Growing plants in greenhouses and pots requires special soil material. The accumulation of high content of soluble salts also restricts the plants growth due to direct ion toxicity (e.g. sodium, chloride, borate, etc.), ionic imbalance of the plants and decreasing of the water availability (physiological drought). The plant growth rate is influenced by the effect of salts on leaf area per plant, on leaf thickness, on daily increment in thickness and area of primary leaves, on the changes in the amount of vascular tissue into stem and other effects. In order to improve physical and chemical properties of horticultural substrata we set up some experiment in which a copolymer of maleic anhydride with vinyl acetate (AM-VA) in form of ammonium salt was used as additive. In our work the main organic components of horticultural substrata –fibrous peat and old manure were wetted with aqueous AM-VA polymer solution or water. After 48 hours we determined the content of soluble salts, the nitrate and ammonium ions. The content of water soluble substances in the extract obtained from old manure treated with polymer solution decreased with 98-118 mg/100g wetted component compared to the control variant. The water-soluble substances in the extract obtained from fibrous peat treated with polymer solutions slightly increased with 3-31 mg/100g wetted component compared to the control variant. The content of the ammonium ion increased in all variants treated with polymer solution. This behaviour was expected taking into account the chemical structure of the copolymer.