Interception of Solar Radiation and Dry Matter Production by Various Soybean Planting Patterns1

Solar radiation interception and dry matter production were studied in soybeans ( Glycine max (L.) Merr.) grown in various planting patterns (4 populations of 26, 52, 104, and 209 thousand plants per acre within each of 4 row widths – 5, 10, 20, and 40 inches). Increased population resulted in increased leaf area index (LAI) and a reduction in the number of days from emergance to 95% (daily basis) solar radiation interception (D 95 ). At corresponding populations of 52,000 or above, D 95 was considerably higher for the 40‐inch row pattern than for the other spatial arrangements, which differed in reaching D 95 by only a few days. Dry matter production was a functin of percent solar radiation interception regardless of planting pattern. However, the efficiency of utilization of intercepted energy differed between years for the mean of all treatments, and was significantly lower for the 208,000 plant population in 1962. These differences were attributed to a limitation imposed by moisture stress. Seed yield was not correlated with total dry matter produced, dry matter produced during seed formation, or solar radiation intercepted. As determined by differences in harvest index, seed yield was a function of differential utilization of photosynthate between vegetative and seed production. In general, high population or close plant spacing in any one direction tended to result in a low harvest index. Thus, while full interception during seed formation is required for maximum seed yield, conditions resulting in a high seasonal interception may not result in highest seed yield.