Bill Cox, Section of Crop and Soil Sciences, Cornell University
Most agronomists agree that growers should plant soybeans at the 1.5 inch depth because the seed is vulnerable to drying out at shallower depths and crusting problems at deeper planting depths, both which result in reduced emergence. We conducted a variety x planning date x seeding depth study at the Aurora Research Farm in 2013 and 2014. We planted two soybean varieties on five dates from late April through mid-June at seeding depths of 1.0, 1.5, 2.0, and 2.5 inches. In addition, we conducted field-scale studies with three soybean growers who planted soybean at the same seeding depths. This news article will report on the trends in the early plant populations taken at the V2 (2nd node) stage, about 2 to 5 weeks after planting (depending upon planting date). Eventually, regression analyses will be conducted on the early plant population and yield data at the conclusion of all the studies.
The 1.0 vs. the 1.5 inch seeding depth emerged 0.25 to 1.25 days earlier on the first three planting dates in 2013 (Table 1). In addition, the 1.0 inch seeding depth had ~2,000 to ~11,000 more plants/acre on all planting dates in 2013 (Table 2). Moist conditions ensued after all planting dates in 2013, which negated drying out of the soybean seed at the 1.0 inch depth leading to more rapid and better early stand establishment. Obviously, planting soybeans at the 1.0 inch depth is not a problem and may be a benefit during wet springs.
In 2014, however, dry climatic conditions ensued from May 20 until June 10 (0.13 inches of precipitation), which probably contributed to the 0.75 to 1.25 day delayed emergence and ~3,000 to ~25,000 fewer plants/acre at the 1.0 vs. the 1.5 inch depth on the May 19 and June 12 planting dates (Tables 1 and 2). Clearly, planting soybeans at the 1.0 inch depth is a problem for stand establishment when dry conditions prevail during the late spring.
The 2.5 vs. the 1.5 inch seeding depth generally required an additional day for emergence (although an additional 3.75 days were required at the late April planting date in 2013). Also, the 2.5 vs. the 1.5 inch seeding depth mostly had >10,000 fewer plants/acre but there were two exceptions where the 2.5 vs. the 1.0 inch depth had ~5,000 to ~10,000 more plants/acre (May 19, 2013 and May 30, 2014). Dry ensuing conditions after the May 30 planting date in 2014 probably resulted in dry conditions at the 1.5 inch seeding depth, thereby reducing emergence at that depth.
Despite the differences in early plant populations among the seeding depths at the five planting dates, soybean yields did differ among the four seeding depths on the first four planting dates in 2013 (What’s Cropping Up?, vol.24, no.1, 2014, p.1-2). Early plant populations mostly exceeded 120,000 plants/acre for all planting depths (except for the 2.5 inch seeding depth on the April 20 and June 12 planting date in 2013), which apparently were adequate to optimize yields in 2013.
The field-scale studies were planted from May 10 to May 27 in 2013 and from May 24 to June 6 in 2014 (wet May conditions delayed planting at all three farms in 2014). Consequently, soybeans only experienced warm conditions after planting in most of these studies. Nevertheless, soybeans at two farms (Livingston and Tomkins Co.) showed pronounced negative linear responses to seeding depth for early plant populations as indicated by ~9,000 to ~17,000 fewer plants/acre at the 2.5 vs. 1.0 inch seeding depth (Table 3). The Livingston Co. study in both years was on a silty clay loam soil so probably crusting or difficulty in emerging through a heavier soil from a deeper depth explains the consistent 17,000 fewer plants/acre.
The Tompkins Co. study, however, was on a gravelly silt loam soil in both years so crusting problems were probably not the major cause for reduced emergence. Consequently, it is not clear why soybean planted at the 2.5 vs. 1.0 inch depth had ~9,000 fewer plants/acre in 2013 and ~17,000 fewer in 2014. Despite the pronounced negative linear response to seeding depth at Livingston Co. in 2013, seeding depth did not affect yield (yields ranged from 57-59 bushels/acre). Surprisingly, soybean yield showed a negative quadratic response at Cayuga Co. in 2013 (67 and 66 bushels/acre at the 1.0 and 2.5 inch depths, respectively, but only 63-64 bushels/acre at the 1.5 and 2.0 inch depths). Likewise, soybean yield showed a quadratic response (positive) at the Tomkins Co. site in 2013 (yields increased from 59 to 63 bushels/acre as seeding depth increased from the 1.0 to 1.5 inches then decreased to 61 bushels/acre at the deeper depths).
In conclusion, seeding depth affects early plant populations in soybean but the response is not consistent. Apparently, the 1.0 vs. the 1.5 inch seeding depth can result in greater plants/acre if wet conditions ensue after planting. If dry conditions ensue after planting, as at the Aurora Research Farm from May 20 to June 10 or at the Cayuga Co. in 2014 (located 1 mile from the Aurora Research Farm and planted on May 24), the 1.0 seeding depth and maybe the 1.5 inch seeding depth is too shallow, which can result in fewer plants/acre. The data indicates that climatic conditions after planting is equally important as the actual seeding depth in determining optimum seeding depths for soybeans. Unfortunately, climatic conditions in the first 10 days after planting are not predicted with great precision so planting at the 1.5 inch depth appears to be the best compromise.