A better knowledge of the plant-availability of nitrogen (N) in organic ammendments is necessary to improve efficient application of these products as fertilizers. In the present study, availability indices for N of four poultry-based organic ammendments obtained by different methods were compared with the plant uptake of N from these products and ammonium nitrate as a chemical reference fertilizer. The laboratory methods included chemical method, HCl extraction, thermal fractionation (heating of organic products), and CaCl 2 -autoclave method. The results of pot experiment with corn ( Zea mays L.) using a double-pot technique over 35 days were used as reference for plant-available N. Relative effectiveness of organic N (REo) was calculated as the fraction of organic N that has the same availability to plants as inorganic N. Available N contents of immature poultry manure, composted manure, Micromix and Amino Micromix were estimated to be on average 4.6, 1.8, 1.0, and 1.1 g kg -1 , respectively. Immature poultry manure had the highest available N among organic ammendments studied. The CaCl 2 -autoclave method reasonably predicted plant available N in organic products while nitrogen extracted by the other methods had poor correlations with available N index (RE 0 ) of the organic products. REo value was the highest for the immature poultry manure. Based on the results obtained, the CaCl 2 -autoclave method might be recommended as a practical laboratory index for plant availability of organic N in poultry manures. Keywords: poultry manure, compost, available nitrogen, double pot method,organic nitrogen, autoclave method