SOYBEAN CULTIVARS RESPONSE TO FLOOD IRRIGATION OF CLAY SOIL

Surface irrigation is widely used to increase seed yield of soybean [Glycine max (L.) Merr.] grown on clayey soils of the southern Mississippi River alluvial plain. A field study was conducted on Sharkey clay (Vertic Haplaquept, very-fine, montmorillonitic, thermic) in 3 yr to measure the effect of flood irrigation of varying duration on yield and yield components of soybean. Treatments were (Treatment 0) nonirrigated; (Treatment 1) 24 h to flood (soil submerged 24 h after onset of irrigation) and then drained; (Treatment 2) 24 h to flood plus 24-h moving water flood, then drained; (Treatment 3) 24 h to flood plus 48-h moving water flood, then drained; (Treatment 4) soil submerged with static water for 24 h, then drained; and (Treatment 5) soil submerged within 2 h of onset of irrigation, and then drained. All irrigation treatments commenced at first bloom and repeated whenever soil water potential of a treatment dropped below - 70 kPa at 30-cm depth. 'Sharkey' soybean was more responsive than was 'Centennial', but all irrigation treatments resulted in significant yield increases of both cultivars in years when rainfall was deficient during reproductive development. Treatment 3 flooding resulted in significantly lower seed yield than did Treatment 1 flooding, while Treatments 1 and 2 floodings achieved similar yield increases. In a rainy year (1988), Treatment 3 average yield was lower than yields from all other treatments. This indicates that even relatively short periods of flooding can be detrimental to soybean if complemented with untimely rainfall. Standing water for 24 h (Treatment 4) produced yield increases that equalled or exceeded those of all the moving water treatments. These results showed that flood inundation of less than 48 h will result in the consistently largest significant yield increase for soybean, and that soybean genotypes differ in their response to flood irrigation.