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A greenhouse experiment was carried out using a Decatur silt loam soil to assess the availability and recovery of algal nitrogen (labeled) which was previously grown in simulated inorganic‐wastewater medium by grain sorghum . Dried green or blue‐green algal biomass providing 175, 350 and 525 mg labeled N/kg of soil was used as sources of nitrogen. Application of 175, 350 and 525 mg N/Kg of soil as algal biomass resulted in the release of 73.2, 126.7 and 181.6 mg N for green and 85.9, 147.6 and 183.3 mg N for blue‐green algal biomass, respectively, via mineralization over a 6‐week period. These values correspond to 35 to 42% of total N applied as green and 35 to 49% of total N applied as blue‐green algal biomass. Grain sorghum plants assimilated 25 to 34% of total N applied as green algae and 24 to 45% as blue‐green algae during 6 weeks of growth. The application of above mentioned rates increased the shoots yield by 94, 82, and 65% for green algae and 88, 61, and 61% for blue‐green algae, respectively. No significant increase was observed for the application of algal biomass at a rate higher than 175 mg N/kg of soil. Total N uptake by grain sorghum increased significantly due to the addition of algal biomass to the soil. About 58 to 65% of total green algal N and 51 to 65% of total blue‐green algal N remained in the soil. Six to 12% of the total N applied as green algae and 4 to 11% as blue‐green algae biomass was unaccounted for. The results indicated that there was no significant difference between green and blue‐green algal biomass with regard to their use as a source of plant nutrients. The results also encourage the use of algae as cleaning microorganisms for wastewater facilities and lagoons because the harvested algal biomass may be used as a source of plant nutrients to help reduce the cost of operations. [ABSTRACT FROM PUBLISHER]