More efficient fertilization practices could conceivably be developed if nutrient availability from manures and crop residues could be accurately predicted under field conditions. Heat units were recently found useful for describing the combined thermal and temporal relationships with papermill sludge C and N mineralization. This study was conducted to describe C and N mineralization of corn (Zea mays L. cv. ‘King 1113’), lupin (Lupinus albus L. cv. ‘ultra’), and potato (Solanum tuberosum L. cv. ‘russet burbank’) residues and to field test the utility of heat units in predicting these processes. Residues of each crop were applied to soil microplots in May, June, and July of 1987. Carbon mineralization was monitored by periodic capture of evolved CO2 in alkali traps. Nitrogen mineralization-immobilization processes were followed by frequent sampling for KCl extractable inorganic N. Net N mineralization from the potato residue occurred almost immediately after residue application, apparently owing to its relatively low C/N of 15/1. Net N mineralization from the corn and lupin residues began 119, 99, and 317 d after application in the May, June, and July application months, respectively. These respective times represented 2346, 1990, and 2360 degree days after application. Heat units appear to provide a mathematically simple, pragmatic approach for predicting crop residue N mineralization under field climatic conditions.