RELATIONSHIP BETWEEN PLANT AVAI LABLE SOIL WATER AND YIELD FOR EXPLAINING SOYBEAN YIELD VARIABILITY

Spatial patterns of crop yield differ from year to year because of spatial and temporal interactions that occur within a field. A clear understanding of spatial soil–water uptake by plant roots is fundamental to understand yield variability and to make management recommendations that maximize profit or minimize environmental impacts. The objective of this study was to investigate variations in water relations within and between soil map units in a field in order to explain spatial distribution of soybean yield. This research was conducted in a 20–ha field in Boone County, Iowa, in 2000. Spatial distribution of soil water was investigated in 30 sites across field using a tube–access TDR probe. Aerial digital photos were taken three times during the growing season to investigate the relationship between plant canopy and resulting yield. Results showed that soybean yield was greatly reduced in the field compared to an average year, probably due to the occurrence of a drier than normal year. The yield variation was about 24%, likely due variation in soil water during pod filling. Soil water balance calculations for selected sites showed that plants likely experienced water stress in mid–July, but the level of stress increased dramatically later in the season and reached its maximum at the end of August. The sites exposed to earlier water stress exhibited lower yield. There was a good correlation (r 2 > 0.48) between plant available soil water and yield for any date during the reproductive phase of the soybean crop. The soil water relations were able to explain more than 48% of yield variability in 30 sites. However, the vegetation index did not correlate well with yield for any of the dates on which remotely sensed images were taken. This poor relationship indicated the variable drought stress that dominated yield variability occurred after full canopy was reached and primarily affected pod numbers, not canopy biomass.