Dormancy of seeds of shepherd's purse in alternating wet and dry, compressed aggregated soil: a laboratory experiment.
The dormancy of seeds of shepherd's purse (Capsella bursa-pastoris) in different soil structures during the 63-day period of establishment of a crop of grass (Lolium multiflorum) was simulated in the laboratory. The seeds were incorporated in 450-g test samples of an artificial aggregate mixture comprising a fixed fraction of each of five sizes of aggregate preparations which were differently crumbled and compressed in cylinders at 0.3 and 1.8 bar before the grass seeds were sown. The soil was subjected to seven cycles of moistening and drying. After 63 days the soil was sieved and aggregates crumbled to particles <2.5 mm diameter, then spread out in germination trays, air dried and remoistened to stimulate germination. Emerged seedlings represented the non-dormant seeds during crop growth and were counted daily. The different conditions that prevailed in the four replicates after growing the crop in the cylinders caused a block effect between the sieved soil of the replications to appear in the fractions of emerged seedlings. A stepwise regression analysis of the transformed emergence percentages on eight interrelated soil structure and moisture features indicated that models with two properties gave a satisfactory explanation. The mean volumetric moisture content (MC) during crop growth had the greatest influence on the fraction of emerged seedlings. The loss of moisture by gravitation immediately after soil saturation represented the volume of large pores. It strongly determined lower MC and was thus connected with higher emerged seedling fractions. However, with MC at a rather low level, the fractions emerging decreased with increasing volume of these large pores. A decrease of soil evaporation and crop transpiration during crop growth in the presence of the MC explained the fractions of emerged seedlings almost as well as did the large pores. The way these factors achieved their influence is discussed. When the models were extended to include soil compression and the initial aggregate mixtures, hardly any influence of these factors was noticed. It is recommended that weed seedling emergence should be studied in relation to soil structure and soil moisture features, and that field conditions should be simulated in the laboratory to develop models for application in the field.