To update our recommendations for soybean population, the Delaware Soybean Board sponsored a study of five different planting populations (60, 90, 120, 150, and180,000 seeds per acre), two row spacings (15 and 30”), and included irrigated and rainfed treatments under variable rate irrigation. Overall, no differences in yield were observed by population, while 15″ rows boosted yields by 10.6 bushels and irrigation boosted yields by 25.9 bushels.

Figure 1: a) aerial image of planting populations and row spacings at our Irrigation research farm, b) rainfed plots senescing earlier under our variable rate linear irrigation field.

Project Summary

Soybeans (maturity group 4.3) were planted at the UD Warrington Irrigation Research farm in May 2022 and harvested in November 2022 with a plot combine. The results were analyzed statistically as a randomized complete block design with three factors (population*row-spacing*irrigation) with means separation by Fisher’s LDS (alpha = 0.1).

There were no interactions between population, row spacing, or irrigation status, which can be observed in Figure 2. For each planting population, the yield pattern between row spacing and irrigation was always the same, where 15” irrigated plots had the highest yields (75-78 bu/acre) and 30” rainfed had the lowest (38-45 bu/acre).

Figure 2: Soybean yield (bu/acre) across different populations sorted by row spacing*irrigation combinations.


When averaged across all irrigation and row spacing, yields by populations ranged from 63 to 68 bu/acre, with no significant differences between full season planting populations. Previous work by the University of Delaware had not observed differences in populations between 90 and 180,000 seeds per acre, but even dropping to 60,000 did not provide any differences.

Row Spacing

Across all of the populations and irrigation conditions, 15 inch rows yielded 10.6 bushels better than 30 inch rows. This is also supported by national research that shows either 7.5 or 15 inch rows often yield similar, but higher yields than 30 inch rows. That this occurred across all population and irrigation combinations supports this practice under many management scenarios.  


Moisture is agriculture’s most limiting factor and the very dry summer of 2022 provided a major boost to zones that received irrigation under our linear field. Across all population and row spacing combinations, irrigation provided a 25.9 bushel difference over rainfed conditions. This would certainly vary by season and rainfall availability, but 2022 was particularly dry from July to August during reproductive stages in Harbeson, and irrigation provided a major advantage.

Drone imagery (Figure 3) also revealed the effect of soil type and water holding, where some rainfed zones have strong growth (greener colors or higher NDVI), revealing possible soil moisture holding that occurs there. Although the differences still held out across treatments, this project does help show the importance of both irrigation and soil moisture holding in maintaining higher yields. However in the case of soybeans, management in terms of populations did not matter with irrigation, while row spacing works best at 15 inch rows and irrigated conditions.

Figure 3: Late season NDVI showing full soybean canopy across row spacing and populations, with drought affected rainfed plots lighter in color.
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