Emergence of seedlings of sugar beet (Beta vulgaris L.) as affected by the size, roughness and position of aggregates in the seedbed

Laboratory and field experiments were carried out to quantify the effects of the size and roughness of aggregates placed in the seedling path of sugar beet, in order to help in decision making for soil tillage and sowing operations. Graded aggregates (10, 15, 20, 30, 40, 50, 60 and 70 mm longest axis) were either laid on the soil surface or included in the soil over the seeds. The percent emergence decreased exponentially with aggregate size over 10 mm when the aggregates were included in the seedbed. The result was the same with aggregates laid on the soil surface, but for aggregates over 30 mm (mass > 10 g). Aggregates on the soil surface could be lifted by the seedlings until their weight exceeded the seedling emergence force. Larger aggregates or aggregates in the soil layer could not be moved. Seedlings which did not emerge remained blocked in small cavities in the aggregate surface. No seedlings were blocked under smooth aggregates or glass beads. The experimental results fits with a model giving the probability to meet a hole according the distance covered and the diameter and density of holes. The results obtained under controlled conditions were similar to those obtained in field experiments for a wide range of aggregate sizes. These results will be incorporated into a computerised seedbed generator to simulate the effects of seedbed structure on seedling emergence.