volume ratio of 2:1:1 to obtain a trace element solution; and    (G) adding algae extraction solution II with the trace element solution at volume ratio of 1:0.1-1 and uniformly mixing to obtain the fermented algae liquid fertilizer synergistic agent.    An INDEPENDENT CLAIM is included for a fermented algae liquid fertilizer synergistic agent used in a fertilizer composition comprising urea, ammonium carbonate, monoammonium phosphate, diammonium phosphate, potassium chloride, potassium sulfate, and trace element, where the synergistic agent is added at 5-100 pts.wt./1000 pts.wt. of fertilizer.