Optimal range of relative bulk density obtained using proctor and axial stress of 200 kPa for sunflower (Helianthus annuus L.) growth

The aim of this study was to determine the critical limits and optimal range of relative bulk density (RBD) obtained using proctor test (RBDProctor) and axial stress of 200 kPa (RBD200kPa) for a silty clay loam soil which were determined based on relative grain yield, relative shoot dry, relative height and relative root volume of sunflower (Helianthus annuus L.). The relative values were obtained by dividing the absolute mean values of mentioned traits to observed maximum values of each trait. A range of soil bulk density (1.35 to 1.59 Mg m-3) was prepared in 10 bins (40 cm × 50 cm with 50 cm depth) using rollers of different weights. The bins were located out-doors after sowing sunflower and the weight of bins and drained water volume were measured daily. The soil water content was retained in the range of field capacity (FC) to 0.6FC through irrigation. The optimal ranges of RBDProctor and RBD200kPa were assumed to be the ranges in which the relative traits of sunflower were greater or equal to 0.95 of maximum value predicted by obtained equations fitted to relative data versus RBD. Sunflower yield was maximum at RBDProctor equal to 0.854 and the optimal range of RBDProctor based on relative grain yield was from 0.823 to 0.884. In this range sunflower relative grain yield was greater or equal to 95%. The corresponding values of RBD200kPa were 0.975, 0.940 to 1.009, respectively. This finding revealed that at the RBD values less than lower limit and greater than upper limit of starting restriction, unsuitable physical conditions reduce sunflower relative grain yield.