Spinach (Spinacia oleracea L.) is known for nutritional value. It is good source of vitamins and minerals. However it can acumulate harmfull substances such as nitrate and oxalate. Oxalate content in spinach is associated with enviromental factors, one of the major is fertilization. Nitrogen fertilizer with nitrate form stimulate oxalate accumulation. Oxalate synthesis is associated with root uptake of nitrate and its assimilation in leaves. Oxalate accumulation is related to OH- production caused by nitrate assimilation. Presence of OH- ions causes pH increase, which induces formation of oxalate. Increasing contentration of nitrates in solution causes oxalate accumulation. Nitrate can inhibit the oxidase of oxalic acid. N fertilizers increase nitrate assimilation. Oxalate and nitrate correlation is not necessarily positive, probably due to metabolic homeostasis.