Quantification of Grain Yield Response to Soil Depth in Soybean, Maize, Sunflower, and Wheat

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Abstract
Risk analysis to identify the more profitable or less risky crops in areas with shallow soil requires quantification of yield responses to physical constraints to root penetration. Grain yield, shoot biomass, and harvest index (HI) were measured in commercial fields of indeterminate soybean (Glycine max L. Merr.), maize (Zea mays L.), sunflower (Helianthus annuus L.), and wheat (Triticum aestivum L.) grown on Typic Argiudols and Petrocalcic Paleudols. At crop maturity, shoots were sampled along transects (approximately 200 m) with soil depths between 0.35 and >1 m as determined by petrocalcic horizon depth. In all four species, shallow soil reduced shoot biomass and grain yield but did not affect HI in wheat. Harvest index was most affected by shallow soil in maize. Seasonal water deficit [maximum – actual evapotranspiration (ET)] accounted for 43 to 90% of the variation in yield. Average water use efficiency (WUE) was wheat, 14.5; maize, 11.7; soybean, 8.9; and sunflower, 7.5 kg grain ha−1 mm−1 ET. In relation to crops on the deepest soil, yield decline per centimeter reduction in soil depth was 0.41% in wheat, 0.45% in soybean, 0.54% in sunflower, and 0.76% in maize. This ranking of grain yield response to shallow soil was mostly accounted for by (i) cropping season (autumn to late spring for wheat vs. spring to autumn for row crops), (ii) timing of the most critical period for grain yield determination (later in soybean than in sunflower and maize), and (iii) plant features related to vegetative and reproductive plasticity, including growth habit.