Spatial patterns of crop yield differ from year to year because of spatial and temporal interactions that occur withina field. A clear understanding of spatial soilwater uptake by plant roots is fundamental to understand yield variability andto make management recommendations that maximize profit or minimize environmental impacts. The objective of this studywas to investigate variations in water relations within and between soil map units in a field in order to explain spatialdistribution of soybean yield. This research was conducted in a 20ha field in Boone County, Iowa, in 2000. Spatialdistribution of soil water was investigated in 30 sites across field using a tubeaccess TDR probe. Aerial digital photos weretaken three times during the growing season to investigate the relationship between plant canopy and resulting yield. Resultsshowed that soybean yield was greatly reduced in the field compared to an average year, probably due to the occurrence ofa drier than normal year. The yield variation was about 24%, likely due variation in soil water during pod filling. Soil waterbalance calculations for selected sites showed that plants likely experienced water stress in midJuly, but the level of stressincreased dramatically later in the season and reached its maximum at the end of August. The sites exposed to earlier waterstress exhibited lower yield. There was a good correlation (r 2 > 0.48) between plant available soil water and yield for anydate during the reproductive phase of the soybean crop. The soil water relations were able to explain more than 48% of yieldvariability in 30 sites. However, the vegetation index did not correlate well with yield for any of the dates on which remotelysensed images were taken. This poor relationship indicated the variable drought stress that dominated yield variabilityoccurred after full canopy was reached and primarily affected pod numbers, not canopy biomass.
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Ирмак et al. (2002) studied this question.