Equations to estimate gross photosynthesis, respiration, and dry-matter accumulation for soybean from daily estimates of intercepted, photosynthetically active radiation (PAR) were developed from field measurements. We found that gross photosynthesis was a function of intercepted PAR and that 24-h dark respiration was a function of gross photosynthesis. In addition, we developed equations relating intercepted PAR to leaf area index (LAI) and solar radiation. We assumed that gross photosynthesis was reduced when soil-water depletion was greater than 65%. The growth model predicted accumulated dry matter for soybean in 1974, 1975, and 1976 to within 2.7 mg cm−2 of observed values.