To suppress the use of antibiotics and agrochemicals in harvesting a commercial crop and enable the harvest of a safe crop by mixing an organic fertilizer raw material with a specific substance, powder or its extract and using the mixture as a main component of the objective organic fertilizer.

This fertilizer is produced by mixing 100 pts.wt. of an organic fertilizer raw material selected from poultry manure, fish manure, cattle dropping, pig dropping, oil cake, pupa cake, vegetable residue and their fermentation products with 2-70 pts.wt. of a seaweed or powdered seaweed selected from among Posidonia austraris, Posidonia ostenfeldii, Posidonia sinuosa and Posidonia augustifolia or 0.005-5 pts.wt. of an extract of the seaweed. The fertilizer is used as it is or added as a fermentation assistant in the case of fermenting poultry manure, etc., and fermented together with the manure. The fertilizer may be incorporated as necessary with peat moss, crab shell powder, chitin, bone meal, magnesia, sawdust, feather meal, etc.